The Roof Deck

There are three main roof deck types, classified by material: metal, wood, and concrete. Roof deck selection is influenced by the building construction type, building use, and regional construction trends. Each has their own implications to roof system selection including roof attachment methods and reaction to moisture.

Substrate Board/Roof Board and Vapor Retarder/Air Barrier

While not every roof assembly requires a dedicated vapor retarder or air barrier, it is important to know when one should be installed as well as recognize when it should not. Most often the roofing membrane functions as both the air barrier and the vapor retarder, however, controlling moisture is critical in roofing assemblies, and the use of a dedicated air barrier/vapor retarder may assist in doing this. A best practice is to install a vapor retarder on a concrete deck or over a building with high interior humidity, such as a natatorium, where there is a higher potential for moisture vapor to enter into the roof assembly. For buildings with fluted steel decks, a substrate board can provide a solid substrate for the vapor retarder when one is needed.

Incorrect placement of a vapor retarder can inadvertently trap moisture, and therefore, it can be a better design choice to not install a vapor retarder than to install it in the wrong configuration. The location and presence of a vapor retarder should be confirmed by hygrothermal analysis or a qualified design professional. Additionally, detailing for any air or vapor barrier is important as it must be continuous across the roof and tied into the wall air or vapor barrier, without interruptions, in order for it to function as intended.

Insulation

Rigid roof insulation is typically installed above the roof deck in most commercial assemblies. The most common insulation materials for commercial low slope roofs include polyisocyanurate (polyiso), expanded polystyrene (EPS), and extruded polystyrene (XPS). Other types such as mineral wool and vacuum insulated panels (VIP) are also available and installed in various roofing assemblies.

The selection of insulation should consider the various properties of the insulations; each of these types have their place in the market. Providing adequate insulation is critical in roofing assemblies for overall energy efficiency of the building. The higher the R-value, expressed per inch, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures. One factor to consider when selecting a material for existing roofs where termination heights are limited, is that a material with a higher R-value per inch can reduce the height of the insulation to help designers meet the height restrictions. Vacuum insulated panels can be well suited for this. Besides thermal resistance and a stable R-value, the insulation’s compatibility with adhesives, its component compatibility, water absorption, and compressive strength all need to be compared prior to selecting a material.

Polyiso has become the dominant insulation for commercial roofing, making up more than 70 percent of the market. Polyiso insulation is manufactured with a facer on each the top and bottom of the board. Two types of facers are available: glass fiber reinforced facers (GRF) and coated glass facers (CGF). GRF are made with organic fibers, and CGF are made with inorganic fibers; CGF provides improved moisture protection and resistance to mold. Due to the facers, adhesives used in many adhered systems do not adversely affect polyiso, allowing it superior solvent compatibility when compared with polystyrene foams.

Improper installation of insulation can impact overall roof performance. Insulation boards are required to be installed so that the joints are staggered and offset, and at least two layers of insulation should be installed rather than just one thick layer. Gaps between boards can decrease insulating ability by allowing thermal loss, as well as an increased condensation potential if air travels into and through the roof assembly. Air flowing between the boards also brings moisture, which if allowed to condense, can saturate the insulation boards.

Coverboards

Coverboards are installed directly below the roof membrane and above the primary insulation layer, to prolong the life of the roof system, as they offer protection against fire, puncture resistance from hail or heavy traffic. High density (HD) polyiso, glass mat, cementitious, and gypsum coverboards are the most common on the market today. Wood fiber and perlite are also available, although installed less frequently. Each has their advantages and considerations, including capability to resist hail and fire, and installation convenience. Coverboard selection also depends on the activities planned on the roof: for example, a higher compressive strength will be needed for solar or overburden installations, areas that experience larger hail, or high foot traffic zones. Coverboards also provide added protection against penetration, including tools dropped by service contractors. Coverboards not only increase the durability and resilience of a roof, but are considered a best practice to protect the entire roof assembly. Incorporating a coverboard into a roof assembly decreases overall lifecycle costs as replacement cycles are lengthened since the addition of the coverboard will mitigate damage from impact or foot traffic.

Coverboard selection may be influenced by ease of installation including weight of materials and ability to easily cut on-site. Wood fiber boards quickly became replaced in the market by newer technologies due to its heavy weight and the need for saws to cut it. Most other technologies only require the use of a utility knife. The weight of the boards also varies by material type. HD polyiso is very light and easy to cut, whereas glass mat gypsum is heavier, but is a good option in areas where there is risk for larger hail stones.

Many insurance companies, including FM Global, may require a coverboard based on building location. FM Global has divided the contiguous US into three hail zones: Moderate Hail Zone, Severe Hail Zone, and Very Severe Hail Zone. The hail zones are divided based on the likelihood that a hail stone of a particular size occurs in that zone. The selection of a coverboard will be determined by its ability to resist damage from hail of a particular size. For example, most coverboards available meet Severe Hail requirements, but only glass mat gypsum coverboards and plywood meet the test requirements of Very Severe Hail.

A word of Caution: Attachment Methods for Single-Ply Membranes and Performance Implications

Single-ply membranes tolerate a wide variety of attachment methods and are able to be mechanically attached, adhered, or ballasted. The types of adhesives can vary depending on membrane type, the ambient temperature during installation, and local VOC regulations. Mechanical attachment and ballast is often more cost effective, but have considerations such as added weight for ballast and membrane flutter.

Mechanically Attached Systems

Mechanically attached systems are often associated with single-ply systems, as these membrane types regularly use fasteners to install both the insulation layers and the membrane. For an appropriate installation, the fasteners are installed at the membrane laps and must penetrate through or engage the proper depth into the structural roof deck for securement. The number of fasteners will depend on project specific requirements, but typically for a mechanically attached system, there is a minimum of six fasteners per 4’x8’ insulation board, and additional fasteners along each membrane seam for membrane attachment. This can lead to a substantial number of fasteners penetrating and creating thermal bridges within the roof assembly, reducing the effective R-value on the roof.

Mechanically attached single-ply systems are also subject to billowing in high wind events. Billowing or fluttering of a membrane is when wind causes a negative pressure by pulling interior air into the roof assembly creating uplift force on the roof assembly. Although this is an acceptable behavior of single-ply membranes, it can cause stress and fatigue on the mechanical attachments and membrane over time. Interior air that is pulled into the roof assembly equates to energy loss since often the temperature controlled air may be warm or cool based on the temperature of the membrane, and may present a condensation risk depending on outside temperatures.

It should be noted that mechanically attached single-ply systems are commonplace in the market and have had many years of successful installations. These systems are also able to achieve high wind uplift ratings due to the direct attachment to the roof deck and have no fumes associated with the installation.

Adhered Systems

Systems that use adhesives to secure the roof assembly and do not use fasteners, greatly reduce thermal bridging by eliminating the heat path from the interior of the roofing assembly to the exterior. Adhering also prevents billowing of the membrane, and mitigates the amount of interior air that can be brought into the roof assembly.

Adhered systems may mean that all layers are adhered or that only the membrane is adhered or it may include a mix of adhered and mechanical attachment of some or all of the layers within the assembly. If thermal bridging of fasteners is a concern, attaching only the substrate board or first layer of insulation and adhering the second layer, cover board, and membrane can significantly reduce interior air loss and thermal bridging.

Induction Welded Attachment

Induction welded fasteners are another type of roof attachment that is installed frequently in the thermoplastic single-ply roofing market. 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 combined insulation and membrane fasteners resist wind uplift forces, so that wind loads are more uniformly distributed versus a conventionally attached system. By definition, this is a mechanical attachment method, but the fasteners can be installed in the field of the sheet since they are welded to the underside instead of penetrating the membrane only at the laps. Thermal bridging is also reduced, compared to traditional mechanically attached membrane systems as there are less fasteners required for this installation. There are no application temperature restrictions and there are no fumes or fire hazards associated with the installation.

Single-Ply membranes can be installed in any climate as long as each component is thoughtfully designed.

In Closing

Roofing assembly selection plays an extremely important role in protecting the building from the elements, impacting energy efficiency, and contributing to the resilience goals of a building. Single-ply roof membranes are the largest sector of the low-slope market and can contribute to these building goals. It is important for the designer to consider material selection for each layer in the assembly, including how the entire roof assembly will perform over time.

Selecting a reflective or light colored membrane is a common choice when a building is opting to divert solar heat gain from entering into the building. Reflecting the solar heat gain can be advantageous in warm climates, but also in cold climates since the sun’s energy is often greater in the summer due to longer days and the angle of the sun. Designers must also select the appropriate amount of insulation for thermal efficiency, including the consideration of a coverboard to protect the system from hail and foot traffic over the life of the roof. Additionally the system attachment method will influence the impact of thermal bridging and subsequent building energy loss, since an adhered system will mitigate the effects of thermal bridging from fasteners.

While system selection is important, as energy performance of building enclosures continues to improve, moisture risks can increase from decreased heat flow across the assemblies if uncontrolled air movement, vapor movement, and discontinuous control layers inadvertently trap moisture into the roofing system. Continuous control layers, including water, thermal, air, and vapor control, across the roof and tied-into the exterior walls will limit the condensation potential within a roof assembly. Designers should incorporate continuous control layers into each roof detail including at transitions and penetrations.

After design, the roof system installation, including quality assurance measures to ensure that seams are installed correctly and slope is provided to drain, are critical to limit water intrusion and increase the life of the roof system after installation is complete.

The key elements for improving energy efficiency are:

1. Selecting the type of single-ply membrane including consideration to attachment and color
2. Selection of the amount of insulation including attachment method
3. Adequate detailing including consideration to continuous control layers
4. Perform QA/QC during construction to ensure quality installation

Single-ply roofing assemblies offer many opportunities to contribute to the energy efficiency and resiliency goals of a building, and it includes careful consideration of each of the assembly components, the overall roof design, and the installation to ensure the roof assembly meets these goals.

Bozeman Public Safety Center

Bozeman, Montana
Complete September 2022
Roofing materials summary

• TPO 60 mil smooth membrane
• Sprayable, solvent-based adhesive
• High-density polyiso coverboard
• Polyiso Insulation, flat and tapered panels
• Self-adhered vapor-barrier direct to deck

The Bozeman Public Safety Center houses the Bozeman Police Department, a fire station, the Municipal Courts, and Prosecution and Victim Services. The ambitious new construction project encompasses 95,000 square feet. The facility needed a roof system that would withstand the tough Montana winter weather, including hail resistance, for decades to come.

The architect on the project, ThinkOne Architecture, specified a TPO system which was installed by Summit Roofing, a Master Select Commercial Roofing Contractor with offices in Missoula and Helena, Montana.

Roof Installation
The 55,000-square-foot steel roof deck was first covered with a self-adhered vapor retarder, which was directly applied to the deck. The intent of the vapor barrier was to function as the air control layer and to mitigate interior moisture from migrating into the roof assembly. Two 3-inch layers of polyisocyanurate insulation were installed, with the first layer mechanically attached and the second adhered. The system attachment method buried the fasteners by mechanically attaching the first layer and decreasing thermal bridging caused by fasteners penetrating the insulation. Tapered polyiso was adhered to flat roof sections to ensure proper drainage. Due to the risk of hail, a half-inch layer of high density polyiso was adhered to the insulation. The high-density polyiso provides an R-value of 2.5 per half inch, as well as resistance to Severe Hail. A 60-mil TPO membrane was fully adhered using a quick spray adhesive. The adhered roof membrane removed the effects of thermal bridging that would have been caused by a mechanically attached system.

Erin Andes, PE, LEED A.P., is a Building Design Manager for GAF, focusing on the Western U.S. As a member of the GAF Building and Roofing Science Team, she works with designers to review project designs to mitigate risk and achieve affordable, durable, watertight, and energy-efficient roof assemblies. Andes is a Professional Engineer in multiple states and holds a Master's Degree in Civil Engineering from Georgia Institute of Technology.

Kristin Westover, PE, LEED AP O+M, is a Technical Manager of Specialty Installations for low-slope commercial roofing systems at GAF. She specializes in cold storage roofing assemblies where she provides insight, education, and best practices as it relates to cold storage roofing. Kristin is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.