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Commercial Rooftop Solar Design Explained

By Jennifer Keegan 13 Feb 2022

When installing rooftop solar, there are a few important questions to consider.

1. Is the life expectancy of the roof system likely to exceed that of the photovoltaic system (PV array)?
2. Has the roof system and PV layout been coordinated to facilitate maximum solar output, maintenance and safety?
3. What are the potential risks of deviating from best practices to reach a lower cost solution?

If photovoltaic (PV), aka solar, arrays were installed on all the commercial buildings in the U.S. with roofs over 5,000 square-feet, they have been estimated to provide enough energy to power nearly 60 percent of the total commercial electricity demand. Commercial rooftops are an appealing option as a platform for installing PV arrays to support energy generation, as well as corporate and community energy initiatives. However, it is important to remember that the roof’s primary function is to protect the building and its contents from the elements.

When considering rooftop solar, the roof system should be designed to have an equivalent or longer lifespan than that of the PV arrays. Whether it’s a new roof that has PV arrays or will have PV arrays installed in the near future (i.e., a solar ready roof), or it’s an existing roof that will receive solar, there are many important considerations for roof system design and panel layout.

PV Roof System Design: Best Practices

PV arrays have an average useful economic life of more than 25 years. For commercial rooftop solar, it is often cost-prohibitive to remove existing PV arrays, install a new roof, and then reinstall the PV arrays. Therefore, the best time to install a rooftop PV system is right after a new roof has been installed or when a building has been newly constructed. The key issue is that the roof system should have an expected useful life that matches or exceeds the expected economic life of the PV array. To specify a roof system that is as durable as the PV array, designers should consider the following:

-- Adhered reflective roof membranes that provide exceptional protection from weather elements and greater thickness;
-- Incorporation of a cover board directly below the membrane to help protect against punctures and damage from rooftop traffic;
--Use of high compressive strength rigid insulation for good traffic resistance and avoid damaged insulation due to the weight of the solar overburden; and Roof system warranty or guarantee that exceeds the life expectancy of the PV arrays.

Roof Membranes

By their nature, reflective roof membranes are beneficial in reducing heat build-up around PV arrays as the temperature of a PV panel can significantly impact how much electricity the panel produces, as shown in Figure 1. As panels get hotter, they produce less power. It is estimated that the efficiency of a PV panel can be up to 13 percent higher when installed over a highly reflective membrane compared to a dark membrane with low reflectance. Also, the use of bifacial PV panels over reflective roof membranes can increase the efficiency by 20-35 percent, as they take advantage of the reflected light.

The National Roofing Contractors Association (NRCA) recommends the use of a roof membrane that offers enhanced protection against the effects of UV radiation and high service temperatures, and can maintain high reflectance over a long period of time to help ensure that the roof life expectancy will match that of the PV arrays. Figure 2 highlights the advantage of utilizing a membrane that maintains high reflectance over a long period of time. The reflection from the lower roof onto the rising wall can be seen clearly in photographs ranging over a 5-year period. The use of bifacial panels would significantly increase the solar output generated from the use of this highly reflective membrane.

Designers and owners may also want to consider an increased roof membrane thickness to match the service life of the PV arrays. A large independent study shows an estimated service life of 30-35 years for an 80 mil high-performance TPO membrane, which is in line with high-performance PV arrays and provides some time buffer to have the roof and PV arrays replaced. Additionally, membrane thickness can provide additional protection against punctures, which is especially important considering the extra foot traffic on the roof due to PV service and maintenance activities. Fleece backed membranes installed with a low-rise foam adhesive provide enhanced protection against impact. However, membrane thickness significantly improves impact resistance by almost 80 percent from 45 mil to an 80 mil membrane.

The use of wider rolls, such as 10-foot or 12-foot single-ply membranes, can minimize the number of seams on the roof and reduce the potential for seams to be obscured below PV arrays. This will allow for more thorough roof inspections and future maintenance efforts, should the need arise.

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