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?

This course will explain the importance of each of these questions and offer actionable insights into roof system design that can maximize solar output and provide long-term performance of the roof system that can outlast the life expectancy of the PV arrays.

Fifty acres of rooftop solar installed at the Port of Los Angeles.

Introduction

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.

Image courtesy of Architectural Science Review

Figure 1: Average roof surface and air temperatures on a dark absorptive roof membrane versus a cool reflective roof membrane.

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.

Figure 2: Example of a highly reflective roof membrane (installed on the lower roof) that maintains its reflectivity as it ages.

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.

Membrane Attachment

The membrane attachment method should be carefully considered. Adhering the membrane will avoid the normal billowing in high wind events of mechanically fastened single-ply membranes, which could cause ballasted PV systems to shift resulting in localized abrasion of the membrane. The use of a protection or separation sheet installed between ballasted supports and the membrane, extending beyond the contact surface area on all sides, can protect the membrane from abrasion and may be required for warranty or guarantee coverage. The protection sheet should be secured to the roof membrane, not to the bottom of the racking system, so that water does not become trapped between the roof membrane and the slip sheet.

Even if the roof membrane is adhered to the cover board or insulation below, many times mechanical fasteners are below the membrane and attach the cover board and/or insulation to the roof structure. These mechanical fasteners and large metal plate washers are very rigid elements, hidden just below the single-ply membrane. Ballasted PV systems inadvertently placed directly over roof system fasteners may cut or puncture the membrane as the PV array shifts during strong wind events. Burying fasteners in the roof assembly, for example by having the upper layers adhered, will minimize the potential of damage to the roof membrane, as well as enhance thermal performance of the roof system. This supports the use of adhered membranes as well as an adhered top layer of insulation and cover board.

Self-Adhered flexible PV arrays installed over a mechanically attached or induction welded membrane will billow and flutter with the roof membrane during high wind events. Over time, this could create additional stress on the PV arrays, their interface with the membrane, and their electrical connections, and may compromise the solar and roof system performance. Therefore, an adhered roof membrane can contribute to a roof system lifespan that will better match that of the self-adhered flexible PV arrays, and help enhance the performance of both. Of course, it’s necessary to verify material compatibility, long-term durability and heat aging capabilities of the adhesive to the roof membrane and PV arrays, as well as compliance with local code and uplift resistance requirements. For self-adhering PV, use a high temperature-resistant roof membrane as heat collects below these arrays, raising the temperature of the membrane in these locations. Where a standard membrane is utilized, flexible PV should be installed to a sacrificial layer of an adhered roof membrane.

For attached or penetrating systems (i.e. non-ballasted) PV rack mounting systems, mechanically attached or induction welded roof membranes could be more suitable than with ballasted PV systems. Attached PV arrays do not move or shift over the membrane and the array attachment points might act as additional anchors for the membrane. Attached and penetrating systems can be structurally connected to the roof deck below the roof membrane; these connections can be designed to provide robust wind-uplift capacity.