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Your roof is a smart place to install solar panels, but without the right precautions, you can void your warranty, compromise thermal performance and ruin waterproofing.
Follow these tips to ensure you get the full benefits of renewable energy without harming your roof.
The Impacts on Roof Performance
Because the decision to place solar panels on the roof is often made by professionals who don’t have a background in roofing, they may not be aware of the impacts on the roof performance or the difficulties and costs involved when completing maintenance or replacement.
The logistical and legal issues associated with installing solar PV on roofs are also complex. This is particularly true when owners rent their roofs to solar providers. If not clearly defined by contract, conflict can result over safety provisions, lost income when the panels are removed during roof work, or liability in the event of theft, vandalism or damage.
Membrane manufacturers typically have their own requirements if solar panels are installed. If any array is installed over an existing roof without the contractor or roofing manufacturer’s permission, your warranty could be nullified.
Lastly, the fire testing of roofs covered with PV panels is far from complete. While roof fires involving PV are rare, they have occurred. Some ballasted systems use plastic components that are far from fire resistant. Polystyrene insulation pads installed under the panels to minimize the risk of damage to the membrane are also cause for concern. (Photo: Fires can obliterate both the solar panels and roof.)
None of these factors means rooftop solar is a lost cause.
Owners and facility managers should simply be aware of the risks and take precautions before investing in an array.
Is Your Roof a Good Candidate?
When evaluating an existing roof for PV potential, look at factors such as the load-carrying capacity of the deck and framing, the type of roofing system and slope, the roof’s age and condition, building height, and wind and seismic loads.
Roofs should be classified as good, better and best based on their ability to comply with the overall design strategy. Good candidates include roofs that can accommodate increased loads without modification. (Photo: Racks can dislodge if they aren’t properly secured against wind.)
Other good options are protected membrane roofs or conventional roof membrane assemblies that have been constructed with high compressive strength, as well as roofs that have been designed with a positive slope for drainage.
Always be sure to use thicker membranes when installing solar on single-ply membranes. Avoid installing solar on roofs with light wood decks or autoclaved aerated concrete as blocks and precast panels.
One of the fundamental choices is whether to install a penetrating, mechanically fastened, ballasted or laminated system.
For low-slope roofs, NRCA recommends the use of a penetrating system where the panels are attached through to the building structure. This approach can be accommodated by installing curbs, sleepers or posts as an integral part of the roof system.
Ballasted systems are also popular as they are less expensive to install, minimize the risk to the building during installation and increase the provider’s return on investment.
If curbs, sleepers or posts are used, they must be designed and attached to the building frame to with-stand the live loads, including seismic and wind. An inability to recognize environmental loads such as snow and wind can lead to system failure or collapse.
Penetrating and mechanically fastened systems can be designed with a high degree of predictability vs. the ballasted system. These options can be designed to accommodate almost any building and roofing system, but due to cost and ease of installation, most PV systems have a ballasted design.
One example of a ballasted system places solar panels on a racking system, usually constructed from aluminum and weighted down to keep them in place. If a ballasted PV system is installed on new construction, most membrane manufacturers require the use of a thicker membrane (such as a minimum of 60 mm in the case of single-ply) and high-density coverboard installed directly under the membrane.
This approach provides added durability and reduces the risk of accidental damage.
A ballasted solar system weighs about 5 pounds per square foot, increasing on the outside row and at the end of panels as an added measure against wind. While this weight can be easily accommodated in new construction, existing roofs often do not have the spare capacity to accommodate the additional load.
Smart Installation Practices
Changing a protected membrane roof or aggregate ballasted roofing system to a conventional roof or using a modified bitumen or single-ply membrane that weighs less than the existing roof membrane will often provide the necessary spare capacity required for the PV installation.
Changing the roof system or membrane just to accommodate solar installation may not always be the best choice. Removing the aggregate surfacing from a built-up roof system and capping it with a single-ply membrane can have risks, including the difficulty of finding leaks if there are punctures and water has spread beyond the point of origin and become trapped.
While economics dictate that it is cheaper to install more panels in less space, this is not in keeping with good roofing or solar design.
Panels should be laid out and kept back from walls or equipment that can cause periodic shading. They should not be installed over or adjacent to equipment where hot exhaust air will impact their efficiency. Panels also need to be kept back from high-wind zones, such as the corners and roof perimeter.
The NRCA’s Guidelines for Roof-Mounted Photovoltaic System Installations include a number of recommendations that address clearance widths, location of access paths and arrangements for smoke ventilations.
Panels installed on a moderate slope can become dislodged by a buildup of ice and snow. To prevent this, mount ballasted systems on a pad to protect the roof membrane from possible damage from the solar panel frame.
(Photo: If the panels block access to roof drains so they can’t be cleaned, ponding water can result.)
Walkways and work areas should receive additional protection to guard against damage from roof traffic. It is important to provide independent locked roof access so that panels can be serviced without entering the building.
If your older roof will not last as long as the solar system, consider replacing the roof prior to installing PV. In addition to following code, the roof must have sufficient capacity or be modified to carry the additional loads.
Keep in mind that BUR and aggregate ballasted roofs typically have spare capacity to accommodate solar if they are replaced with lighter-weight membrane systems. Solar panels should be located away from roof edges, snowpiling (drifting) areas, trees and overhead wires. When practical, place the system in a way that prevents shading from higher buildings, equipment and adjacent panels.
Douglas Fishburn, Registered Roof Observer and Green Roof Professional, has over 40 years of roofing and building envelope construction and design experience. He is president of Fishburn Building Sciences Group Inc. The full version of this article was originally published in the 2014 Proceedings of the RCI 29th International Convention & Trade Show and can be found at rci-online.org.
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