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Related: Class A Fire Rating Explained
TLDR: Most Colorado homes don't burn in wildfires because a wall of flame reaches them. They burn because embers, carried ahead of the fire by wind, find their way inside through vents, gutters, deck spaces, and gaps in the building envelope. By the time the fire front arrives, ignition has often already started from the inside. This article explains where embers enter, why attic vents are the highest-priority vulnerability, and what you can actually do about it.
Most people picture wildfire destroying homes the same way it destroys forests: a wall of flame moving through and burning everything in its path. That image is accurate in some fire scenarios. But it's not what destroys most homes in Colorado wildfires.
Embers do.
What does an ember storm actually look like?
In a wind-driven wildfire, the fire generates a continuous stream of burning material that gets lofted into the air and carried ahead of the fire front by wind. These embers can travel a mile or more from the fire itself.
In the Marshall Fire, which destroyed more than 1,000 homes in Boulder County in December 2021, winds were gusting over 100 miles per hour. Embers were landing in neighborhoods well ahead of the visible fire front. Some homeowners reported their roofs igniting while the main fire was still blocks away. The ember storm preceded the fire, not the other way around.
Where do embers actually enter a home?
Embers don't need a door or a window to enter a home. They find their way through the small openings that exist in every house.
Attic vents are the most common pathway. Standard roof vents typically have screening that is not ember-resistant. It either has openings large enough for small embers to pass through, or it's combustible itself. Once an ember gets into an attic, it lands on insulation or wood framing. Attic spaces have ample oxygen and very little moisture. Smoldering can go on for a significant time before anyone sees smoke.
Understanding how your attic is ventilated, and whether that ventilation system is protecting or exposing you, is one of the most important home hardening questions you can ask.
→ Attic Ventilation 101: What It Is, Why It Matters, and Why Most Colorado Homes Get It Wrong
Soffit vents are another common entry point. Embers accumulating in gutters or on low-slope roof areas near soffits can ignite material that enters the soffit cavity. Gaps at exterior wall penetrations, including dryer vents and exhaust fans, are also pathways. And underneath decks, a wood deck creates an enclosed space where embers accumulate and smolder against structural members that connect directly to the house.
Why does ignition often start from inside the structure?
What makes ember intrusion particularly difficult to manage is that the ignition often happens inside the structure, not on its surface.
A Class A roof can resist a burning brand landing on it for a significant period. But if an ember has already entered through a vent and is smoldering in the attic, the roof assembly is essentially being attacked from below. By the time the roof is visibly involved in fire, the interior structure may already be burning.
This is why vent protection is treated as one of the highest-priority home hardening upgrades in wildfire research.
What did the Marshall Fire show us about how homes actually burn?
Post-fire analysis of the Marshall Fire identified a consistent pattern: homes that survived more often had ember-resistant vent covers, non-combustible or fire-resistant cladding, and cleared zones in the first five feet adjacent to the structure.
Homes that burned more often had standard vents, combustible decks or fences connecting directly to the structure, wood or vinyl siding, or accumulated dry debris in gutters. The fire happened in December, on grassland, in suburban neighborhoods. It wasn't a remote mountain scenario.
What does vent hardening actually involve?
Ember-resistant vent covers are metal vent covers with 1/8-inch or finer non-combustible mesh screening, sometimes combined with intumescent materials that expand and seal the vent opening when exposed to high heat. Products tested to ASTM E2886 and E2912 are the relevant standards for ember and flame intrusion resistance.
The Marshall Fire brought much-needed awareness to the real vulnerabilities in how homes are built and how little attention has been paid to wildfire resiliency in residential construction. One of the biggest culprits is unprotected attic ventilation. Embers from a burning structure, even one at a significant distance, get pulled into a home through unprotected vents, and that's often what starts the spread. When we're replacing a roof, thoroughly evaluating the ventilation system and making sure every component is protected with ember-resistant covers is something we take seriously on every single job. It needs to be something every contractor takes seriously too.
Replacing standard vents with ember-resistant versions is one of the more accessible home hardening upgrades available. It doesn't require tearing off a roof or replacing siding. Pairing ember-resistant vents with a non-combustible roofing system addresses two of the most common ember contact points at once.
→ Stone-Coated Steel Roofing in Colorado: A Long-Term Solution for Hail, Snow, and Energy Efficiency
How do I know if my roof type is appropriate for a WUI zone?
For homeowners in wildland-urban interface areas, roofing system selection is as important as vent protection. Materials, assembly ratings, and how a roof handles ember accumulation all vary significantly by product type. The Roof Type Guide covers how location in Colorado affects which systems make sense.
→ What Roof Type Is Best for My Home?
Where should I start if wildfire is my primary concern?
Vent protection and the first-five-foot zone are the highest-priority starting points. They address the most common ignition pathways and don't require waiting until the next major roof or siding project.
The Home Hardening Quiz includes a section on vent and roof assembly vulnerability and gives you a score showing where your home stands overall.
→ Home Hardening Quiz
