Cover Boards and Insulation

The advantages of greater membrane thickness have been demonstrated. However, roof durability is system dependent so we must look beyond the membrane and consider the entire roof assembly. Rooftops with PV arrays are burdened with more trades and increased foot traffic on the roof, and therefore, are more susceptible to degradation and potential leak sources. A logical starting point is to protect high traffic areas with walkway pads.

The addition of a high-compressive-strength cover board below the single-ply roof membrane will also enhance system protection and extend the life expectancy of the roof. Cover boards provide added protection against penetration by objects, including tools dropped by service contractors, wind-borne debris, and hail. Increasing the thickness of the cover board will increase its penetration resistance. The use of high-density polyiso or gypsum cover boards can significantly increase penetration resistance by nearly 250 percent, as shown in Figure 3.

Figure 3: Puncture resistance of varied membrane thicknesses, and with added support of cover boards.

In addition to increased foot traffic, concentrated loads from ballasted PV systems can exceed the compressive strength of the roof system’s membrane and insulation. Specifying a rigid insulation board with high compressive strength, such as an ASTM C1289 Grade 3 polyiso, with a 25psi minimum compressive strength, will distribute loads and help prevent crushing that may occur with lower compressive strength materials.

Design Review

Designers must understand that with the installation of rooftop solar, the roof system becomes a permanent platform for the continuous operation, service, and maintenance of the arrays. As many solar designers may not be intimately familiar with best roofing practices, it is helpful to specify solar layout requirements for rooftop access that align with not only code requirements (i.e., International Fire Code, (IFC) and National Electric Code (NFPA 70)), but with best practices for roof maintenance and safety of rooftop workers. This can include requirements such as prohibiting PV arrays from crossing building expansion joints, and setting PV arrays and rack heights such that the roof membrane seams, drains, and penetrations are accessible for emergency responders and maintenance workers.

Designers and owners also have an opportunity to specify the type of PV attachment, which includes attached, ballasted or adhered. Each option will impact decisions for the roof system design so project teams should take a holistic approach to any value engineering discussions regarding the roof and PV arrays.

As the cost to install PV arrays is significant, specifying an integrity test of the roof membrane prior to installing the solar overburden is a worthwhile investment of time and resources. This could include the incorporation of a metallic primer on the cover board to facilitate electronic leak detection testing, or the installation of a permanent monitoring system, so the owner can continuously monitor for water intrusion, which may be especially useful over critical areas such as uninterruptible power supplies, operating rooms and data storage.

PV Roof System Design: The Real World

While following best practices can result in enhanced long-term performance and minimize designer, owner and contractor risk, not all projects follow these recommendations. As an industry, we regularly see deviations from best practices. These deviations from best practices include, but are hardly limited to:

• Mismatching the expected remaining useful life of an existing roof system with a new PV array installation
• Specifying a 45 mil single-ply membrane
• Installing a mechanically attached membrane and/or insulation boards with adhered or ballasted PV systems
• Eliminating the cover board below the single-ply membrane
• Loose laying the slip sheet below a ballasted PV system

While these modifications may have been made in the name of “value engineering,” understanding the risks associated with these deviations from best practices, and the reduced value they bring to long-term performance may lead to informed decisions rather than decisions based solely on dollars.

A significant risk in PV roof system design is that the life expectancy of the PV system exceeds that of the roof system. Replacing a roof with installed PV arrays may be cost-prohibitive. Therefore, aligning the life expectancy of the roof with the PV system makes financial sense, as well as best roofing practice. For instance, the decision to offset the investment of a PV system by extending the life of an aged roof membrane by the use of a maintenance coating may lead to long term complications and expense t when the PV system needs to be dismantled and reinstalled to facilitate a roof recover or replacement. Worst case scenario? The PV system remains in place on a roof that is no longer protecting the building. Instead of just replacing the roof, the owner may have to also deal with structural repairs, water damage, and mold remediation. Removal and replacement of the PV system, as well as damages incurred to the PV system in the process. This often includes leased PV systems or systems that are simply renting space on the rooftop. In addition, some contract clauses leave the building owner with ownership of the PV system at the end of the lease, which means the building owner is responsible for removing and disposing of the PV system from the roof.

In line with the intent of installing a roof system that exceeds the life expectancy of the PV system, refrain from saving a few pennies by installing a 45-mil single-ply membrane. Best practice is to increase roof membrane thickness to match the service life of the PV arrays. A 60-mil thick membrane should be the baseline design. A good design would include an 80-mil thick membrane. A great design would include a high-performance membrane that has enhanced heat and UV resistance. Again, the risk of installing a thinner single-ply membrane could include the cost of replacing the roof before the PV system needs to be replaced. The cost becomes significantly more than just the cost of a roof due to the cost of dismantling and reinstalling the operational PV system.

Mechanically attaching the roof membrane when installing an adhered or ballasted PV system can result in damage to both the roof and PV systems. The normal billowing during high wind events of mechanically attached roofs could cause ballasted PV systems to shift and damage the membrane. This billowing could also create additional stress on the solar arrays and its connections, compromising the long-term performance and life expectancy of both the roof and the PV system.

Eliminating the cover board from the roof system is sometimes the first step in initial cost savings. The cover board is there to enhance the protection of the roof membrane from the overburden, foot traffic, and maintenance activities. This is especially important with ballasted PV systems that move around on the roof during wind events. A common compromise includes the use of 25 psi top layer of polyiso insulation and standard polyiso insulation installed below. While a cover board can extend the life expectancy of the roof, this approach may be reasonable, specifically for attached or adhered PV systems. However, understand that the puncture resistance of this roof system will be dramatically reduced, putting the integrity of the roof at risk every time maintenance personnel steps onto the roof. Oftentimes tools are inadvertently dropped or a shoe forces a loose object into the membrane (like a piece of gravel or dropped screw). Damage from these instances often go unnoticed until water shows up on the inside of the building. Note that the removal of the cover board for ballasted PV systems is not recommended due to the shifting around of the arrays during wind events, and the resulting repetitive impact on the roof.

Loose laying the slip sheet below a ballasted PV system runs the risk of water collecting between the slip sheet and the roof membrane. Trapped moisture that has no means of evaporation or drainage can have negative long-term effects on some membranes. Additionally, slip sheets could blow away if ballasted PV arrays are lifted up during severe wind events. Adhering the slip sheet to the roof protects the membrane and PV arrays from damage.

Roof Systems Guarantees

The importance of matching the life expectancy of the roof and the PV arrays has been discussed. At a minimum, the roof system must be designed such that the roof system guarantee will meet or exceed that of the PV arrays. Roofing guarantees are a valuable tool for the building owner, but they carry important limitations and conditions that must be addressed for successful rooftop PV installation and operation.

From the start, the roofing system must be designed and installed in accordance with the manufacturers published specifications, and PV system details must be accepted by the manufacturer prior to installation to avoid any lessening of coverage of the guarantee.

Manufacturers typically do not guarantee the solar mount system utilized in the solar installation. Therefore, most manufacturers specifically disclaim any liability arising out of or in connection with the integrity, installation or performance of, or damages sustained by or caused by the roof mount or PV systems. This emphasizes the importance of collaboration between the roof system designer and the PV designer.

PV System Design and Detailing: Best Practices

The layout of the PV system can also influence the long-term performance of the roof. Drainage, equipment access, roof maintenance and safety should be considered, in addition to PV layout and positioning to maximize solar output.

The Port of Los Angeles has a 50-acre bifacial solar array on an industrial facility that collects reflected light from the surface of the roof in addition to direct sunlight, gathering 45 percent more power than traditional designs. The array is expected to produce 565 million kilowatt hours of electricity over 20 years—saving more than 440,000 tons of CO₂. This is enough to power 5,000 homes each year and the carbon emission savings are estimated equivalent to taking 6,000 cars off the road. The layout shown in Figure 4 illustrates best practice in terms of PV layout to maximize solar output and provides the necessary walkway paths, fire safety, and access to mechanical equipment and roof drains.

Figure 4: PV layout on the Westmont Industries Distribution Center at the Port of Los Angeles.

Target erected rooftop solar over cool roof membranes on more than one-quarter of its stores, on the way to achieve 100 percent renewable electricity in its stores. The more energy-efficient the roof system performance, the less energy needed to cool stores, and the more sustainable rooftop solar becomes. Figure 5 is the rooftop solar on Target’s distribution center in Phoenix, Ariz. In addition to the clever incorporation of brand in the PV layout, notice the walkway paths and spacing around mechanical equipment. This will facilitate the service and maintenance of rooftop equipment and drains.

Image courtesy of the New York Times

Figure 5: PV layout on Target’s distribution center in Phoenix, Ariz.