Mitigation Techniques

Recent wildfire experiments and investigations have shown that buildings are much more resistant to heat transfer and ignition from wildfires than has been expected and perceived. Even relatively minor construction and retrofit techniques have demonstrated their ability to significantly increase the ignition resistance of buildings. There is no longer any doubt that planning, design, proper placement and the use of selected building materials effectively reduces the count of houses burned in wildfires.

The desired condition in these situations is to eliminate the combustible areas of the structure and to manipulate vegetation in the HIZ to reduce the possibility of high heat or flames reaching buildings. Flame contact to houses is an extremely critical situation but can be prevented by the judicious use of building materials and vegetative fuel arrangement within the HIZ. Houses that survive severe wildfire indicate that when the probability of ignitions near structures is reduced; the wildfire passes, but firebrands do not ignite roofs and flammable vegetation, embers do not enter structural openings causing internal ignitions; flames do not burn vegetative or man-made fuels and contact structures, and homes don’t ignite and burn!

The most effective wildfire mitigation construction and retrofit solutions are not necessarily found in building code and fire performance materials testing applications**. Relying on the ‘ignition performance’ of fuels is a preferred method of reducing structural ignitions rather than fire performance materials testing (which are designed for structural fire performance rather than wildfire performance and conditions). Fire performance testing is based on actual flame contact, and that is the situation that must be avoided in preventing WU structures from burning when protection forces are not available to assist.

Unlike mitigating a structure from a high-velocity sustained wind event, WU fire mitigation offers more options than the application of construction standards and codes alone. By reducing the flammable fuels that exist in the HIZ, the need for fire-resistive materials and building design can be reduced. For example, rather than retrofitting wooden siding on a building, reducing the adjacent flammable vegetation in the HIZ may be equally effective in eliminating flame contact to the building, achieving the desired result, and being less costly to the owner.

Wildfire Mitigation Construction Recommendations for WU Homes

Locating Buildings in the HIZ

- Select a site away from heavily vegetated areas or create buffers that create space between the structure and thick, flammable fuels.

- Locate structures near natural areas that reduce fire behavior, like rock out croppings, wetlands and stands of deciduous trees.

- Locate structures a minimum of 30 feet back from slopes; increase the distance if the structure will exceed one story in height.

- Avoid locating structures near dangerous topographical features including:

• Valleys and canyons that channel local winds.

• Saddles between peaks on a ridgeline where fire behavior is intensified.

• Ridge tops above canyons that create areas of fast moving fire.

- Maintain a 30 foot separation between primary and secondary structures.

Construction in the HIZ

- Install only burning brand exposure rated (Class A, B or C) roof assemblies using materials such as asphalt shingles, slate or clay tile, or metal roof coverings.

- Ensure roof-covering assemblies do not contain openings that allow firebrand entry.

- Install fire-resistive or noncombustible construction materials on siding and walls.

- Construct eaves that extend beyond exterior walls with fire-resistive materials.

- Cover all vent openings to internal structure areas (attic, eave/soffit, foundation, etc.) with a corrosion-resistant, non-combustible wire mesh or screen that prevents firebrands from entering structures.

- Provide concrete block, cement walls, or other fire-resistive materials for foundation walls and mobile home skirting.

- Construct overhanging projections (decks, balconies, unenclosed roofs, etc.) of heavy timber, non-combustible or ignition resistant material.

- Minimize the size and number of windows on the downhill side of the house or the side most likely to be exposed to wildfire. Both the size and the type of materials used are crucial in windows and sliding-glass doors. Multi-paned glass or tempered glass is recommended.

- Do not locate decks at the top of a hill. A fire’s spread is generally up hill. Enclose the undersides of balconies and decks with fire-resistive screening so that firebrands and fine fuels cannot accumulate under these extensions.

- Cover chimneys and stove pipes with a nonflammable screen (mesh openings approximately 1/4 inch).

Fuels in the HIZ

- Flammable native and landscape vegetation should be trimmed and made fire-resistant or removed from the HIZ.

- Flammable vegetation and combustible debris within 30 ft. of structures should be removed.

- Ladder fuels, that will spread a surface fire into tree canopies should be removed.

- Flammable tree crowns should be spaced from other aerial fuels to prevent their ignition.

- Vegetation must be regularly trimmed and maintained to prevent the buildup of hazardous fuels.