Additionally, roofs are required by codes to provide “live-load” capacity, a measurement that includes people, snow and other weight-bearing temporary scenarios that may occur. The weight of a PV system is typically be- low the live-load capacity, however in the absence of a structural analysis, the live-load capacity will be reduced by the addition of the PV system.
A final consideration is whether a PV installation will create new locations for drifting snow, which may add considerable weight to a roof.
When determining key considerations for wind load and fire safety, best practices require deferral to the PV supplier.
EQUIVALENT SERVICE LIFE
Ideally, a roof system and the PV system should have the same expected service life. Removal (decommissioning) and reinstallation (recommissioning) of a PV system is costly, and the cost should be weighed relative to the residual service life of the existing roof and cost of roof replacement at the time of PV installation. Ballasted, rack-mounted PV systems are difficult, if not impossible, to reroof (or recoat) under and around. Elevated racks with adequate space be- neath may be able to be left in place when reroofing.
For example, a PV system that covers 10 percent of the rooftop will be easier to relocate during reroofing than a PV system that covers 75 percent of the rooftop. Building owners should be advised of future reroofing and maintenance costs with roof-mounted PV systems.
Drainage on rooftops is important for safety of the structure and longevity of the roof. PV arrays often have many points of contact with a roof, and these are possible locations that will block or slow drainage. PV racking should be positioned to minimize ponding water and/or include methods, such as notched pads, to allow drainage under points of contact, especially for ballasted systems.
Roof systems used as platforms for PV systems must be tough and durable, and generally speaking, SPF has greater compressive strength as density increases. Higher-density SPF systems may be preferred, especially when ballasted support systems are used.
An SPF system will be stressed during the installation of the PV system and coatings and granules will help protect the roof during this time, as well as during scheduled maintenance. Because a roof surface below PV panels will likely not dry as fast as non-covered portions, coatings that stand up better to standing water and biological growth should be selected.
PV SYSTEM ACCESS
All roof-mounted PV systems should be inspected and maintained at least twice a year. Wiring, attachment points and flashings should be inspected, and cleaning of the top surface of the PV panels may be required.
To maintain and service the roof and PV system, workers must be able to access both. PV systems should not block access to drains, penetrations, flashings, mechanical units or other rooftop equipment. Similarly, PV systems should be installed so maintenance workers can access wiring, inspect panel-to-racking connections and properly clean top surfaces without stepping on PV panels.
Installation of PV systems on SPF roofing will inevitably create additional foot traffic. It is important to protect heavily trafficked areas with additional coating and granules or walk pads. The cost to do so is low and will protect the service life of the roof.
Although there are many additional considerations to the application of PV systems in combination with SPF roofs, the energy generation and conservation provided by the combined solution is well worth the extra effort.
PHOTOS: Spray Polyurethane Foam Alliance