June 15

Assessing Wildfire Hazards in the Home Ignition Zone

The Sacramento Metropolitan Fire District (Metro Fire) sponsored a free workshop on assessing wildfire hazards in the home ignition zone that took place on Friday & Saturday June 13 & 14, 2014. Madera County’s Firewise Coordinator Roger Maybee was able to attend.
The instructor was Pat Durland of the NFPA presenting “Assessing Wildfire Hazards in the Home Ignition Zone” and he also teaches the one-day “Assessing Residential Wildfire Hazards”. Both of these mitigation courses will be available here locally.
Mr. Durland has studied many fire sites after the event to determine why homes survive, versus homes that don’t. The answer is logical and simple. If the conditions for ignition in the home ignition zone do not meet the requirements of combustion, the home may survive.
Pat Durland is the principal of Stone Creek Fire, LLC

Embers or Firebrands, Not Only Flames & Radiant Heat

One very interesting and important result of much of the work of the NFPA and Firewise Communities program has been the determination that large flame fires are not what generally destroy homes in the Wildland Urban Interface, embers are.
The biggest concern is not radiation from big flames but firebrands which land up to fourteen miles away under perfect conditions. Embers traveling a mile away from a fire front is not uncommon. If your home and the area adjacent to your home is Firewise, then there is a strong possibility you will not lose your home.
In a timber crown fire, radiation doesn’t affect wood structures as much as you think, according to many studies. If you have trees limbed up, and no forest litter, a hardened Firewise structure, a crown fire passing through only lasts seconds, and goes over the top. Wood siding takes longer to ignite than one would think, but better still is Hardiplank or other completely non-combustible material. The embers are the big threat.

Focus on Embers Too

These firebrands, or embers aka “clinkers”, burn far more structures than do large flames or radiant heat. They do this by igniting vegetation and litter around the home, the siding, the gutters, the roof, and also by getting inside the home, igniting and burning from the inside out. These embers have been seen to travel many miles ahead of a large fire event.
The majority of firebrands that enter the house enter directly, meaning level or horizontally, depending on wind behavior. 1/8 inch screening is becoming more common, but as you’ll see in the video below still allows embers to penetrate. Turbo vents on a roof are also a potential for penetration because they are spinning. In a wind, embers can get sucked in igniting the home.
Watch this interesting video on an ember storms affect on a typical structure.


How to do Home Ignition Zone (HIZ) Assessments

When you do a HIZ assessment, you start with the home at the center. The class was taught that the structure at the center of the home ignition zone is the starting point for assessment and mitigation. Even addressing the interior of the structure can be part of the assessment.
The class went out and did an assessment of a home. The team then compared notes. Then they critiqued each others’ notes. Everybody offered their opinions and ideas.

More Than One Way

The instructor taught that the conditions for combustion can be reduced or removed in many different ways. The instructor said that there are a lot of ways to do things. You can adhere to a strict mandated prescription, or, you can compromise to an extent, and still effectively stop combustion.

Trees Can Stay

Trees are a good example. You don’t have to remove all trees within ten feet of a structure to stop combustion. That’s a misconception. Even California’s PRC 4291 for clearance of vegetation around structures mandates that the specific condition of the structure, and surrounding vegetation, be considered in determining mitigation needed for that specific structure. The instructor’s position was that if you don’t have the conditions for ignition, then you are okay.

Limbs Over Roof?

Limbs over a roof are another example. If the roof is fire resistant Class A, B or C then the risk is minimal and a blanket prescription for removal of all limbs above a structure may not make sense. What’s wrong with branches over your roof? Nothing, if they don’t meet the criteria for ignition. If there is no fuel underneath the branches to sustain combustion, a fire passing through can’t ignite the structure.

Clear Combustible Debris!

In assessing the structure start at the top and go down to the ground. Roof material, vents, gutters, soffits, siding material, windows all play a role. Any debris on the ground under the siding or the roof eaves should be removed. Flammable trees or bushes under the eaves or against the siding should be removed as well.
Clearing flammable materials 5 feet back from the house and reducing the fuels 30 feet from the house will radically reduce the conditions necessary for combustion.  In an area with big flame potential such as dense forest or brush relatively nearby, then a hundred feet of fuel reduction may make more sense.
For the trees and vegetation located away from the structure, if you limb the trees up, ideally 6 to 10 feet, have no combustible litter, and there is no fuel to ignite the vegetation, you will be in good shape. Greening around structures also eliminates combustion potential.
Better than greening even, are patios with concrete, brick or tile that simply can’t burn when kept clear of combustible debris. Avoid combustible decks around the home of any kind. The higher they are, particular with vegetation under them, the worse off you’ll be. A wood deck is organized kindling waiting to ignite and take your home with it.

Requirements for Combustion Vary

The requirements for combustion vary depending on the fuel. In terms of HIZ assessment, a home is nothing more than fuel for fire. If the home has a shake roof, has pine needles on the roof, and in the gutters, the gutters are plastic, has older outdated non-reinforced vinyl windows, vinyl screens, combustible vegetation and litter nearby, then the home meets the requirements for combustion and should be mitigated.
A fire resistant roof does not meet combustion conditions. Even with pine needles in the gutters a proper fire-resistant roof is at minimal risk of combustion. If the gutters are vinyl or plastic however, they can ignite exacerbating the situation.
Without the right roof, with spaces in the roofing material, or broken shingles, no metal drip cap at the edges, then needles in the gutters can ignite the roof even with metal gutters. Of course you remove needles from the gutters in any case. But the risk they pose varies depending on the roof and gutter material.

Common Sense

The point taught was that conditions vary widely, and common sense is needed to determine the highest priorities for mitigation.
Wildland houses in the home ignition zone can be built to remove the conditions for combustion. With the correct chimney, roof, siding, window frames, whether single or double pane (though double is required for new construction), the conditions of combustion will not be met.
Studies and real world observation have shown that radiant heat through glass in a large fire is not enough sustained heat to shatter the glass as long as you have wood or metal frames. If you have older non-reinforced vinyl frames, they can melt away opening the structure to heat and flames, and again, embers. Newer vinyl windows have been shown to withstand wildfire better. http://ucanr.edu/sites/Wildfire/Side_of_House/Windows/
The sames applies to window screens. Vinyl window screens will melt and allow the entry of embers potentially igniting the home.

IBHS Research Center

IBHS Research Center ember storm highlights. See how embers can insidiously penetrate your home and set it on fire on the inside.

So doing an effective assessment of the HIZ depends on little details to keep embers out and stop ignitions.
Nylon window screens instead of aluminum are at risk. Older vinyl window frames can melt exposing the interior. Plastic gutters can melt away igniting the eaves. Gable and eave vents with too large vent holes allow embers in as shown in the ember storm video. Even 1/8 inch screening dramatically allowed embers through to the attic space.
Firebrands or embers can ignite any combustible material. Most structure home losses in large fires are caused by firebrands landing inside the home. Even homes in otherwise good shape are lost due to simple things like gable vents allowing embers in, bam! the home burns from the inside out. Use common sense with simple mitigation efforts and greatly increase your home’s chances of survival.

California & Australia Tops

Interesting and somewhat alarming is the fact that California is number one for fire fatalities, with Australia number two. Surprisingly, we concentrate on the foothills and mountain areas a lot, but there are places that are more urban that need focus as well. Don’t let your neighbors grassfire become your house fire is one saying.

Some Interesting Related Resources

Jack Cohen Articles on the Wildland Urban Interface Problem


Dan Bailey on “WUI Wars”


Examining Home Destruction in the Wildland Urban Interface DVD

You can order a copy. The Cascadel Ranch Fire Safe Council will be holding screenings.


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