PV Array Layout

The logic behind the layout of PV arrays applies to all attachment options. PV arrays are usually oriented and laid out for maximum solar energy collection. This includes keeping panels away from walls or equipment that provides shading and away from hot air exhaust that will impact PV efficiency. Designers should also consider the appropriateness of PV arrays installed in high-wind uplift pressure areas of a roof, such as corners and perimeters, to avoid potential uplift failures of the PV arrays or the roof system. Arrays should also be configured in such a way to avoid additional snow accumulation.

The layout of PV arrays should also address access for solar installation, PV array and roof maintenance, and fire safety. Safe access for the solar contractors and electricians during installation, as well as service and maintenance over the service life of the PV arrays must be accounted for in the design. This may include davits or other tie-off points, perimeter access and consideration to proximity of overhead power lines. Roof maintenance is also important so PV arrays generally should be kept away from mechanical units, catwalks, anchors, drains and other rooftop elements to provide access for service.

The NRCA Guidelines summarize the requirements from the International Fire Code (IFC) and National Electric Code (NFPA 70). Generally, they require a 4-foot perimeter around roof edges, hatches and a pathway between the two, as well as a pathway along both centerline axes protected by walkway pads or pavers. They also recommend a 4-foot wide pathway to skylights, ventilation hatches and roof stacks for future serviceability, as illustrated in Figure 6. Best practices for fire safety include 8-foot wide pathways for smoke ventilation between panel arrays, and getting approval from the local fire chief.

Figure 6: Rooftop solar layout guidelines published in NRCA Guidelines.

Roof system maintenance is also an important consideration for PV array layout. It is important to align PV arrays and set rack heights such that there is enough clearance to service the roof membrane, especially drains and penetrations. While a PV layout that covers the entire roof system may maximize solar output, this makes maintenance of both the roof and PV systems nearly impossible (see Figure 7). Considering these panels have a life expectancy in the range of 25 years, these panels need to be cleaned and debris and plant growth that accumulates between panels will need to be removed each year.

Figure 7: A tightly packed rooftop PV array without space to maintain the panels or the roof.

Providing additional space between the arrays and the roof membrane also increases ventilation and reduces heat build-up, which results in more efficient panels. Some consider that the most efficient PV arrays are installed in conjunction with vegetative roofs, as they provide a better climate and temperature for PV arrays to function, which improves electricity production. The relatively small payback period and the environmental benefits of combining these two sustainable approaches could balance out the initial investment.

Drainage

Installation of a PV system should not interfere with the drainage of the roof system. However, it is not uncommon to see PV arrays covering the roof drains, which significantly hinders access for clearing of debris from the drains. The layout design should consider how the drains will be accessed for maintenance. Adhered PV arrays and ballasted PV systems inherently run the risk of impeding rooftop drainage. Drainage should be considered during the PV layout which is most often specified by a solar designer, not the roof designer. This is just one of many examples of the importance of early project coordination and how critical it is for a successful project.

Consider the following:

1. Elevate framing and conduits above the roof surface to allow drainage.
2. Install crickets on anything perpendicular to drainage flow and greater than 24 inches wide.
3. Projections through the roof system should not be located within 2 feet of valleys or designated drainage areas adjacent to drains, scuppers, or gutters.
4. Consider modifying solar racking heights to counter the varying thickness of tapered insulation.
5. Review solar array layout such that drainage is not impeded.
6. Provide clear access to roof drains and scuppers to allow for maintenance.

Following these drainage best practice guidelines will reduce the potential for ponded and trapped water on the roof, keep the surface cleaner and potentially extend the life of the roof membrane.