Commercial roof fires cause significant property damage and business interruption — and most of them are preventable. The leading ignition sources are hot work during installation or repairs, electrical faults in rooftop HVAC equipment, and open-flame torch application of modified bitumen. Understanding where fires start, what fire ratings mean, and which maintenance steps reduce risk gives commercial property owners the information they need to protect their buildings before a problem develops.
Where Commercial Roof Fires Start
Three ignition categories account for the majority of commercial roof fires:
Hot work. Welding, cutting, grinding, and torch-applied roofing are the most common sources. According to NFPA hot work safety data, U.S. fire departments responded to an average of 3,396 structure fires per year caused by hot work between 2017 and 2021. Sparks from these activities can travel up to 35 feet and are small enough to fall through gaps around penetrations, into insulation cavities, or enter HVAC ductwork and travel to other areas of the building. Critically, sparks can smolder for hours before flaring — a fire that starts at 2 p.m. may not be visible until well after a roofing crew has left the site.
HVAC and rooftop equipment. Commercial buildings typically have multiple rooftop units — HVAC systems, exhaust fans, and sometimes kitchen hood exhausts that vent directly through or near the roof. Electrical faults, grease accumulation in exhaust fans, and overheated compressor components are all recognized ignition sources. Equipment curbs and penetration flashings that have deteriorated also create gaps where heat or sparks from equipment can contact the membrane or insulation.
Improper torch application. Modified bitumen membranes are often installed using open-flame torches. When a crew rushes the work, heats a section unevenly, or fails to check beneath laps and at penetration details, the underlying insulation or vapor barrier can begin smoldering without visible signs at the surface. This is a well-documented installation-phase risk, distinct from hot work on an existing building.
Understanding Commercial Roof Fire Ratings
Not all roofing membranes are equal in fire resistance — and the rating system is more nuanced than most property owners realize.
Commercial roofs are classified as Class A, B, or C under UL 790 and ASTM E108, the standard test methods for fire tests of roof coverings. Class A represents the highest resistance — effective against severe fire exposure. Class B is moderate, and Class C is the minimum (light fire exposure). The key point: no membrane earns a fire rating in isolation. The classification applies to the complete assembly tested from deck up — the deck type, insulation board, any cover board, the membrane, and the roof slope all affect the final rating. A TPO membrane on the wrong insulation substrate may not carry the Class A listing you assume it does.
Colorado's Division of Fire Prevention and Control has adopted the 2021 International Building Code, which sets minimum fire-resistance requirements for commercial roof assemblies based on occupancy type and construction classification. Most commercial occupancies require at least a Class B assembly; many require Class A, particularly in higher-occupancy or mixed-use structures.
When a commercial roof is being re-covered or replaced, verifying that the proposed system — membrane, insulation, and any cover board — is tested and listed as a complete assembly for your deck type is a basic compliance step that is frequently skipped by less experienced contractors.
What NFPA 51B Requires from Your Contractor
If any roofing work on your building involves open flames, welding, or grinding, NFPA 51B — the Standard for Fire Prevention During Welding, Cutting, and Other Hot Work — applies. The 2019 edition tightened the fire watch requirement: a trained fire watch must now remain on site for at least one hour after hot work is complete (the previous requirement was 30 minutes). The fire watch person must have fire-suppression equipment immediately available and must inspect the work area and any adjacent spaces for smoldering.
Before any hot work begins, a hot work permit should be issued that documents the scope, the fire watch assignment, and the site conditions. Professional roofing contractors know this and follow it. If a contractor dismisses the permit requirement or the fire watch as unnecessary, that is a red flag.
Practical Prevention Steps for Property Owners
You cannot control everything that happens on your roof, but you can significantly reduce fire risk through the following:
- Schedule annual rooftop HVAC inspections. Have your HVAC service contractor inspect all rooftop unit electrical connections, exhaust fan grease traps, and compressor components. Grease buildup in kitchen exhaust fans is a particularly common ignition source that accumulates faster than most owners expect.
- Keep the area around rooftop units clear. Debris — leaves, paper, packaging — accumulates around HVAC curbs and provides ready fuel if a spark or overheated component is the ignition source.
- Inspect membrane at penetrations annually. The flashing details where pipes, conduit, exhaust fans, and HVAC curbs penetrate the membrane are the highest-wear points. Deteriorated flashing creates gaps where heat or sparks can contact insulation below.
- Require hot work permits from any roofing contractor. Before any repair or re-roofing work involving open flame or welding begins, ask to see the hot work permit and confirm the fire watch plan. This is standard professional practice, not an unusual request.
- Know your roof assembly's fire rating. Ask your contractor for the UL or FM listing number of the installed system — not just the membrane brand. The listing documents which combination of components was tested and what Class rating it carries.
Commercial Roofing in Colorado Springs
Colorado Springs sits at 6,035 feet elevation, and at that altitude the combination of intense UV radiation, dry climate, large daily temperature swings, and frequent severe weather creates a punishing environment for commercial roofing membranes. UV degradation accelerates membrane aging faster than at lower elevations, which means seams, penetration flashings, and equipment curbs that appear adequate at year five may be significantly compromised by year ten. Compromised flashings and aged membrane are not only water-infiltration risks — they reduce the effective fire resistance of the assembly by creating gaps and delaminated layers that behave differently than the tested system.
Annual inspections by a qualified commercial roofing contractor are the most cost-effective fire and water risk management tool available to Colorado Springs commercial property owners.
Ready to schedule a free commercial roof inspection? Call L&N Construction LLC at (719) 355-0648 or request an inspection online. We serve commercial and multi-family properties across Colorado Springs, Fountain, Falcon, Monument, and the surrounding Front Range, and we have done so for 15 years.
For information on our full range of commercial roofing services, visit our commercial roofing page or learn about storm damage and emergency repairs.
Frequently Asked Questions
What is the most common cause of commercial roof fires?
Hot work — welding, cutting, and open-flame torch application of modified bitumen — is the leading cause. According to NFPA data, U.S. fire departments responded to an average of 3,396 structure fires per year caused by hot work between 2017 and 2021. Sparks can smolder for hours after work stops, which is why NFPA 51B now requires a fire watch for at least one hour after any hot work is complete.
What does a Class A roof fire rating mean for a commercial building?
Class A is the highest fire resistance classification under UL 790 and ASTM E108, indicating the system can withstand severe fire exposure. The rating applies to the entire roof assembly — deck, insulation, and membrane — not the membrane alone. Most modern commercial membrane systems (TPO, EPDM, PVC) can achieve Class A as part of a tested assembly, but the actual rating depends on how they are installed together.
Can HVAC equipment cause a roof fire?
Yes. Rooftop HVAC units are a recognized ignition source. Electrical faults, grease buildup in exhaust fans, and overheated compressors can all ignite the roofing membrane or underlying insulation. Annual HVAC servicing and keeping the area around rooftop units clear of debris significantly reduces this risk.
Does Colorado require a specific fire rating for commercial roofs?
Yes. Colorado's Division of Fire Prevention and Control has adopted the 2021 International Building Code (IBC), which sets minimum fire-resistance requirements for commercial roof assemblies based on occupancy type and construction class. Most commercial occupancies in Colorado require at least a Class B assembly, and many require Class A.
Does L&N Construction LLC inspect commercial roofs for fire-safety issues?
Yes. L&N Construction LLC has 15 years of experience serving commercial and multi-family property owners across Colorado Springs, Fountain, Falcon, Monument, and the surrounding Front Range. A free roof inspection covers membrane condition, flashing at penetrations, drainage, and the condition of rooftop equipment curbs — all common fire-risk points. Call (719) 355-0648 to schedule.