Understanding the UL 790 Fire Test for Rooftop Solar Arrays
Simply put, the purpose of the UL 790 fire test is to evaluate and classify the fire resistance of roofing assemblies, including those with photovoltaic (PV) modules installed on them. Its primary goal is to determine how a roof system with solar panels contributes to the spread of fire, providing critical data for building codes, firefighters, and installers to ensure occupant safety and protect property. This isn’t about testing the panels in isolation; it’s about testing the entire roof system as a cohesive unit under fire conditions. The test answers a fundamental safety question: If a fire starts on or beneath a roof with a solar array, will the system slow the fire’s progress or dangerously accelerate it?
The development of this standard was driven by the rapid adoption of rooftop solar. As more arrays were installed, fire safety officials and insurance companies needed a standardized, scientific method to assess risk. The UL 790 test, often referred to by its building code designations—ASTM E108 or UL 790—fills this role. It simulates three critical fire exposure scenarios that a roof might face: a spreading flame across its surface, burning brand ignition (simulating wind-blown embers in a wildfire), and intermittent flame exposure. A PV module and its mounting system must perform adequately in all three to earn a classification.
The Three Pillars of the Fire Test: Class A, B, and C Explained
The test results classify the roof assembly into one of three classes, with Class A being the most resistant. These classifications are not just labels; they are rigorous performance benchmarks.
- Class A is the highest rating. It signifies the assembly is effective against severe fire exposure. To achieve this, the system must prevent the flame from spreading more than 6 feet beyond the center of the burning brand application. It also must not develop any flying brands (burning pieces that can spread the fire) and cannot allow the roof deck to develop any holes during the test.
- Class B is effective against moderate fire exposure. The allowable flame spread is increased to 8 feet.
- Class C is effective against light fire exposure, with an allowable flame spread of 13 feet.
Most building codes in high-fire-risk areas, such as California’s Wildland-Urban Interface (WUI), mandate a Class A rating for roofing. Consequently, for a solar installation to be permitted in these zones, the entire system—roof covering, underlayment, PV modules, and racking—must be tested and listed together as a Class A assembly. You cannot simply install Class A-rated panels on a non-rated roof and expect to meet the requirement; the system is only as strong as its weakest link.
A Deep Dive into the Testing Procedure and Critical Metrics
The UL 790 test is a controlled but severe simulation. The test occurs in a specialized furnace where a 12-foot by 12-foot section of the roof assembly is constructed, complete with the actual PV modules, racking, and roofing materials. The test consists of three sequential parts:
- Intermittent Flame Exposure: The assembly is exposed to a gas flame for 2 minutes, followed by a 2-minute pause. This cycle is repeated three times to simulate fluctuating wind conditions during a fire.
- Burning Brand Test: This is often considered the most challenging part. A wooden “brand” (a 17×17-inch grid of wood sticks weighing 9 lbs for Class A) is placed on the test specimen and ignited. The test measures how far flames travel from the brand and whether the fire penetrates through the roof deck. This simulates the threat from wind-blown embers during a wildfire.
- Spread of Flame Test: A large gas flame is applied to the edge of the test specimen for 10 minutes to see how far the flame travels up the sloped roof surface.
Throughout these tests, technicians monitor and record key data points, including:
| Metric Measured | Significance | Class A Threshold |
|---|---|---|
| Lateral Flame Spread | How far the fire travels horizontally from the ignition point. | ≤ 6 feet |
| Vertical Flame Spread | How far the fire travels up the roof slope. | ≤ 8 feet |
| Burning Brand Penetration | Whether the fire burns through the roof deck. | No penetration allowed |
| Flying Brands | Production of small, burning debris that can spread fire. | None allowed |
| Temperature Rise | Increase in temperature on the unexposed (attic) side of the deck. | ≤ 325°F (163°C) above ambient |
The Role of System Components in Achieving a Fire Classification
Passing the test is a team effort. Every component in the assembly plays a vital role in its fire performance.
PV Modules: The panels themselves are a key factor. Their backsheets, encapsulants, and frames must be made of flame-retardant materials that resist ignition and limit flame spread. The gaps between panels are critical; they can act as chimneys, channeling flames if not properly managed by the racking design. The glass-glass module construction is often highlighted for its inherent fire resistance due to the non-combustible backsheet.
Mounting Systems (Racking): The racking system dictates the airflow and spacing. Systems designed for fire safety often include features like intumescent materials (which swell when heated to seal gaps) or specific geometries that block flame pathways. The attachment method—whether it’s penetrating (bolts) or ballasted (weights)—also influences how heat is transferred to the roof deck.
Roofing Materials: The underlying roof covering (e.g., asphalt shingles, clay tile, metal) and its underlayment are the first line of defense. A Class A-rated roof covering is a prerequisite for achieving an overall Class A rating with solar. The underlayment must be non-combustible or fire-resistant to prevent embers from reaching the wooden deck below.
Real-World Implications for Installers, Builders, and Homeowners
This technical standard has very practical consequences. For installers and developers, compliance is a matter of legality and liability. Using a pre-tested and UL-listed system combination is the only way to guarantee compliance with local codes. It streamlines the permitting process and provides documentation for insurance companies. Trying to mix and match un-tested components can lead to permit denials and significant safety risks.
For firefighters, the classification provides vital information for tactical decisions. Knowing a roof has a Class A solar system indicates there is a reduced risk of rapid fire spread across the roof, potentially allowing for safer interior firefighting operations. It also informs them about potential fall hazards and the location of conduit runs.
For homeowners and building owners, especially those in wildfire-prone regions, specifying a UL 790 Class A-rated solar system is a direct investment in property protection. It can lead to lower insurance premiums and provides peace of mind that the system designed to save on energy costs does not inadvertently increase fire risk. The initial investment in a fire-rated system is a long-term safeguard for the property’s value and the safety of its occupants.