As commercial building design seeks to reduce energy costs and environmental impact, rooftop solar installations are rapidly gaining ground. The U.S. alone saw a 21% year-over-year increase in solar installations in Q3 of 2024 with 8.6GW of new installations, and has a total of nearly 450GW capacity expected by the end of 2029. But while solar panels offer environmental and economic benefits, they also introduce new challenges for the roofing systems that support them – particularly on low-slope commercial roofs.

The demands of a solar-ready roof call for important material specifications to help mitigate long-term risks these installations create for waterproofing, insulation performance, fire resistance, and maintenance access. Designing a roof that can coexist safely with a photovoltaic (PV) array means understanding the full system and choosing materials that can handle the added pressures (literally and figuratively) that come with rooftop solar equipment.

Solar-Ready Roof Assemblies: Layering for Resilience

A commercial roof doesn’t just provide shelter for the building below; it supports critical systems, protects valuable assets, and increasingly serves as real estate for rooftop mechanicals, gathering spaces, and now solar photovoltaic (PV) arrays.

While most commercial roof assemblies are designed with durability and efficiency in mind, they’re not always optimized for the additional stresses introduced by rooftop solar. The weight of mounting systems, increased maintenance foot traffic, and elevated fire risks all call for careful material selection.

A standard low-slope commercial roof assembly typically includes:

• Structural Decking: Commonly made of fluted steel for speed and cost efficiency.
• Vapor Retarder: Specified based on climate and interior humidity loads.
• Insulation: Typically rigid polyisocyanurate (polysio) valued for its high R-value per inch and compressive strength.
• Gypsum Cover Board: Installed above the insulation to provide a suitable flat bonding substrate, resist punctures, add durability, and support the waterproofing. Non-combustible gypsum cover boards can also add fire resistance required by codes.
• Roof Membrane: Usually a single-ply TPO, EPDM, or PVC membrane adhered to the cover board or mechanically attached through the cover board and back to the structural deck. Some installations may skip the cover board and adhere the membrane directly to polyiso insulation.

This assembly is a commonly accepted configuration for many commercial roofing applications, but may not be enough to withstand the four big risks that come with rooftop solar installations:

The 4 Biggest Risks to Roof Assemblies with Solar Panel Installations

1. Moisture and Waterproofing Failures

It should come as no surprise that PV installations elevate the risk of damage your commercial roof’s waterproofing layer. Any penetrations through the membrane create a potential path for moisture, and this is amplified with solar installations that require fastening panels, racking systems, or conduit attached to the roof deck.

Add to this the maintenance requirements of solar arrays (more on that in a moment), and your roofing membrane needs to account for even more potential for puncture. Dropped tools or equipment, improperly placed ballasts, or accidental punctures from technician activity can compromise the membrane, potentially leading to costly water damage to the roof assembly. Damage that’s made more difficult by the presence of a large PV array.

2. Thermal and Structural Load Impacts

PV panels may be mounted using ballast systems or mechanical attachments. Regardless of method, the roof must bear not only the static weight of the panels but also maintenance traffic and dynamic loads from wind. Over time, this pressure can compress insulation layers in the roof assembly, degrade thermal performance, and cause ponding or cracking of the membrane.

Typical commercial rooftop solar panels can weigh 40 to 55 pounds each. With large installations requiring dozens (or even hundreds) of panels to meet the building’s power generation needs, the weight of a rooftop solar system can add up quickly. For example, a 100kW array using 400-watt panels would need 250 panels; at 50lb each, your roof now needs to support 12,500lb of panels, plus the racking system to support them.

3. Fire Risks: Hidden but Significant

Fires caused by rooftop solar systems occur at a rate of fewer than 29 fires per GW annually, according to a study by the Journal of Building Engineering. Though solar panel fire risks are generally low, increased PV capacity means fire prevalence is likely to increase as well. What’s more worrying is that PV equipment itself can exacerbate fire conditions and make firefighting challenging. The reasons are three-fold:

Electrical Arcing: Poor connections at DC isolators, inverters, or junction boxes can create high-temperature arcs exceeding 6,000°C. This excessive heat is more than enough to ignite roofing membranes or adjacent materials.

Flame Trapping Under Panels: PV panels mounted close to the roof surface can trap heat, soot, and gases, accelerating flame spread and magnifying the fire’s intensity. Research from the University of Edinburgh showed that fire under PV modules can spread up to 38 times faster than fires on exposed roof membranes due to this semi-enclosed cavity effect.

Rooftop Access to Firefighting: From a practical perspective, solar arrays can take up a great deal of space, creating crowded rooftop environments that could make firefighting difficult. Not to mention, panels continue generating electricity during daylight, even when disconnected from the building’s main utility, which can pose shock hazards for first responders.

4. Maintenance Demands and Damage Potential

With regular inspections, repair, and cleanings, PV installations increase the foot traffic a roof assembly must endure for equipment maintenance. Unlike typical commercial roofs and mechanical equipment that may only require a few inspections a year, this added foot traffic for PV places repeated stress on the roof assembly membrane and insulation, leading to greater potential for failures.

A Better Roof Assembly to Reduce Rooftop Solar Risks

With these four risks in mind, consider reevaluating your commercial roof assembly to keep fire, and maintenance damage at bay in rooftop solar situations. Upgrading your cover board, insulation, and membrane are all smart considerations:

High-Performance Cover Boards

Opting for glass mat-faced gypsum boards is the easiest and most beneficial roof system upgrade you can make when designing for PV. These enhanced cover boards offer better water resistance compared to standard paper-faced gypsum boards, and increased structural strength to help improve load distribution, resist foot traffic damage, and offer a fire-resistant layer between the membrane and the insulation. Additionally, their rigid surface can help maintain a consistent bond with the waterproofing membrane, helping to preserve the roof’s performance under load.

Industry research has shown that the inclusion of a glass mat-faced cover board beneath the roofing membrane significantly improves puncture resistance. In one modified ASTM D4833 slow-speed puncture test, a polyiso insulation with 60 mil TPO membrane was penetrated at just 35 lbf. Adding a glass mat-faced gypsum cover board beneath the membrane increased resistance to 72 lbf, more than doubling the force required to puncture the membrane. In addition to this added protection against puncture damage, the consistent bonding surface provided by a fiberglass facing can enhance membrane adhesion and support the overall wind uplift performance of the roof assembly. By reinforcing the membrane and distributing loads more effectively, the cover board can also help reduce the opportunity for damage that might compromise the waterproofing layer.

Non-Combustible Insulation

Swapping out rigid polyiso insulation for non-combustible mineral wool can improve fire resistance in the roof assembly to mitigate risks. Depending on the manufacturer, mineral wool can withstand temperatures up to 2,150F, whereas polyiso can melt or burn at a temperature of 390F.

While the thermal performance of polyiso allows roof assemblies to achieve NFPA 276 in standard installations, the increased fire risk with PV demands higher standards. Keep in mind, though, that unfaced mineral wool board insulation has a substantially lower compressive strength of 11 psi compared to polyiso at 20 psi. So while it offers a high level of fire resistance, the weight of a solar array could compromise mineral wool’s insulation properties moreso than it would with polyiso. When prioritizing fire safety then, using a strong glass mat-faced gypsum cover board becomes even more important in helping distribute the weight of the PV array and reduce the risk of compression.

Non-Combustible Roof Membranes

Between electrical arcing and flames trapped under PV panels, upgrading to a non-combustible roof membrane can further help mitigate rooftop solar fire risk. Fiberglass-reinforced membranes can offer enhanced fire resistance by maintaining more of their structural integrity under high temperatures and helping to reduce flame spread. This is an added benefit to the membranes’ durability against puncture and foot traffic.

Code Compliance and Insurance Implications

Beyond day-to-day durability, solar-ready roofs must meet evolving codes and third-party testing standards. FM Approvals for instance, under FM 4470, evaluates entire roof assemblies for resistance to:

• Fire
• Wind uplift
• Water leakage
• Foot traffic
• Hail
• Corrosion
• Puncture

Assemblies that includes glass mat-faced gypsum cover boards often perform better across all of these metrics and can also help building owners secure better insurance rates. To that end, some insurers have issued guidance suggesting PV systems be installed over noncombustible roof assemblies to reduce risk. Risk guidelines from AXA state, “do not install PV systems on combustible roofs,” and “do not install panels over roof assemblies that contain foam plastic insulation below the roof covering system,” precluding the use of polyiso in rooftop solar environments.

Building for the Future: Should You Overbuild in Anticipation of Solar?

With solar installations expected to increase dramatically in the next decade, commercial roof design and engineering teams need to consider the value of pre-designing roofs to accommodate future solar installations.

While it’s not always practical to anticipate every future use, there’s a case to be made for incorporating solar-ready features in today’s new roof designs and retrofitting existing roofs prior to installing solar. A roof built to last 20–30 years could easily be fitted with a solar array during that time. Likewise, a recent NFPA global workshop found global experts agreeing that existing roofs should be retrofitted prior to installing solar.

Preparing your roof assembly with resilient materials now, including glass mat-reinforced cover boards, and perhaps non-combustible insulation and membrane, could help extend the life of the entire roof system and reduce long-term operational costs. One FMI study found that cover boards increased the median lifespan of a single-ply roof by four years and lowered maintenance costs by an average of $1.40 per square foot over 20 years.

Final Recommendations for Solar-Ready Roof Design

With industry research and best practices in mind, minimizing the risks of rooftop solar installations on commercial buildings comes down to five key recommendations:

1. Design for Fire Safety: Incorporate noncombustible layers like glass mat-faced gypsum roof cover boards to slow flame spread and reduce ignition risk.
2. Protect the Membrane: Use cover boards to improve puncture resistance, foot traffic durability, and long-term membrane performance.
3. Plan for Maintenance: Create access paths, schedule inspections, and build the roof to withstand frequent service visits.
4. Think Like an Insurer (or Firefighter): Follow guidance from insurance and fire safety authorities to improve insurability and emergency response.
5. Build to Last: Ensure the roof assembly is designed to meet or exceed the life expectancy of the solar panels.

As rooftop solar grows more widespread, these guidelines will help commercial construction teams ensure that the building enclosures beneath these arrays are ready to handle the heat, weight, and wear they bring.