Commercial Rooftop Solar Design Explained

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Sponsored by GAF
By Jennifer Keegan, AAIA
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PV Array Attachment

Designers have many PV mounting technologies in the market and not all systems are considered equal. It is critical that the designer match the right mounting method with the right roof system. The NRCA recommends the use of attached or penetrating systems, which are mounting systems that are attached through the roof to the structure, as shown in Figure 8. Penetrations and flashings must be well detailed and coordinated with the roofing contractor, solar contractor, and electrician. These details are critical to the success of the installation and must be designed to align with the life expectancy of the PV array and roof system.

Figure 8: Example of attached PV arrays.


Figure 9: Example of ballasted PV arrays.


Image courtesy of Das Energy

Figure 10: Example of adhered PV arrays.

While ballasted PV systems (Figure 9) are cost effective and easy to install, they can add a significant amount of weight to the roof based on the uplift pressures. While this loading can be incorporated into the structural design for new construction, it may exceed the capacity of an existing building. Additionally, concentrated loading of systems beneath the ballast can exceed the compressive strength of the roof insulation. Therefore, the use of a higher compressive strength insulation such as ASTM C1289 Grade 3 polyiso, should be strongly considered. Better yet, consider specifying a cover board such as HD polyiso for added protection.

Ballasted PV systems can shift and flutter and move during high winds and seismic activity. This can result in localized surface abrasion of the roof membrane as it rubs the edges and corners of a PV mounting system which can be “detrimental to satisfactory long-term roof system performance,” according to the NRCA. If the project team accepts the risks associated with ballasted PV systems, a protection or separation sheet should be installed between the ballast supports and the membrane, and should be adhered to the roof membrane via heat welding or with adhesive.

Ballasted trays can block or inhibit drainage, which can result in trapped water on the roof membrane. This can undermine the performance and service life of the roof system. Given all the performance challenges with ballasted PV systems, “NRCA is of the opinion ballasted rack systems do not satisfy the equivalent service life criteria necessary for successful roof system performance throughout the useful life of rooftop-mounted PV systems.” However, as ballasted PV systems are often used, it is important to understand potential issues and use an appropriate design approach with your roof system so the lifespan of the roof can exceed that of the PV arrays.

Another attachment option for PV arrays includes the use of self-adhered flexible panels, which are adhesively applied directly to the roof membrane, as shown in Figure 10. The low-profile application makes this system attractive to many designers, however such panels can expose the underlying membrane to temperatures as high as 195 degrees Fahrenheit. Best practices that extend the life of adhered panels include the use of high temperature-resistant membranes. Where a standard membrane is utilized, self-adhered flexible PV arrays should be installed to a sacrificial layer of membrane. Documented compatibility between the roof membrane and the adhesive is critical. Of utmost concern is the potential for adhered PV arrays to detach from the roof membrane over time as the adhesive ages, and exposure to elevated temperatures and repetitive wind uplift forces.

As previously discussed, adhered PV arrays are best applied to an adhered roof membrane. And given the increased heat load on the roof system with adhered arrays, a roof membrane with enhanced heat aging properties is even more critical.

Thermal Movement

PV arrays and electrical conduits are subject to thermal movement. The expansion and contraction of these elements runs the risk of damaging the roof membrane, premature failure of the PV system, and damage to the electrical connections. The effect of thermal movement may be reduced by:

  1. Splitting larger arrays into smaller sub-arrays.
  2. Using racking clips and attachments that accommodate thermal expansion without transferring movement to the underlying roofing systems. Some racking manufacturers have provisions for thermal expansion built into the PV systems.
  3. Using flexible flashing details for penetrating or attached racking systems.
  4. Incorporating thermal expansion fittings in straight electrical conduit runs.
  5. Locating racking systems such that they do not cross over expansion joints.

These strategies apply to all PV systems and can be considered for mounted, ballasted, and adhered systems. Accounting for thermal expansion and contraction is necessary for long-term performance of the PV system.

Racking Systems

The PV racking system should also be designed so that external forces acting on the rack do not compromise the waterproofing integrity of the roofing system. Loads to be considered include horizontal wind load, vertical wind uplift load, vertical dead load, and vertical snow load, as well as vibration loads from external forces or building equipment.

Racking systems should have:

  1. Thermal and dynamic movement provisions within the racking system.
  2. Minimal UV exposure of washers/gaskets.
  3. Base mounts of ballasted PV systems with sufficient area to distribute vertical loads without cutting or compressing the underlying roofing system.
  4. Support stands fabricated from round steel pipes or square tubes to facilitate effective flashing installation methods in accordance with NRCA or the roof manufacturer’s construction details.
  5. Ballast pavers that have been tested for long-term durability (for ballasted PV systems), including freeze-thaw durability.
  6. Compatible corrosion-resistant rack components.
  7. Sufficient racking clearances for inspection, maintenance, repair or reroofing.

Racking systems should allow ease of removal by trained personnel without exposing the roof system to the possibility of damage. This may require collaboration between the PV system designer and the roof designer to be sure the penetrations are properly flashed and allow for maintenance.

Electrical Connections

Best practice dictates that the number of roof penetrations are kept to a minimum. Cables passing through the roof assembly should not travel horizontally within a roof system, such as directly under the roof membrane or in a notched-out section of the insulation. Adhered PV Systems present unique challenges for cable runs and interconnections. To avoid these issues, consider the use of enclosed electrical conduit that is raised above the roof to facilitate drainage and maintain the integrity of the roof system.

Penetrations of electrical conduit into the building should be properly flashed to the roof system. Electrical conduit passing through a roof assembly should be run inside a sheet metal enclosure with roof curbs. A gooseneck-type penetration detail can be used where a cable needs to pass through a roof assembly and be flashed. All flashings should be installed in accordance with NRCA or the roof manufacturer’s construction details.

Penetration pockets (pitch pockets or pitch pans) are acceptable as a last alternative for flashing PV system electrical connections passing through a roof assembly. Clear access to these penetrations should be provided as more frequent maintenance will be required to maintain the integrity of these penetrations. Again, these flashings should be installed in accordance with NRCA or the roof manufacturer’s construction details.

 

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Originally published in November 2020

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