Examining attachment options
There are more adhesives and application methods for single-ply roof assemblies available today than ever before. This article looks at the various options and discusses the increasing number of advantages to using adhered systems, including:
- Potential for lower installation costs with newer adhesives and application methods, along with long-term energy efficiency improvements.
- More uniform distribution of wind uplift resistance across the roof deck.
- Substantially reduced (or eliminated) billowing of single-ply membranes, minimizing condensation risks from air intrusion in colder climates.
- Better aesthetics v. mechanically attached roofs.
- Improved impact resistance when fasteners are buried lower in the assembly and upper layers are adhered.
The attachment of thermoplastic single-ply membranes such as TPO and PVC used to involve just a few clear choices. By fastening along the weld area with screws and plates, the membrane was regarded as being mechanically attached. The alternative was to use a solvent-based adhesive to coat the substrate and the underside of the membrane, after which the membrane would be pressed down onto the substrate. The result was known as an adhered system.
As concerns grew about the solvent content of adhesives and their impact on regional ozone pollution, both water-based and low volatile organic content (VOC, i.e., solvents that increase ozone) adhesives became available. But, attachment choices were still regarded simply as being mechanically attached or adhered, with the latter requiring some minimal decision regarding the acceptable VOC content.
However, there's been a proliferation of attachment options more recently, including inductively heated fastener plates, low rise foams, sprayed adhesives, and so on. And, taking low rise foams as an example, there have been variations in application methods, such as ribbon versus splatter pattern. Some of these adhesive systems were intended for insulation attachment, but their use has often been expanded to include fleece-back membranes.
The proliferation of single-ply attachment methods can be baffling, but the general terms mechanically attached and adhered are also adding to the overall confusion. While a purely mechanically attached system is easy to comprehend, many roof assemblies have some layers adhered and some mechanically attached. For example, it is not unusual to mechanically attach the first insulation layer and then adhere all subsequent insulation layers and the membrane. Or, the first two layers of insulation could be mechanically attached and a cover board and membrane both adhered above the insulation. In this article, the terms mechanically attached and adhered are used to describe how individual components are attached.
The criteria that can be used to select the appropriate attachment method for a commercial roofing project are described and discussed here. There isn't a best approach that will apply to all situations. Rather, each attachment method has pros and cons and the final decision must be made by the specifier with knowledge of the specific building type, use, time of year of the installation, and location.
Mechanical Attachment of All Layers
For thermoplastic single-ply membranes, this is frequently regarded as the value option. Screw and plate fasteners are used to attach one long side of the membrane, and they are in turn covered by the overlapping adjacent sheet. The weld essentially protects the screws and plates by sealing them inside the system, minimizing risk of fastener corrosion. The following diagram and schematic show the essential features of this type of assembly.
This photograph shows how this appears in practice during installation:
Mechanical Attachment of Single-Ply Membranes
Wind loads are handled differently by mechanically attached single-ply membranes as compared to adhered membranes. During wind events, mechanically attached membranes are lifted up as indicated in the following schematic:
The wind uplift forces create a low-pressure zone above the roof membrane that acts to lift it up. Air from within the building flows up through the gaps in the insulation and offers little resistance to the uplift forces. As a result, the membrane billows up, and the forces are directed to the fastening points as shown below during a wind uplift test.
This results in large stresses at the fasteners and these are experienced repeatedly over the years in high wind areas. Partly as a result of this, membrane manufacturers typically provide shorter warranties or guarantees for mechanically attached membranes.
The membrane itself experiences three forces at each of those fasteners:
Single-ply membranes are tested for resistance to these forces as part of conforming to the ASTM specifications. It's important to note that the insulation fasteners do not contribute to any wind uplift resistance. They are in place simply to keep the insulation boards flat and in-place during installation. In fact, there is little to be gained by having adhered insulation boards combined with mechanically attached membrane.
Key features of systems with mechanically attached membrane and insulation are:
- Low initial cost – the total material cost is typically the lowest of any of the attachment methods.
- Efficient labor – mechanically attached systems are fast to install and may require fewer installation man-hours than adhered systems
- Wind load resistance is provided by the membrane fasteners only – this gives rise to billowing of the membrane during wind events and stresses the membrane around each fastener plate.
- Fasteners that are immediately beneath a single-ply membrane, such as membrane and insulation fasteners, tend to result in punctures when struck by ice-balls. For ice-ball impact resistance, consider systems with adhered upper layers of insulation and/or coverboard, and membrane.
- Fastener densities are high – for a 125,000 square foot roof, typically over 50,000 fasteners are used, leading to significant thermal bridging.
Solvent-based liquid adhesives were the original means of adhering membranes to insulation. Due to environmental concerns, low VOC and water-based types were introduced later. The solvent and low-VOC types of adhesives are essentially rolled or broomed out over the insulation and membrane underside, allowed to dry down or flash-off before the membrane is then applied to the insulation. The following picture shows a typical installation in progress.
Water-based adhesives are similar, but require the use of fleece-backed membranes. Fleece-backed membranes promote adhesion to water-based adhesives and improve adhesive drying.
While adhered applications are often considered labor-intensive, recent innovations like spray applied formulations can lower the overall man-hours required. The following picture shows the use of a spray-applied adhesive, such as EverGuard® TPO Quick Spray Adhesive:
Adhered membranes resist wind uplift forces differently than mechanically fastened membranes. The following schematic shows that membrane billowing doesn't occur:
The wind uplift forces are primarily resisted in a more uniform fashion by the adhered membrane and top layer(s), and the buried insulation fasteners. The membrane, membrane adhesive, and adhered upper layer(s) of insulation act in a more monolithic way to resist uplift forces. Also, the fasteners are distributed more evenly across the rooftop versus mechanically attached systems.
Key features of systems with adhered membrane are:
- Wind load resistance is uniformly distributed across the roof deck. When combined with an adhered second layer of insulation and adhered cover board, the roof membrane and assembly are more interconnected.
- Membrane billowing during wind events is eliminated and as a result, condensation risks from air intrusion are lowered.
- Overall fastener usage is lower and therefore thermal bridging is reduced. Systems with an adhered membrane generally have improved energy efficiency, which is further improved by adhering the upper layers of insulation and cover board where used.
- Aesthetics – adhered membranes, especially those installed over adhered cover board or insulation, tend to appear flatter and smoother versus mechanically attached or membrane adhered over mechanically attached cover boards or insulation.
- Material costs and labor rates vary widely depending on the type of adhesive and will be discussed later in this article. When hidden costs associated with thermal bridging are taken into account, adhered systems can be competitive.
While single-ply membranes can be adhered to mechanically attached insulation, it is more typical to also adhere the upper layers of insulation. Low rise foam can be used for both insulation and cover board installation. These adhesives are two-part systems and the resultant foam is a type of polyurethane. The original method of application was as a ribbon or beads typically 12" apart and the following picture shows a typical installation in progress using one of GAF's low rise foams.
When the insulation boards are applied, the rising foam spreads out and bites into the insulation facers to maximize adhesion. The ribbon pattern is effective to adhere insulation boards down and can be used to adhere membranes to the insulation. While the ribbon application method appears to be slow, as with other adhesives there have been innovations to speed up application and reduce the man-hours required. When adhering just the membrane, a spatter pattern can also be used, which helps prevent the ribbon pattern from translating through and becoming visible from above the finished roof installation. The following picture shows a splatter pattern approach, which is basically a coarse spray:
Key features of systems with adhered insulation are:
- Airflow up through the assembly is limited and the risk of condensation from air intrusion in cold climates is low. Similarly, membrane billowing is minimized due to the restricted airflow up through adhered insulation layers.
- Thermal bridging is minimized when only the first layer of insulation is mechanically attached.
- Wind load 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 aesthetically pleasing. When applied in accordance with the manufacturer's instructions, the membrane should be flat with no insulation fasteners to telegraph through and mar the appearance. Also when the structural roof deck below is "exposed" to the interior finished space for aesthetic reasons, a combined adhered insulation and membrane roof system does not have fastener tips protruding to the interior space in a regular (or sometimes haphazard) pattern.
With the proliferation of adhesive types and application methods, the choice can at first appear confusing. In terms of performance, generally, all of the adhesives and application methods result in robust systems that meet required wind uplift requirements. The main criteria for basing decisions include :
- Environmental requirements - traditional adhesives emit solvents and other VOCs during drying and curing. These emissions are regulated at a local level and it's important to stay in compliance with those regulations. In general, where such emissions are most tightly regulated and where reroofing over sensitive occupancies, water-based adhesives and low rise foams are the better choices. However, the formulations of other adhesives are improving and it's recommended that design professionals and contractors consult with manufacturer's representative to determine the best choices for their area.
- Material and labor costs - the various adhesives each have different costs associated with the material itself, the application rate (i.e. the number of square feet covered by an adhesive), labor requirements, and, in some cases, the investment needed for application tools. The latter can range from simple carts to hold adhesive boxes to spray and pumping equipment. The table shown later in this article provides a rough guide as to the relative costs, but for specifics, it is best to consult with the manufacturer's representative. Many of the more recent approaches, such as spray-applied adhesive and splatter pattern low rise foam were designed to enable fast application rates with low labor.
- Application temperature – traditional adhesives generally required application temperature to be at least 40°F and rising. This is due to a combination of the time required to dry or flash off the solvent and the viscosity of the adhesive. At low temperatures, most adhesives become too viscous and won't flow sufficiently to apply and roll out. Some of the newer systems, such as GAF's Low Rise Foam M Low Temp, can be used at temperatures below freezing.
Induction welded fastening is a type of mechanical attachment for a roof system with the predominant type being the Drill-Tec™ RhinoBond® system. By definition, this is a mechanical attachment method but it can have many of the features of adhered systems. The technique fastens TPO and PVC thermoplastic 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 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 plate, causing it to weld to that plate's surface coating.
For true mechanical attachment of single-ply membranes, 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 insulation fasteners resist wind uplift forces as shown in this picture during a test of wind uplift resistance:
This means that more insulation fasteners must be used, but no in-seam fasteners are used. For a typical big-box store requiring 120 psf wind uplift resistance, 8, 15, and 20 fasteners per 4 x 8 ft. board are required for the field, perimeter, and corner areas respectively.
Key features of systems with induction welded membrane attachment are:
- Performance is more similar to adhered systems in terms of the distribution of loads across the rooftop.
- Thermal bridging is reduced compared to traditional mechanically attached membrane systems. (more here on roof system r-value optimization)
- 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.
- Especially when combined with a 12-foot wide membrane such as supplied by GAF, a Drill-Tec™ RhinoBond® system can improve cost competitiveness due to the speed of installation, compared to 10-foot wide traditional mechanically attached membrane systems.
Adhered single-ply membrane and insulation have many advantages over mechanically attached systems. These include wind uplift performance, lowered condensation risk from air intrusion, reduced thermal bridging, and aesthetics. While adhesives were traditionally viewed as having high labor costs, more recent innovations such as spray adhesive and splatter pattern low rise foam are making adhered systems more competitive.
There is no one best attachment method and the various options all provide good wind uplift resistance. Key considerations are shown in the following table.