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Coatings and Liquid-applied Membranes— what's in a name?

Liquid-applied roof membranes (LAM) and roof coatings (aka, maintenance coatings) are not only here to stay, their use is on the rise. This blog takes a look at how the building code and the roofing industry generally differentiate between liquid-applied roof membranes and roof coatings. There is confusion because the intended use of each is different, yet many of the materials are the same for both applications. Here's what you need to know to help understand and differentiate between the two.IntroductionCoatings have been used in the construction and roofing industries for a very long time! They have been made from many different materials--from beeswax and pitch some 5000 years ago, to lacquers and varnishes just a couple thousand years ago, to our current polymer-based materials. According to the Roof Coating Manufacturers Association, "the most dramatic advance in coating properties has come in the past 40 years, with the development of polymers1." Polymer-based coatings are used on plaza decks, parking garages, balconies, playgrounds, and roofs, for example, to provide a level of water-resistance and an aesthetically pleasing surface. Polymer-based liquid-applied membranes are used as the water-proofing layer for new roofs, replacement roofs, and roof re-cover systems. The common polymer-based materials include acrylics, silicones, and urethanes. More information about these materials can be found here.The spotlight is on these types of polymers because the materials we use for coatings are quite often also being used as liquid-applied membranes. How do we categorize and define these different installations that have different intended uses when both applications use essentially the same set of materials? This blog takes a close look at each of these product categories—coatings and liquid-applied membranes—to find their similarities and differences. And hopefully to provide clarity around the use of terms and definitions of use.Market ShareIn 2017, The Freedonia Group published a research study titled, "Liquid-Applied Roof Coatings in the US by Product and Subregion." According to that report, 11.85 million squares (1.185 billion square feet) of liquid-applied roof coating were installed in 2016. Approximately 40% was installed in the South, with the remainder essentially evenly split between the Northeast, Midwest, and West regions.The Freedonia Group reported a number of key findings that help explain the increased use of coatings."The South will be the leading US regional market for roof coatings in 2021, boosted by a high level of interest in cool roofing products and in protecting roofs against storm damage.The West will see solid growth as communities amend building codes to mandate the use of cool roofing.Liquid-applied roof coating demand in the Midwest and Northeast will be supported by rising use of roof coatings to rejuvenate older roofs instead of engaging in more costly reroofing projects."Note: The Freedonia Group's report does not separate market share based on liquid-applied materials used as roof coatings versus liquid-applied materials used as roof membranes.The use of coatings and liquid-applied membranes is increasing for a number of additional reasons as well.The use of materials that can be applied at ambient temperature is welcomed by an installer. There are no super-heated materials or open flames therefore reducing specific safety concerns.Materials are typically provided in containers sized for easy transport to and from rooftops.Common low-cost installation tools are used—brooms, brushes, squeegees; and simple, low-cost spray equipment.Using liquid-applied membranes can reduce waste created by a tear off.These materials are commonly light colored so they are reflective to help improve energy efficiency.Depending on the design (intent) and application of polymer-based materials, they can be used to extend the life of an existing roof when used as a coating, or to provide a warranted or guaranteed, waterproofing roof covering when used as a liquid-applied membrane.Defining the TermsOne way to help sort out the difference between coatings and liquid-applied membranes is to understand current definitions used in the industry. The International Building Code (IBC) is a good place to start since it is considered to be consensus-based.International Building CodeThe International Building Code does include a definition for coating, but does not include a definition for liquid-applied membrane."ROOF COATING. A fluid-applied, adhered coating used for roof maintenance or roof repair, or as a component of a roof covering system or roof assembly."IBC's definition of Roof Coating tells us three things.Coatings are fluid-applied and adhered (to a substrate)Coatings are used for maintenance or repair "Roof Repair" is defined as "Reconstruction or renewal of any part of an existing roof for the purposes of correcting damage or restoring pre-damage condition." Coatings can be a component of a roof system or roof assembly (which are the same according to ICC's definitions) "Roof Assembly" is defined as "A system designed to provide weather protection and resistance to design loads. The system consists of a roof covering and roof deck or a single component serving as both the roof covering and the roof deck. A roof assembly can include an underlayment, a thermal barrier, insulation or a vapor retarder." "Roof Covering" is defined as "The covering applied to the roof deck for weather resistance, fire classification or appearance." "Roof Covering System" is a "Roof Assembly" per IBC. Realistically, IBC's definition of Roof Coating doesn't get us that much closer to differentiating coatings and liquid-applied membranes, except that coatings are intended for maintenance and repair. And per IBC's definition, coatings can be used for Roof Repairs to "correct damage or restore pre-damage condition," but that is not how coatings are generally intended to be used.Taking a look at how Chapter 15 of IBC is arranged gives a bit of insight into IBC's perspective on coatings and liquid-applied membranes. Section 1507, Requirements for Roof Coverings, has and continues to include all low-slope and steep-slope materials used as roof coverings that are recognized by the code. This includes materials such as asphalt, wood, and slate shingles, as well as modified bitumen and single-ply roofing (and myriad others). The ICC has always included a section specifically for Liquid-applied Roofing within Section 1507, but there has never been a section for Coatings (until this year—more on that in a bit). To that end, the IBC is essentially saying Liquid-applied Membranes are categorized similarly to all other membranes that are used as roof coverings and their intended use is for "weather resistance, fire classification or appearance" (from IBC's definition as shown above). Because liquid-applied membranes are considered to be roof coverings, roof systems that use a liquid-applied membrane need to be tested for fire, wind, and impact… like any traditional membrane roof system.The liquid-applied membrane subsection within Section 1507 includes ASTM standards for materials not only used as liquid-applied membranes, but it includes the polymer-based materials (e.g., acrylics, polyurethanes, silicones) that are also intended to be used as coatings. This led to confusion within the code requirements, specifically how code officials would enforce the application of a coating product on an existing roof--as a new roof or as a maintenance item.To help with clarification and code enforcement, new language was added to the 2018 IBC in the Reroofing Section that stated a roof coating can be applied to (essentially) any existing roof without triggering reroofing requirements. The 2015 IBC and earlier versions only stated that coatings could be applied over an existing Spray Polyurethane Foam without removing any existing roofs. The IBC 2018 code language is as follows:"Section 1511.3, Roof Replacement. Exception 4: The application of a new protective roof coating over an existing protective roof coating, metal roof panel, built-up roof, spray polyurethane foam roofing system, metal roof shingles, mineral-surfaced roll roofing, modified bitumen roofing or thermoset and thermoplastic single-ply roofing shall be permitted without tear off of existing roof coverings."The additional language in the 2018 IBC was a very important step in distinguishing between coatings and liquid-applied membranes.The I-Codes were further revised regarding coatings and liquid-applied membranes in the 2021 IBC; a new section was added--Section 1509, Roof Coatings. This was an entirely new section, and importantly, Roof Coatings are not a subsection within Section 1507, Roof Coverings. This strengthens the differentiation from a code perspective that coatings are not considered to be a new roof covering. However, the IBC 2021 remains without a definition for liquid-applied roofing or liquid-applied membrane. The code ultimately relies on manufacturers' intentions for their products as the differentiating factor between coatings and liquid-applied membranes.ASTMUnfortunately, ASTM D1079, "Standard Terminology Relating to Roofing and Waterproofing" does not define either term.Industry PerspectiveWhat does GAF, a leading supplier of both systems, say about each? From GAF's page, Liquid-Applied Coating Solutions, the following descriptions are provided."What is a Liquid Membrane Roofing System?A liquid-applied roofing system consists of multiple components that come together to form a fully adhered, seamless, and self-flashing membrane. Components include liquid applied coatings and mesh membranes to create a true liquid membrane system that preserves and protects the integrity of the building." Examples of some of the leading products can be found here."What is a Roof Coating System?Roof Coatings are designed for extending the life of existing structurally sound roofs. GAF Roof Coatings are specially formulated to extend the life of roofs while protecting them from damaging effects of weather and the environment such as UV light, water and wind. GAF offers roof coatings in a variety of different technologies such as acrylic, silicone and polyurethanes to meet many different building needs and budgets."According to GAF, a liquid-applied roofing membrane protects the integrity of the building (like any traditional membrane-type roof system) and coatings are designed for extending the life of structurally sound roofs.The Roof Coating Manufacturers Association (RCMA) has a thorough description of a roof coating. RCMA is appropriately focused on the makeup of a coating (i.e., higher solids content, high quality resins) to differentiate roof coatings from what is commonly called "paint." One concept from RCMA in particular stands out—because roof coatings are "elastomeric and durable films," they provide "an additional measure of waterproofing" and can "bridge small cracks and membrane seams." The roofing industry recognizes a coating's ability to provide an amount of weather resistance / restorative properties, but this characteristic (i.e., crack bridging) is difficult to test for and quantify. And it is worth repeating, a roof coating is primarily intended to extend the service life of structurally sound roofs, not necessarily be the waterproofing layer. That is the intent of a liquid-applied membrane.FM ApprovalsLiquid-applied membranes are considered to be roof coverings by the IBC, and therefore they must be tested and have approval listings. Approval listings are used to show that systems have been tested and comply with the code requirements for roof system properties like fire-, wind-, and impact-resistance.RoofNav—New ConstructionTo that end, performing a search using the Assembly Search function within FM's RoofNav software results in a number of Approval Listings for "Liquid Applied Systems" used for New Roofs. With no manufacturer selected, the RoofNav search resulted in more than 10,000 Approval Listings for liquid-applied roofs used for new construction!Performing a second search using GAF as the manufacturer results in nearly 250 Approval Listings for "Liquid Applied Systems" used as new roofs. The nearly 250 Approval Listings include applications primarily over DensDeck™ and spray foam. When a liquid-applied membrane is used over a substrate board, such as a DensDeck™ board, a reinforcing fabric embedded between two foundation coats is used. The use of the substrate board is more common for new construction or roof replacement projects and is not common when re-covering an existing roof.An example RoofNav listing is shown here. It includes a finish coat and foundation coat with fabric over DensDeck that is adhered to polyiso, and the polyiso is adhered to a concrete deck.Wind-uplift capacity of liquid-applied membrane roof systems can be quite high. The example above has a wind uplift rating of 270 psf! Where would such a high-capacity roof system even be needed? Here's a blog that discusses design wind pressures.RoofNav—Re-coverIn addition to their use as new roofing, one of the primary attributes of liquid-applied membranes is their use over an existing roof. Searching RoofNav using GAF and "Re-Cover" as the Application results in nearly 200 Approval Listings.If a liquid-applied roof system is used in a re-cover application, the use of the reinforcing fabric seems to be tied to the specific substrate. Looking through GAF's RoofNav Approval Listings for Re-cover Liquid-Applied Systems, reinforcing fabric is used when re-covering traditional multi-ply asphaltic membrane roof systems, or TPO and PVC membranes. However, when the substrate is a standing-seam type metal roof panel, a metal-faced composite panel, or spray foam, the fabric is not listed as a necessary component of an Approval Listing.It's important to recognize that an FM Approval Listing also provides information about the internal fire rating, exterior fire rating, and hail ratings. Many liquid-applied roof systems achieve Class A Exterior Fire ratings as well as Moderate or Severe Hail ratings. For a short tutorial on using RoofNav's Assembly Search feature, watch this video.In SummaryThe following chart is intended to provide examples of similarities and differences between coatings and liquid-applied membranes.ConclusionSimply put, coatings are used to provide protection from the elements and help extend service life. Coatings are not installed as 'membranes' so they are not intended to seal leaks or be considered "waterproof". Liquid-applied membranes are considered to be just that—membranes—and are used as the covering in new and re-cover roof systems. Liquid-applied membranes are tested as systems and have approval listings just like traditional asphaltic, modified bitumen, and single-ply roof systems.References:1RCMA.org/history-of-roof-coatings

By James R Kirby

April 07, 2023

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  • wind
    Building Science

    Prevailing Winds and Prevailing Codes: A Summary of Roof Related ASCE 7-22 Changes

    Several changes have been included in the 2022 version of ASCE 7 as they relate to the roof. You may be thinking, 'as soon as I mastered ASCE 7-16, an updated version is set to be released!'. As with any Standard, it can be expected that updates will be made to include current research or trends. While the inclusion of tornado loads and the resulting changes in the load combinations may be the most significant, there are other updates that affect roofing as well. From minor updates to basic wind speed maps, to stepped roofs, and pavers, we have compiled a summary to help you navigate the updates. Not to fret, the changes are likely to not be incorporated until the 2024 version of IBC. However, that does not preclude incorporating these changes on current and upcoming projects.Why are these Changes Significant?The goal of the calculations in ASCE 7 are to determine the uplift pressures on a roof given project constraints including building height, location, and Risk Category. The resulting uplift pressures are then compared with tested roof assemblies to determine which assembly(ies) can be installed on a roof for each unique building. The assembly selection will also dictate the fastening method, whether it be mechanically fastened or adhered, and also the number and spacing of selected attachments. Changes in the Code may affect calculated uplift pressures that may influence the available roof assemblies.What is the Update? Updated Basic Wind Speed MapsWind speeds vary rapidly and are continuously being recorded at various locations around the world. Wind speeds are recorded in miles per hour (mph) in 3-second intervals and are collected over the course of a year. Then, the maximum values of the 3-second intervals are recorded and tracked to see the occurrence throughout the year. Since wind is a random phenomenon and the speeds vary not only by day, but also by year, the maximum 3-second intervals for each year are compared over several years. Rather than simply taking an average of wind speeds over time, the average wind speeds are analyzed by determining the Mean Recurrence Interval (MRI), which is how frequently a wind speed is equaled or exceeded during any given period of time (in years). Generally, higher wind speeds have a lower recurrence interval over time, and lower wind speeds have a higher recurrence rate over time. Analyzing this data is how wind speeds for a particular location are established.Wind speed (mph) has to be translated into units of force (psf) for design purposes in accordance with ASCE-7. The resulting forces are different in various roof zones.ASCE 7-16 has published basic wind speed contour maps for each Risk Category. The wind speed maps have contour lines that show the wind speeds throughout the United States as they vary by geographical location. It is allowable to interpolate between the contour lines, or the larger value listed on the contour line may be used. Additionally, if a location-specific wind speed is desired, an online Hazard Tool developed by ASCE will give an exact wind speed based on an address or GPS coordinates. The 2022 version updated wind speeds, primarily along the coastline. Wind speeds have generally either decreased by a few miles per hour or remained unchanged from ASCE 7-16, with the exception of several cities along the Gulf of Mexico coastline that have increased wind speeds due to recent hurricanes that have made landfall.Basic wind speeds from select coastal locations in hurricane prone regions at 33 ft for Exposure Category C are shown below. Wind speed data is shown for both ASCE 7-16 and ASCE 7-22 as a comparison.What is the Update? Wind directionality factor, Kd was movedThe wind directionality factor, Kd, was removed from the Velocity Pressure Coefficient and inserted into the Components and Cladding design wind pressures equation.The directionality factor (Kd) is a load reduction factor intended to take into account a reduced probability that the maximum wind speed will exactly coincide with the weakest direction of a building. According to ASCE 7, it accounts for the probability that the wind speed will come from any one direction given a location, and the maximum wind speed will occur from the direction that produces the maximum wind pressure on the building or its components. The directionality factor has constant factors to be used for Main Force Resisting Systems or Components and Cladding, but also accounts for various shapes of structures such as arched roofs, circular domes, and chimneys or tanks.In ASCE 7-16, Kd was located in the Velocity Pressure Equation (eq. 26.10-1) where velocity pressure, qh, is evaluated at the mean roof height, h. Where qh= qz .qh=0.00256KhKztKdKeV2 Which provides a velocity pressure in pounds per square foot (psf).The resulting value of qh is then multiplied by the external pressure coefficients (GCp) and then used to determine the wind pressure coefficients for each roof zone (perimeter, corner, field, etc.)p = qh [(Gcp) - (GCpi)] (lb/ft2)In ASCE 7-22, the velocity pressure, qh, is evaluated at the mean roof height, h. Where qh= qz by the following equation (eq. 26.10-1):qh=0.00256KhKztKeV2 Which provides a velocity pressure in pounds per square foot (psf).As you can see, the directionality factor, Kd, has been removed from the equation. In calculations for both the Main Wind Force Resisting System and Components and Cladding, Kd, has been inserted into the equations. However, since we are focusing on the roof, this falls within the Components and Cladding (C&C) calculations in Chapter 30. The design wind pressures of C&C elements are calculated with the following equations:For low rise buildings with h≤ 60 ft: (eq. 30.3-1)p = qh Kd[(Gcp) - (GCpi)] (lb/ft2)For low rise buildings with h＞60 ft: (eq. 30.4-1)p = q Kd[(Gcp) - qi Kd(GCpi)] (lb/ft2)What is the Update? Velocity Pressure Coefficients, Kz and Kh updatedThe Velocity Pressure Coefficients, Kz and Kh, for Exposures B and C were updated. Kz is the velocity pressure coefficient evaluated at height z, and Kh is the velocity pressure coefficient evaluated at height z = h. The velocity pressure at mean roof height h uses Kz. The variables are inserted into equation 26.10 to determine Velocity Pressure, qz (see equations above).What is the Update? Simplified Methods for Calculating C&C RemovedThe two simplified methods, Part 2 and Part 4 were removed from Chapter 30, Wind Loads, Components and Cladding.In the 2016 version there were two simplified methods that allowed for reduced calculations:Part 1 included calculations for Low-Rise BuildingsPart 2 was for Low-Rise Building (simplified)Part 3 included calculations for buildings with h> 60ftPart 4 was for buildings 60 ft<>Part 5 is Open BuildingsPart 6 included Building Appurtenances, Rooftop Structures, and Equipment.The 2022 version has removed the two simplified methods:Part 1 for Low-Rise BuildingsPart 2 is for buildings with h> 60ftPart 3 is Open BuildingsPart 4 includes Building Appurtenances, Rooftop Structures, and Equipment.What is the Update? Section 30.12 was added to address roof paversRoof pavers are used in IRMA (Inverted Roof Membrane Assembly) and PMRA (Protected Membrane Roof Assembly) assemblies where the roofing components are installed at the structural roof deck level and then the pavers are installed as ballast on top of the roof assembly. Roof pavers vary in size, thickness, material, and spacing, in addition to installation method and pedestal size. Pavers are considered air permeable since the gaps between pavers and the space beneath the pedestals allow for partial air pressure equalization between the surfaces. Roof pavers were only addressed in the Commentary Section C30.1 of the 2016 version and are referred to as Air Permeable Cladding. However, siding, pressure equalized rain screen walls, shingles, tiles, and aggregate roof surfacing were all included in this category. In ASCE 7-16, 'because of partial air-pressure equalization provided by air-permeable claddings, the C&C pressures services from Chapter 30 can overestimate the load on cladding elements. The designer may elect to use the loads derived from Chapter 30 or those derived by an alternate method.' While equations and methods are not included in this edition, several references are included where calculation methods may be found.In the commentary of ASCE 7-22, air permeable cladding is still defined as roof or wall claddings that allow partial air pressure equalization between the exterior and interior surfaces, with the same listing of claddings to include siding, roof pavers, and vegetative modular trays. The designer may elect to calculate the net uplift pressures of the pavers with recognized literature as noted in ASCE 7016 or Section 30.12. Section 30.12 was added to include Roof Pavers for Buildings of all heights with roof slopes less than or equal to 7 degrees. The Section includes an equation (eq. 30.12-1) to calculate design net uplift pressures:p = qhKdCLnet (lb/ft2)What is the Update? Roof Zones were revised for Hip and Gable RoofsAs wind blows over roof surfaces, it creates suction, or uplift, on the roof assemblies. The amount of uplift varies by building height, location (associated wind speed), and other factors unique to each building. The uplift created on the building is not uniformly distributed, and will vary depending on factors such as roof shape. Wind uplift is highest at the corners, then perimeters, and is the least in the field, or center of the roof; these varying wind uplift locations are called roof zones.Hip Roofs are where all four sides of the roof slopes down to connect to the exterior walls at the eaves. Gable roofs have two slanted sides that form a ridge that connect to the vertical walls that extend to the ridge. Chapter 30 of both ASCE 7-16 and 7-22 include roof zone diagrams and graphics that can be used to determine the External Pressure Coefficients (GCp).Image 1: Hip roof Image 2: Gable roofImages 1 and 2: Hip roof and gable roof, images courtesy of roofingcalc.comIn ASCE 7-22, both the roof zone diagrams and the graphics to determine GCp are updated. The roof zones are simplified to have three zones, Zone 1, Zone 2, and Zone 3 (in lieu of Zone 1, Zone 2r, Zone 2e, and Zone 3), and the accompanying zone layout was modified to include the Zone changes. The External Pressure Coefficients are determined using graphics, and those also have been updated and simplified.Components and Cladding, h ≤ 60 ft, Gable Roofs Roof Slopes 7°≤Θ≤ 20° and 20°≤Θ≤ 27°Components and Cladding, h ≤ 60 ft, Gable Roofs Roof Slopes 27°≤Θ≤ 45°Hip roofs have one roof zone layout plan for all roof slopes, however, External pressure Coefficients graphs were updated for each.What is the Update? Roof Zones were revised for Stepped RoofsStepped roofs are where buildings have multiple flat roof levels, which are often seen on large hospitals, offices, and school buildings. While stepped roofs have wind uplift pressures and corresponding roof zones, it is important to note that wind on the lower roof is affected by the neighboring higher roof sections. At the intersection of the higher roof section with the lower roof section, the wind uplift pressures are lower.New diagrams have been inserted into Chapter 30 of the ASCE 7-22 version. The primary change to note is that the corner zones, zone 3, have been changed from square shapes to L shapes. This is a reflection of the standard roof zones which have been updated from square corners to L shaped corners in more recent versions of ASCE as well.ASCE 7-16, Figure 30.3-3 Components and Cladding, h ≤ 60 ftASCE 7-22, Figure 30.3-3 Components and Cladding, h ≤ 60 ftWhat are the next steps?It is anticipated that ASCE 7-22 will be adopted in the 2024 version of the IBC, so there is plenty of time to get comfortable with the updates. And as always, feel free to reach out to the Building & Roofing Science team at buildingscience@gaf.com with questions.

    By Kristin Westover

    September 07, 2022

  • Woman sitting in open window under steep-slope shingle roof
    Building Science

    Ventilation of Steep-Slope Roof Systems and Transitions

    Ventilation for steep-slope roof assemblies is often misunderstood. One must not only understand the code requirements, but be able to translate them into real-world installations. Building codes have requirements for ventilation of steep-slope attics and enclosed rafter spaces. Balanced ventilation — nearly equal amounts of intake and exhaust — typcially provides efficient ventilation. Transitions between low-slope and steep-slope roof areas require more distinct intake and exhaust details than traditional eaves/soffits and ridges. This blog provides information relating to ventilation for educational purposes only. Designing ventilation to meet the specific needs of a given project remains the responsibility of the architect, specifier, design professional or roofing contractor. Damage due to inadequate ventilation is typically excluded from coverage under manufacturer warranties. Introduction Residential attic ventilation was a requirement in the very first edition of the Building Officials Conference of America's (BOCA's) model building code that was published in 1948! Even though this requirement has been around for decades, it is still often misunderstood. Perhaps it's the words used and perhaps it's because the code isn't quite specific enough. When discussing residential construction, we often hear something like "We need to vent the roof," when we really mean that we need to vent the attic. We don't ventilate steep-slope roofs themselves; we ventilate the space beneath the roof. More specifically, ventilation is needed for the space under the roof system that is above the insulation in the attic floor. That's the space we know most commonly as an attic (when the insulation is located in/on the floor of the attic). Benefits of attic ventilation Ventilation of an attic space provides a couple of benefits: it lowers the attic temperature and also helps reduce excess moisture that can accumulate. These benefits occur when the air in an attic space is replaced by outside air that is a lower temperature and has less moisture in it (i.e., lower relative humidity). While this seems obvious for most parts of the US, even in warm, humid locations like Miami and Houston, the majority of the time the ambient air is cooler and contains less moisture than the air in an unconditioned attic. Code requirements The International Residential Code (IRC) applies to one- and two-family dwellings, and because of that, most in the roofing industry relate attic and rafter ventilation with residential steep-slope construction, which is a valid and correct presumption. However, the International Building Code (IBC), which covers all buildings other than one- and two-family dwellings (e.g., commercial, industrial, institutional, large residential), also includes information about attic and rafter ventilation because a large number of these types of buildings also include steep-slope roof systems. To that end, both the IBC and the IRC have requirements that apply to the ventilation of attics and enclosed rafter spaces. These requirements are included in Chapter 8, Section R806, Ventilation in the 2018 IRC, and in Chapter 12, Section 1202, Ventilation in the 2018 IBC. (Free versions of the codes are found here.) Both the IRC and IBC include nearly identical requirements, albeit the code sections are arranged slightly differently. The following summarizes the requirements: The requirements for ventilation are specific to enclosed attics (insulation on the floor of the attic) and enclosed rafter spaces (where ceilings are applied directly to the underside of roof rafters/framing members and insulation is between rafters above the ceiling). Vents should not allow the entry of rain and snow. Vents are to be protected from the entry of small 'creatures' such as birds and rodents. Corrosion-resistant materials are to be used, and minimum and maximum sizes of vent openings are provided. The minimum net free vent area is 1/150 of the vented space. The minimum net free vent area can be reduced to 1/300 when both of the following conditions are met: In climate zones 6, 7, and 8, a Class I or II vapor retarder1 is installed on the warm-in-winter side of the ceiling (i.e., attic floor). A "balanced ventilation"2 method is used. 1Vapor retarders — An example of a Class I vapor retarder is a polyethylene sheet, and an example of a Class II vapor retarder is kraft-faced fiberglass batt insulation. The polyethylene sheet or the kraft-paper side of the insulation should be installed immediately below the attic floor insulation layer in order to meet the requirements shown above, regardless if it's a traditional attic or an enclosed rafter space. Importantly, but not specifically required in the codes, these vapor retarders should be installed and detailed to also act as air barriers to prevent warm, moist air from the interior spaces from leaking up into the attic. 2Balanced ventilation — "Balanced ventilation" means 40% to 50% of the required ventilation area is located in the upper portion of the attic, and the remainder is used for intake at the eave or within the bottom 1/3 of the attic area. Commonly, exhaust vents consist of continuous ridge vents or static vents no more than 3 feet from the ridge (measured vertically). Intake vents within soffits or eaves are common, and in-plane intake vents (such as GAF Cobra IntakePro®) are used when eaves and soffits are not built to include intake vents. Current construction methods commonly incorporate the balanced ventilation method for residential attic construction and, therefore, the 1/300 ratio is used to calculate ventilation amounts. The 1/300 ratio means 1 square foot of attic ventilation (evenly split between intake and exhaust) is needed for every 300 square feet of attic floor space. The intent of the requirements for balanced ventilation is that there is more intake than exhaust. This is quite important! Having more intake than exhaust means there will be proper convective flow from eave to ridge. Because warm, moist air is more buoyant than dry air, the warm, moist air rises and is exhausted at the upper portion of the attic. When there is less intake than exhaust, the lack of intake can "choke" the system, reducing the overall effectiveness of the attic ventilation system. Balanced ventilation and reroofing Balanced ventilation is not only important for new construction, but it is an important objective for steep-slope reroofing projects, especially for residential construction. During reroofing, if the amount of exhaust is increased (e.g., by adding a ridge vent with more total exhaust capacity than the previous static exhaust vents), the amount of intake ventilation should be determined and increased as necessary to create a balanced system. If the amount of intake is too little, intake air will come from other sources! A lack of intake at the eave/soffit can lead to air being drawn into the attic from the interior of a residence through can-lights, ceiling vents, and attic-access locations. Believe it or not, air can be pulled from basements and crawl spaces through the cavities in interior walls up into the attic spaces. These "interior" sources of air can contain warm, moist air that can be detrimental to attics, causing condensation and other moisture problems that didn't previously exist. The interior air may not have been drawn into the attic if the system was previously balanced, even if undersized. So, be cautious when increasing the exhaust amounts on existing buildings without assessing the intake amounts. Addressing any 'intake' deficiencies during steep-slope reroofing projects can help ensure that ventilation is balanced and functioning as intended. This post isn't going to dive into calculating the required amounts of ventilation. To better familiarize yourself with that calculation, use the GAF Attic Ventilation Calculator. The calculator determines the minimum amount of exhaust and intake, and the minimum lineal feet of specific GAF products, such as Cobra Rigid Vent 3 for warmer climates, Cobra SnowCountry for cold and snow climates, and Master Flow Undereave Vents, is provided to meet those calculated amounts per the 1/300 ratio. Modern changes to construction: Cathedral ceilings Historically, given that attic ventilation requirements go back decades, the code originally applied only to the traditional attic space under a steep-slope roof — that is, attics with insulation located at the floor of the attic/in the ceiling of the upper floor of a residence. Today, and in the recent past, the traditional attic space is often now a usable, conditioned space. That means the ceiling is attached to the underside of the sloped rafters creating a cathedral ceiling, or some form of that. The traditional attic is turned into occupied space, and the result is an enclosed rafter space. (Remember the code language from earlier that says "attics and enclosed rafter spaces"?) Chapter 8, Section R806, Ventilation in the 2018 IRC, and Chapter 12, Section 1202, Ventilation in the 2018 IBC provide an option for ventilation when a cathedral ceiling is installed with insulation under the roof deck in the enclosed rafter space. The specific requirement for this type of construction states that there must be a minimum 1" vent space in each rafter space directly beneath the roof deck above the insulation. This can be somewhat difficult to construct and maintain continuous air flow. Also, once constructed, inspection and repair is difficult without removal of interior drywall and/or exterior soffits and eave components. The graphic, from the International Association of Certified Home Inspectors, is an example of ventilation of the construction method that incorporates enclosed rafter spaces. The 1" minimum required air space (under the deck between the rafters) is considered to be the vented space, and that means the requirements for the protection of openings from snow, rain, and small creatures, as well as corrosion resistance and sizes of vent openings, are applicable. The minimum net free vent area requirements may also apply when there is a vent cavity/air space under the deck and above the insulation between the rafters. In other words, the vent space size is calculated the same way as the traditional attic space. Specifically, the 1/150 ratio still applies, and in order to reduce the amount of ventilation to 1/300, the additional requirements for Class I and II vapor retarders in Climate Zones 6, 7, and 8, and balanced ventilation also apply. At no time can the vent space between the rafters above the insulation and below the roof deck have less net free vent area than is required for intake and exhaust vents. The depth of the air space may need to be greater than 1" deep to accommodate enough air flow to provide proper ventilation. For example, if the 1/300 ratio determines that 10 square inches per lineal foot of net free vent area (NFVA) is required, a 1" deep air space is appropriate. However, if 20 square inches per lineal foot of NFVA is required, then a 2" deep air space is needed to provide appropriate air flow. Calculating the required depth of the air space to match the amount of NFVA for eave intake and ridge exhaust should take into account the ratio of rafter-to-open air space for continuous eave and ridge vents. Tricky transitions There are many options to vent eaves and ridges on traditional residential construction. However, where a steep-slope roof transitions to a low-slope roof (and vice-versa), the methods to provide intake and exhaust ventilation can be a bit trickier. Where a low-slope roof abuts the low edge of a steep-slope roof, a good option for intake vents is to use a "deck-level" intake vent, such as GAF Cobra Intake Pro. This type of intake vent is intended for use where there are no eaves or soffits available to install traditional intake vents. Due to the potential for water to build-up at the transition from the low-slope roof to the steep-slope roof due to rain, sleet, or snow, or some combination thereof, it's logical to install a "deck-level" intake vent up-slope at least 2 courses. It is best to locate an intake vent far enough up-slope to help prevent snow from blocking the vents, as well. The National Roofing Contractors Association (NRCA), in The NRCA Roofing Manual: Steep-slope Roof Systems—2017, provides the following detail for a "Steep- To Low-Slope Roof System Transition." A key element is that NRCA shows the bottom edge of the shingle roof is a minimum of 10" from the low slope transition point. This helps prevent water intrusion through the steep-slope roof. And if the "deck level" intake vent is up 2 courses, the intake is some 20" from the surface of the low-slope roof (albeit measured along the slope, not vertically). Where a low-slope roof abuts the upper portion of a steep-slope roof, detailing and constructing the exhaust vent is needed in order to properly terminate the low-slope roof. The concept, in general, is to use one-half of a ridge vent, and that likely means this detail is built in place (it does not appear that there are pre-manufactured vent devices for this type of installation). A gap is needed at the top of the sloped deck to allow air to move from the attic or enclosed rafter space up and out the vent material. As shown in the detail below, wood blocking and vent materials are installed on top of and along the upper edge of the steep-slope roof covering. A nailable top layer (e.g., a 2x6) is installed to keep the vent material in place and to act as a nail base for the termination of the low-slope roof. In addition to the ventilation details needed at these types of transitions, it's important to remember the transition details need to consider the continuation of the water, air, thermal, and vapor boundary conditions. You can refresh your knowledge with this GAF blog post. What the codes mean but don't say Simply put, ventilation of attics and enclosed rafter spaces occurs outside of the thermal layer. The code requirements have been developed and instituted based on this, but codes don't explicitly state it. That leads to confusion by some who ask if low-slope roofs need to include ventilation. Let's think about that. For membrane roofs with insulation above the deck (that is, compact roofs), where exactly would the ventilation space be located? Between the insulation and the membrane? That's not how low-slope roofs are constructed. The next possible location for a ventilation space would be under the deck, which means the ventilation is on the conditioned side of the thermal layer for a low-slope, compact roof system, and that is illogical. Expensive conditioned air would easily escape from the building, and unwanted exterior air would easily enter. That would be like leaving doors and windows wide open while air-conditioning or heating a space. One very important point — even if there was a way to provide intake and exhaust vents as part of a low-slope roof system, a horizontal air space provides no path for warm moist air to rise to an exhaust vent. Another way to say it — natural convective flow does not really happen in a horizontal space. In conclusion We ventilate our attics and enclosed rafter spaces to remove unwanted heat and moisture. According to the GAF Pro Field Guide for Steep-slope Roofs, attics can reach up to 165° F, and for asphalt shingles, excessive heat can reduce shingle life. The Guide provides information why venting makes sense, and there are a couple other details available for review and use. Keep your ventilation balanced!

    By James R Kirby

    June 07, 2021

  • Polyiso installation
    Building Science

    Designing with Polyiso

    Thermal insulation is an important part of commercial roofing assemblies. As with anything, there are ways to design with and install polyiso insulation — a better way, a best way, and many variations in-between! What may be best in terms of lowest up-front costs, may prove only good or worse over the long-term life of the building.As discussed later, important points to remember about polyiso and its use include:Due to polyiso's very low air permeability, good design and installation practices can result in low risks of condensation, even for buildings with somewhat higher than normal humidity levels.Building designers can use the label R-value, stated at a mean temperature of 75°F, for projects in most ASHRAE climate zones. However, for climate zones 6 and 7, it may be appropriate to use published values at 40°F.Electricity is four times as expensive as gas, on an equivalent Btu basis. Therefore, polyiso's greatest value is seen during summer months when air conditioning usage is at its highest.IntroductionPolyiso has become the go-to insulation for commercial roofing with about 75% market share. The reasons include:Lowest cost per R-value.High R-value per inch, meaning it doesn't require as much thickness as other types of rigid insulation to achieve the same insulation performance.Good fire resistance. Being a thermoset unlike polystyrene foams, it doesn't melt during a fire. Melting of thermoplastic foams and subsequent flowing down of molten material through the deck into a building can be hazardous for occupants and fire crews.Solvent compatibility — adhesives used in adhered systems don't adversely affect polyiso. This is not the case with polystyrene foams.Reduced condensation — When installed correctly in a properly designed system, polyiso helps to resist interior air from reaching the underside of the membrane, thereby reducing condensation risks.However, just because polyiso is specified and used doesn't mean that the ideal outcome is reached. There are installation details that need to be considered.Single-ply membrane combined with polyiso can make for a good roof, but additional factors can improve performance:The BasicsPolyiso boards have straight cut edges (the boards are not tongue and grooved). These butt joints between boards can allow for vertical air movement.Polyiso was often installed as a single layer:As shown in the figure above, a single layer of butt-jointed insulation is generally a bad idea and is no longer allowed by the International Building Code (IBC) in the 2018 and newer versions. For roofs with a single layer of butt-jointed insulation boards:There is little restriction for air flow and so, during wind events, wind uplift forces result in membrane billowing for attached systems.Membrane billowing draws interior conditioned air up to the underside of the membrane and may create a risk of condensation within the assembly.Billowing may place additional stress on membrane fasteners, a factor recognized by membrane manufacturers by the issuance of shorter guarantees or warranties for mechanically attached versus adhered membrane.Essentially unrestricted air flow up into the assembly diminishes the insulation value and lowers energy efficiency.In contrast, two layers of polyiso restrict the free flow of air into the roof assembly. Care must be taken to seal between roof penetrations and the polyiso, or air is still free to move up into the assembly as shown here:Spray foam is a good choice for achieving a seal between the polyiso and the penetrations.Two layers of polyiso with staggered joints and sealed penetrations is a good system, but two of its features still reduce its insulation efficacy. These are shown in the following magnified view of a roof assembly cross-section.This cross-section illustrates lower energy efficiency resulting from:Insulation and membrane fasteners both act as thermal bridges, conducting heat through the assembly.Gaps between insulation boards that allow thermal convection to occurWhen both the insulation and membrane are attached and fastener densities are at their highest, R-value reductions of up to 29% have been predicted.Adhered versus AttachedThe alternative to mechanical attachment is to adhere the insulation with adhesive. Typically, low rise foam is the adhesive used for both insulation and cover board installation. The original method of application was as a ribbon but for labor savings it is often sprayed in a spatter-pattern as shown in the following picture of a typical installation in progress using one of GAF's low rise foamsUsually, when polyiso is adhered, the roof membrane is also adhered. For a steel deck substrate, fasteners are typically only used for the first layer of insulation, and then the adhesive is used on the subsequent layers of insulation and membrane. While it may be possible to adhere directly to a steel deck and eliminate fasteners totally, this approach is more common with concrete decks, as shown in the schematic here:Key features of systems with adhered insulation are:Air flow up through the assembly is limited and the risk of condensation in cold climates is low. Similarly, membrane billowing is minimized due to the restricted air flow up through adhered insulation layers.Thermal bridging is minimized since only the first layer of insulation might be mechanically attached. With concrete decks, the first layer of polyiso can be adhered thereby eliminating the use of fasteners totally.Wind uplift resistance is uniformly distributed across the roof deck. A system with adhered membrane and insulation can act as a monolithic system with excellent wind uplift resistance.When combined with an adhered membrane the finished roof appearance is usually aesthetically pleasing. When applied in accordance with manufacturer's instructions, the membrane can be very flat and there will not be any insulation fasteners to telegraph through and mar the appearance.Induction WeldedInduction welded fasteners are another type of roof attachment, the largest type being the Drill-Tec™ RhinoBond® system. By definition this is a mechanical attachment method but it has many of the features of adhered systems. The technique fastens TPO and PVC membranes to the substrate below using a microprocessor controlled induction welding machine. The thermoplastic roof membrane is welded directly to specially coated fastening plates used to attach the insulation. The picture below shows such a system being used:The induction machine is placed above each plate in turn and activated for approximately 10 seconds. As the machine is moved to the next position, a weighted magnet is placed over the plate and acts to squeeze the membrane down onto the hot fastener plate causing it to weld to that plate's surface coating.For typical mechanical attachment of single-ply membranes with fasteners along the seam lines, the insulation boards are simply secured with five fasteners per 4 x 8 ft. board to keep them flat. When using a Drill-Tec™ RhinoBond® system, the combined insulation and membrane fasteners resist wind uplift forces as shown in this picture during a test of wind uplift resistance:This means that wind loads are more uniformly distributed versus a conventionally attached system.Key features of systems with induction welded membrane attachment are:Performance is very similar to adhered in terms of the distribution of loads.Thermal bridging is reduced compared to traditional mechanically attached membrane systems.There are no application temperature restrictions and so this approach can be used in place of adhesive attachment regardless of how cold it might be.Drill-Tec™ RhinoBond® systems can be cost competitive due to the speed of installation as compared with traditional bucket and roller adhesives.Dew Point CalculationsThe dew point is the temperature at which moisture vapor forms condensation. It's a function of the relative humidity and the ambient temperature. The evaluation of a roof assembly where the dew point might be reached is an important step towards designing a roof with minimized condensation risks. The topic has been covered in greater depth elsewhere, but this section summarizes the key steps. Examine the chart below (this is a simplified form of what is used by HVAC engineers).Locating 40% relative humidity in the first column and then going across to the 70°F, design dry bulb temperature shows a dew point of 45°F. This means that in an environment that is 70°F and 40% relative humidity (RH), water in the air will condense at a temperature of 45°F. A roof assembly separates the interior conditioned environment from the outside and the insulation layer in the roofing system resists heat loss or gain to/from the outside, depending on the season. Within the insulation layer the temperature has a gradient between the hot and cold side, i.e. between inside and outside.As an example, consider a building in the winter to illustrate the point. The interior is 70 °F with 40% RH, like the example on the chart above. The temperature gradually drops from the innermost part of the insulation until at the outermost part it will be at the exterior, cold temperature. The plotting of temperature through the insulation thickness is referred to as the temperature gradient of that system. Using the example, if the temperature gets to the dew point of 45°F at any point in that system then water would be expected to condense on the nearest surface. This is shown in the following diagram:Summarizing, in this example the interior air has 40% of the total water vapor that it can support. But as the air migrates up through the roof system, it gets cooler until the point where it can no longer hold onto the water vapor and condensation occurs. In the example shown above, that's at 45°F and just inside the insulation layer.As will be discussed later, dew point calculations can be used to inform the placement of a vapor retarder layer when used.When a dedicated vapor retarder layer is not used, it is good practice to ensure that the dew point is in the upper layer of polyiso insulation. This will lower the risk of interior air migrating up through the roof assembly and condensing on a surface below its dew point.Using Vapor Retarders with PolyisoVapor retarders can be used to limit humidity ingress to the roof assembly. Under certain circumstances, depending on the building use and location, condensation can occur as discussed in the previous dew point discussion. In general there are four basic building conditions that can be considered:1. Buildings with Standard Amounts of Occupancy-Generated MoistureThese are the most common situations covering for example, office, retail, and warehouse spaces. The risk of having a condensation issue is low and good roofing practices such as sealing around penetrations may likely suffice.2. Buildings with Larger Amounts of Occupancy-Generated MoistureThis category includes apartments and other multiple residency buildings, paper mills, laundries, buildings with indoor swimming pools, and the like. In fact, anything that doesn't fit into category 1 above should be evaluated to determine humidity levels. The building's air handling and ventilation systems should be carefully specified to take into account the moisture loading.3. Construction–Related MoistureMost construction practices release some amount of moisture into the building space. These can be relatively short term such as drywall installation and painting. However, some practices can release large amounts of water over a considerable time frame into the building. These include poured in place concrete floors and roof decks.4. Concrete Roof DecksThese can present a challenge for roof system designers especially in new construction. Regardless of the type of concrete, significant amounts of water remain after curing is completed. Allowing concrete to thoroughly dry is most appropriate; however, it is often reasonably impractical. Dealing with potential moisture in concrete decks is beyond the scope of this article, but guidance can be found elsewhere.It is recommended that a building science professional experienced in designs for Categories 2, 3, and 4 be involved to determine whether a vapor retarder should be used and what type.Specification of R-ValuePolyiso is manufactured to meet the ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. ASTM C1289 specifies the thermal resistance at a mean temperature of 75°F for various product thicknesses and requires that the values at 40 and 110°F be made available upon request.Important features of manufacturer's published R-values are:The values are an average across a temperature range. The test methods need the insulation specimen to have a hot and a cold side at least 40°F apart. Most reputable test labs use a difference of 50°F for accuracy. A published value at 75°F is actually an average R-value across the range of 50 to 100°F.To take into account the diffusion of gas out of, and air into the polyiso foam, the values are based on projected "Long Term Thermal Resistance" or LTTR, obtained by rapidly aging thin slices of the foam.The Polyisocyanurate Insulation Manufacturers Association, PIMA, conducts a third-party certification program to independently validate LTTR values. This is referred to as the PIMA QualityMark™ program. The LTTR values are considered as "labelled R-values" to be used by building design professionals.Label R-values represent a 15-year time-weighted average value. They can help a design professional estimate a building's energy efficiency without having to be concerned about long-term loss of performance, which is already factored into the value.Building design professionals designing roofs for ASHRAE climate zones 6 and 7 may need to use the R-value reported for a mean temperature of 40°F.Attachment PatternsThe various fastener patterns for polyiso have been mentioned in the previous sections. However, due to the impact of fasteners on thermal bridging and wind uplift resistance, the key points are summarized here:For systems that have both mechanically attached membrane and insulation, the membrane attachment provides the wind uplift resistance. The polyiso insulation fasteners are simply there to hold the insulation flat during the roof installation and to resist long term lateral movement.Typical fastener patterns are shown here:For systems with adhered single-ply membrane and mechanically attached polyiso boards, the insulation fasteners provide the wind uplift load resistance. This is the case whether both layers of polyiso are mechanically attached or only the bottom layer (in which case the upper layer would be adhered).Manufacturers have tested the fasteners per board required to meet wind uplift resistance requirements for these combined mechanically attached and adhered systems. The number of fasteners needed depends on the board size and thickness. For common systems, the numbers are shown in the table below:For each of these combinations above, manufacturers' handbooks provide fastener patterns.The number of fasteners for these combined mechanically attached and adhered systems is very large, for example a 125,000 s.f. big box type roof could require around 50,000 fasteners, resulting in significant thermal bridging.When installing over a steel deck, to reduce thermal bridging and to make for a more robust system with reduced condensation risk, it is advisable to only attach the first layer of polyiso and to adhere all subsequent layers and the membrane.If the first layer of polyiso is attached and the rest of the system adhered, then using a 1.5" thickness for that first layer would help to bury the thermal bridging fasteners. It could also put the dividing line between first and second polyiso layers below the dew point, which is advisable.ConclusionsPolyiso is a cost-effective roof insulation and has the advantage that its permeability is low. Good design and installation practices can result in low risks of condensation even for buildings with higher than normal humidity levels.When designed correctly, mechanically attached components with two layers of polyiso having staggered and offset joints can be part of a successful roof system.Adhered insulation and membrane roof systems have advantages including reduced or eliminated thermal bridging, lowered condensation risks, and better wind uplift resistance.In cases where building use anticipates higher interior humidity levels and/or the local climate suggests higher condensation risk, then a building science professional should be consulted as to vapor retarder use and specification.Building designers and specifiers are advised to use the labelled R-values shown for a mean temperature of 75°F. For projects in ASHRAE climate zones 5 and 6, values at 40°F could be used depending on the building's geometry and local energy costs.

    By Thomas J Taylor

    March 17, 2021

  • GAF Cedar City plant
    Building Science

    How will ASCE 7-16 affect the low-slope roofing industry? A Tale of Two Buildings

    A common question being asked in the roofing industry is whether or not the 2016 version of ASCE 7 is going to increase the design wind pressures acting on a building. The answer is "yes" in many cases. So, the follow up question is "by how much?" And, that leads to the next question, "how much more capacity will roof systems be required to have when wind design follows ASCE 7-16?"This blog looks at 2 buildings of different sizes and heights in two cities in the U.S., one centrally located and one along the Gulf coast, and compares design wind pressures based on ASCE 7-10 and ASCE 7-16. By assessing the required roof-system capacity for all roof zones in conjunction with the increases in the size of roof zones, we can begin to put some real numbers to the question "by how much will ASCE 7-16 affect the low-slope roofing industry?"While analysis of two building types in two cities doesn't represent a significant study, this study offers an example of how the 2016 version of ASCE 7 is going to affect installed roofing systems over the next decade.Case StudiesDesign wind pressures (DWP) were determined for two building types in two cities. DWPs were based on varying the Risk Category and Exposure. The following table shows the building types, cities, variables, and constants.The big box store is 290' long x 169' wide x 24' tall, and the apartment building is 100' long x 40' wide x 55' tall. Wind speeds were determined using an online tool.DWPs can be determined by using proprietary third party methods (e.g., RoofNav), simplified calculation tools (e.g., RoofWindDesigner), or hand calculations following the methods within the ASCE 7 standard.For this case study, DWPs were determined using hand calculations via an Excel spreadsheet. The DWPs are slightly more refined since there was no rounding of wind speeds for ASCE 7-16. Both RoofNav and Roof Wind Designer round to the nearest 5 or 10 mph. The maps in ASCE 7-16 have been revised, not only from a wind speed perspective but from the number of wind isobars on the maps. The ASCE 7-16 maps are more refined which allows for more exact wind speed values.ASCE 7-10 Map, Risk Category II, showing a limited number of wind-speed isobars.ASCE 7-16 Map, Risk Category II, showing many additional wind-speed isobars relative to the ASCE 7-10 wind map shown in the above figure.In total, 72 different sets of DWPs were calculated. Buildings over 60' tall are designed the same in ASCE 7-16 as in ASCE 7-10; therefore, this article focuses on buildings less than 60' tall.Design Wind Pressure ResultsThe following charts provide the DWPs for each of the scenarios that were analyzed for this article.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Kansas City, MO that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Mobile, AL that is 24' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Kansas City, MO that is 55' tall.Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Mobile, AL that is 55' tall.Comparing the 2010 and 2016 versionsThe primary intent of this article is to start generating data to help answer the question "how much did the DWP increase from the 2010 version to the 2016 version of ASCE 7?" Using the DWP calculated for each roof zone within each of the four scenarios, we calculated the percent increase in DWP per zone. The percent increases in DWP were calculated by dividing the ASCE 7-16 value with the corresponding ASCE 7-10 value for each roof zone. However, because there are 4 roof zones in ASCE 7-16 (and 3 roof zones in ASCE 7-10), both Roof Zones 1' and 1 were divided by the value for ASCE 7-10 Roof Zone 1 to determine the percent increase. Values in the following charts that are below 100% (shaded gray) indicate a reduction in DWP from ASCE 7-10 to 7-16.Percent increases in DWP per roof zone for a big box store in Kansas City, MO that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a big box store in Mobile, AL that is 24' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Kansas City, MO that is 55' tall, focusing on Exposure C values.Percent increases in DWP per roof zone for a 5-story apartment building in Mobile, AL that is 55' tall, focusing on Exposure C values.For the big box store scenarios, the percent increases in DWP per roof zone varies from 84% to 178%. The percent reductions (values less than 100%) are all in Roof Zone 1' while Roof Zones 1, 2, and 3 have increased DWP. As seen in the Figures, Roof Zone 1 percent increases are greatest while Roof Zone 3 increases are smallest.For the 5-story apartment building scenarios, the percent increases in DWP per roof zone varies from 84% to 177%. The percent increases in Roof Zone 1 are the largest. The percent reductions are all Roof Zone 1'.It is worth noting that the increases in Pressure Coefficients from ASCE 7-10 to 7-16 are, in fact, relatively indicative of the increases in DWP. For more on Pressure Coefficients and the wind design process.The case studies in this blog also show that the 2016 version of ASCE 7 imposes higher DWPs in Roof Zones 1, 2, and 3. However, given some of the substantial reductions in Roof Zone 1', it could be expected that installed roof system capacity may be lower. While that is hopeful, the roofing industry has a minimum-capacity backstop of 60 psf. This means that any calculated DWP at or below 60.0 psf (even for DWP values as low as 23 psf) will have a 60-psf-capacity roof assembly installed to meet building code requirements. The reality is many buildings are required to use a 60-psf-capacity roof system when in fact '60 psf' is more capacity than needed based on calculated DWPs.Let's take a closer look at one subset for each of the 4 scenarios.Example 1: Big Box; Central USThe big box store in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 38 psf to 56 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 32 psf in Roof Zone 1'. Regardless of the increases or decreases, a 60-psf-capacity roof should be installed in both Roof Zone 1 and Roof Zone 1' per 7-16, which is unchanged relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 2 and 3, the DWPs are not only increased, but the sizes of the roof zones that require higher capacity are also increased. Roof Zone 2 per ASCE 7-10 required a 75-psf-capacity roof, and a 75-psf-capacity roof will be needed per ASCE 7-16. However, approximately 9% of the roof area will need a higher capacity because of the increase in roof zone size. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 2: Big Box; Gulf CoastA big box store in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 69 psf to 109 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 63 psf in Roof Zone 1'. A 75-psf-capacity roof should be installed in Roof Zone 1', and higher-capacity roofs will be required in Roof Zones 1, 2, and 3 relative to ASCE 7-10.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-10 using Risk Category II and Exposure C.Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-16 using Risk Category II and Exposure C.For Roof Zones 1, 2, and 3, the DWPs are not only increased, but the size of the roof zones that require higher capacity are also increased. A larger perimeter roof zone (combining Roof Zones 1 and 2) with higher required capacity is created (relative to 7-10). Approximately 30+% of the roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 3: 5-story; Central USThe 5-story apartment building in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 45 psf to 66 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 38 psf in Roof Zone 1'.However, because the method used to determine the sizes of roof zones in ASCE 7-16 is changed, Roof Zones 1' and 1 do not exist on this building! Decreases of DWP in Roof Zone 1' are rendered meaningless, and the lack of a Roof Zones 1' and 1 means the entire roof consists of only Roof Zones 2 and 3, the perimeter and corner roof zones.Roof zones and pressures for a 55' tall apartment building in Kansas City, MO based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWPs for the roof zones significantly increase the roof system capacity requirements for this scenario. Almost the entire roof area will need a higher capacity roof. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Chart showing required roof capacity and the roof area (%) where each is required.Example 4: 5-story; Gulf CoastThe 5-story apartment building in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 81 psf to 129 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 74 psf in Roof Zone 1'. Similar to Example 3, due to the size of the roof zones relative to the size of the roof, Roof Zones 1' and 1 do not exist in this scenario.Roof zones and pressures for a 55' tall apartment building in Mobile, AL based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C.The increases in DWP for the roof zones significantly increase the roof system capacity requirements for this scenario. The entire roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario.Performance versus PrescriptiveThe case studies in this analysis are presented using a performance method for wind design. That is, a specific DWP is determined for each roof zone and a roof system with appropriate capacity is selected for use in the associated roof zone. Another method that is often used in the roofing industry is a prescriptive method for wind design. The DWP for Roof Zone 1 (or Roof Zone 1' in certain localities) is used to select a roof system with an appropriate capacity for Roof Zone 1 (similar to a performance method), and then Roof Zones 2 and 3 use an installation method/layout that is prescriptively increased relative to Roof Zone 1 installation methods. Prescriptive installation methods for Roof Zones 2 and 3 are regularly used with mechanically attached (MA) roof systems, and for MA roofs, Roof Zone 2 increases are commonly 50% and Roof Zone 3 increases are commonly 100%.ConclusionASCE 7-16 DWPs are generally increasing relative to ASCE 7-10, but this impact is better understood by comparing the required roof system capacity for different building types and dimensions in different cities. Looking only at DWPs does not provide enough information; a comparison of required roof system capacities is needed. Not only did the DWPs change, but the methodology for determining the size of the perimeter and corner roof zones changed significantly in the new ASCE 7-16 version; the amount of roof area contained by the perimeter and corner roof zones increased. The case studies—of two buildings less than 60 ft tall in two cities—and corresponding analysis in this blog are just one additional small step to understanding how the ASCE 7-16 will affect wind design over the next decade.

    By James R Kirby

    February 21, 2021