Protecting Residences From Wildfires

Structural Design and Construction

Because of the behavior of wildland fires, how a building is designed and constructed is the most important factor in providing fire safety for a home or other structure. Properly built homes can survive conflagrations even if many of the other protective measures discussed in this report are absent. Even homes apparently well separated from wildland vegetation will be destroyed if poorly designed and if constructed without regard to fire safety. The architect and building contractor, therefore, are key figures in providing safety. Many others (e.g., legislative and governing bodies, building inspectors, financers, insurers, buyers) need to be knowledgeable and be able to exert pressure that will guarantee adherence to fire safe practices in design and construction. 3i,5


The roof is the most vulnerable part of a building during a fire- especially one in chaparral or oak areas. Because of its horizontal component, a roof can catch and hold the flying firebrands almost invariably associated with the strong winds and convection columns characterizing these fires. Unlike ground fire, these firebrands soar beyond any type of firebreak, natural or artificial, and thus endanger structures as far as a mile away from the wildfire (Alger 1971, Los Angeles County Fire Dep. 1970, Howard and others 1973, Lowden and Degenkolb 1972, Smaus 1978b, Wilson 1962).

Recognizing this vulnerability to fire from external sources, the Uniform Building Code requires `fire retardant roof coverings' in Fire Zones I and 11, the high value and high life hazard areas in or near the business sections of cities. Many local jurisdictions have adopted the UBC by reference or by basing their own code on it. A few local jurisdictions have amended the UBC or their own codes to require "Class C" or better roofing as defined in UBC Standard 32-7 in wildfire hazardous areas. Most wildland areas still have no requirements regarding roofing materials (Build. News, Inc. 1977, San Bernardino County Bd. Sup. 1977, Los Angeles County 1937, Intl. Conf. Build. Off. 1976, Holmes 1971).

Most structures at or near the urban/wildland interface are either not covered by a building code, are in Fire Zone 111, or are permitted to have any type of roofing material, or at most, Class C roofing. In the past 30 or 40 years, wood shingles or shakes have become popular with architects and buyers alike. Various fire-retardant treatments have been available for about 20 years, but only in the past 10 years or so have any of them been made relatively permanent (i.e., will retain a significant degree of fire-retardancy for 5 years or more). Although shakes and shingles with Class C rating are available, none meet the Class B requirement for "fire-retardant" roofs as defined in UBC Standard 32-7. Treated shakes or shingles cost more than untreated, and in the absence of a local code requiring them are seldom installed. Thousands of homes and other buildings exposed to the threat of wildland fires, therefore, are roofed either with untreated shakes or shingles, with ones which were merely dipped in fire-retardant chemical, or with ones from which the treatment has been leached by the weather. These roofs are not only serious hazards to the buildings on which they are installed but also to any other buildings downwind from them which are similarly roofed. Once a shake or shingle roof catches on fire, shakes or shingles peel off and are carried as new firebrands on the convection column and the wind. 6

Many types of firesafe roofing materials are available. Some are less expensive than wood shakes or shingles, others more costly. They include Class A and B builtup assemblies, Class A and B prepared roofing, properly installed Class C mineral surfaced asphalt shingles, asbestos cement shingles or sheets, concrete slabs, metal, slate shingles, fiber glass shingles, and clay or concrete tile. Although most of these materials are not currently popular for residences, many can be made quite attractive- especially if the rest of the building is designed to accommodate them (Oreg. St. Dep. For. 1978a, Intl. Conf. Build. Off. 1976, Smaus 1978a).

The probability that a house of a given roof type and with brush clearance will be burned can be estimated from records compiled by the Los Angeles City Fire Department for the 1961 Bel Air Fire, in southern California. These records cover a sampling of 1,850 homes. For the probabilities shown it is assumed that houses are exposed to the rate of wildfire destruction observed in the Bel Air Fire. Values have been interpolated to match the brush clearance categories of insurance industry (Howard and others 1973):

  Brush                  Roof type 
clearance     Approved by        Unapproved by 
(ft):      insurance industry  insurance industry 

  0 to  30      0.243                0.495
 30 to  60       .054                 .286
 60 to 100       .016                 .144
100+             .007                 .148
The most cost-effective means of protecting homes from destruction by fire in or near the wildlands is a combination of approved fire-resistive roofing and clearance of 100 feet or more from the native brush for each home (fig. 6).

Proposed Standards: Base roofing materials required on the following fire hazard severity classification: moderate-Class C, high-Class B, extreme-Class A.


Another Achilles' heel to the attack of homes by windborne firebrands is an unprotected attic or underfloor vent. Although unprotected vents are not as well-documented as a cause of structure fires as flammable roofs, flying embers can easily enter a structure through such vents. If they land on any ignitable material, the inaccessibility of the interior to suppression efforts almost certainly will lead to destruction of the house. The presence of flammable materials (e.g., dry leaves or grass, waste paper) is almost assured by the same mechanism-strong wind-that brings the firebrand. To remove this hazard, vents can be screened to prevent the entrance of flammable materials and firebrands but still allow the passage of air.
3k 7

Proposed Standards: Use vent screens of corrosion resistant wire mesh with a mesh size of one-quarter inch.

Overhangs and Stilt Construction

Two other common architectural practices present serious fire hazards to a home built in or near the wild lands: (a) overhanging or projecting members (e.g., eaves, balconies, raised sun decks), which are likely to be found anywhere and are always dangerous; and (b) stilt construction. The latter, although dangerous anywhere, is particularly so on sidehill sites because the uphill side forms a trap for heat and flames. The danger, directly caused by vegetative fuels being under the building, is then aggravated by the wind accompanying conflagrations (County Sup. Assoc. Calif. 1966, Colo. St. For. Serv. 1977, Wilson, 1962).

Structures with overhangs or stilt construction, or both, are usually ignited by flames sweeping against the underside of the projection or the building itself. A wildfire running uphill ahead of a strong wind through heavy brush or timber to within a few feet of a house built on stilts and with a cantilever balcony and 4- or 5-foot eaves is a positive prescription for disaster. This sequence is not as uncommon as it might sound. Hillside homes often offer spectacular views, and balconies are often provided to take full advantage of the view. Wide eaves are commonly built to shade windows. On a hillside, one side of a house may be at ground level or below while the other side is 15 or 20 feet, or possibly more, above ground level (Deeming and others 1977, Helm and others 1973, Smaus 1978b, Wilson 1962).

These architectural fire hazards can be overcome by a number of measures. One is to construct any overhanging member or building of nonflammable materials (e. g., metal or concrete) of at least 1-hour or 2-hour fire resistive materials as defined in the UBC. Another is to encase stilts so that there is no opening below the floor of the building, balcony or sun deck. Roofs can be built without eaves. Sidehill houses can be designed with two or three floor levels, thus utilizing what otherwise would be not only a fire trap but wasted space.3l 4b

Proposed Standards:

  1. Construct eaves, cantilever balconies, and other similar overhangs with undersides of materials that meet the following UBC specifications, depending on fire hazard severity class: moderate-fire-retardant pressure treated wood or equivalent; high- 1 hour; extreme-2 hour.
  2. For structures supported wholely or in part on stilts, encase all underfloor areas to the groundline with materials meeting the standards for exterior vertical walls in the same area or zone.


Windows can easily be a weak point in the fire protection of a home for two reasons. They allow entrance of radiated heat of such intensity that interior materials (e.g., furniture, drapery, clothing, papers) are ignited. And they admit convective heat, firebrands, or flames when they are open or broken. Large picture windows and sliding glass doors are particularly vulnerable to these hazards. The orientation of the glass surface will determine the degree of hazard it represents. If it is on the windward side of the building or is facing toward a concentration of vegetative fuel, the danger is heightened. If it is downwind or shielded by some nonflammable obstruction, the danger will be reduced (Alger 1971; Los Angeles County Fire Dep. 1970; Lowden and Degenkolb 1972; Smaus 1978a, 1978b; Wilson 1962).

Windows cannot be abandoned or prohibited. The opportunity to enjoy a spectacular view or the feeling of spaciousness afforded by a sliding glass door opening onto a patio, sun deck, or swimming pool is hard to give up. But the danger of fire can be reduced by installing nonflammable shutters or fire-retardant drapes; by orientating away from concentrations of vegetative fuels; by shielding with nonflammable balconies or decks; and by using tempered or double-paned windows or both, or wire glass (Orange County Bd. Sup. 1976; Oreg. St. Dep. For. 1978a; Task Force on California's Fire Probl. 1972; Lowden and Degenkolb 1972; Smaus 1978a, 1978 b).

Proposed Standards:

  1. Hold to a minimum the size and number of glazed openings on the side of the house facing the normal fire carrying wind or the downhill side or both.
  2. Use extra-strength glass (thick, safety, tempered, or double-paned, or both).
  3. Protect windows and sliding glass doors with nonflammable shutters, balconies or decks, and fire-resistant drapes.
  4. Orient any glazed openings so that they do not face concentrations of vegetative fuels within 100 feet, unless such openings are provided with fireproof shutters.


Most home fires in urban areas have internal sources of ignition. Therefore, the materials of which their exterior walls are constructed are of relatively minor importance from a fire protection standpoint. The architect, therefore, has considerable latitude in design. By contrast, home fire ignitions during a wildfire are almost entirely external (assuming all openings are protected as discussed earlier). Thus fire-resistance of exterior walls becomes of great importance, and the choice of materials must be restricted. Contrary to this safety requirement is the strong tendency on the part of both architects and buyers toward an increasingly rustic appearance as the site gets deeper into the chaparral or woods. As a result many structural fires in rural areas have started by direct ignition of external wood siding, garage doors, and porches. This hazard is heightened if concentrations of fuels (e.g., vegetation, firewood, wood fences) are too close to the wood-encased house (Alger 1971, Los Angeles County Fire Dep. 1970).

Many materials are available for exterior wall use that have varying degrees of fire-retardance up to 2-hour ratings or even higher. Some of these can be made to appear quite rustic. Others have unique visual qualities of their own. They vary in ascending order of fire safety from wood siding or panels pressure-treated with fire-retardant chemicals (same leaching problems as for shingles and shakes) through stucco, metal siding, brick, asbestos-cement shingles or corrugated panels to concrete block or poured concrete and rock walls. To achieve full fire-resistant effect, all but the solid concrete or rock walls must be properly applied over suitable base materials, as defined in the UBC, continuously from the ground or foundation to the roof or eaves (Orange County Bd. Sup. 1976, Oreg. St. Dep. For. 1978a, County Sup. Assoc. Calif. 1966, Intl. Conf. Build. Off. 1976, Smaus 1978a).

Proposed Standards:

  1. Build exterior walls of such materials as to provide the following degrees of fire resistance based on fire hazard severity class; moderate-fire-retardant pressure-treated wood or equivalent; high-l-hour, extreme-2-hour.
  2. Extend the applicable degree of fire-resistance of all exterior walls from ground level to roof line.

External Sprinklers

Sprinklers permanently mounted on the roof have been suggested as a defensive measure, particularly in order to offset the hazard of wood shingles or shakes, either treated or untreated. Such sprinklers could be either automatic, similar to internal fire sprinklers, or activated manually by the occupant. However, most authorities argue against the value and advisability of sprinklers as a viable alternative to safe roofing materials. Sprinklers could, theoretically, prevent roof ignition from flying firebrands, but rather large volumes of water would be required at a time when loss of water pressure can normally be expected. Also, if activated too soon, a sprinkler system will contribute to the loss of water pressure and very likely cease to function before the actual danger arrives. In this case its benefits would be almost totally lost by evaporation, and it does not appear to be an adequate substitute for approved fire-resistant roofing material
3m (Alger 1971, Los Angeles City-County Fire Bd. Inquiry 1971, Smaus 1978b, Wilson 1962).

Proposed Standards: Allow permanently installed roof sprinklers only where the structure has its own independent water source that will not deplete the supply of water for nearby properties nor for use by organized firefighting manpower and equipment. Such systems, when installed, should have a dependable water source (gravity or alternate power).

Safety of Human Life

Of even greater importance than protecting homes from destruction by wildland fire is avoiding the loss of human life. No structure should be designed and built in any way that could trap people inside while it is burning. Occupants should have ample warning and positive means of exit (Governor's Study Comm. 1966).

Proposed Standards:

  1. Install in each dwelling unit one or more smoke detectors-at least one of which shall be in or near sleeping areas.
  2. Install in each dwelling unit at least two door exits by means of which ground level may be reached.
  3. Install in each room, especially each bedroom, two means of exit-one of which must be to the outside of the building.
  4. If a window provides one of the exits it must be no more than four feet above floor level and capable of providing an opening no less than 22 inches in least dimension and 5 square feet in area.

Mobile Homes

Mobile homes have grown remarkably in numbers in rural areas in the past 10 years, both in mobile home parks and as substitutes or replacements for detached single family residences. They are subject to all the fire problems and vulnerabilities discussed earlier for conventionally constructed houses and a few of their own. Insofar as possible mobile homes should conform to the same recommendations and standards as conventional homes in the interests of fire safety for both the building and its occupants (Los Angeles County Fire Dep. 1970, Oreg. St. Dep. of For. 1978a).

Mobile homes usually stand a reasonable chance of avoiding roof fires from windborne firebrands because most of them have insulated metal roofs. On the other hand, since the metal involved is usually fairly thin aluminum, heavy long-burning firebrands can melt through the roof and fall inside where they may ignite furnishings.

Of greater concern and probability is the danger of fire coming under the structure and igniting the floor. The best way to avoid this is to provide complete skirting, preferably metal, from floor-line to ground-line all the way around the home. Such skirting requires proper venting which should meet the same mesh or opening standards as for conventional homes (Oreg. St. Dep. For. 1978a).

Other fire problems common to mobile homes are unprotected sliding glass doors and picture windows, wide carports and porch canopies attached to the main structure, and carpeted or exposed wood porches or sun decks. Large panes of glass need some kind of protection from breakage which would allow an external fire to enter the structure. Any overhang will trap and intensify heat and flames. Flammable horizontal surfaces will collect windborne firebrands.

Probably the greatest fire safety problem of mobile homes (and this also applies to all types of recreational vehicles) is the life hazard. The fatality rate for this type of structure per 100,000 fires is several times that for conventional homes. There are three causes for such a high fatality rate: (a) the flash nature of interior fire spread often encountered in these units; (b) highly toxic and very thick smoke and gasses are given off by the many plastics commonly used in the interior; (c) the generally inadequate exit or escape capability of mobile homes. Mobile homes rarely have more than two exterior doors. Most of their windows or the individual panes, except for a picture window in the living room, are too small and too high off the floor to allow a person to climb out.

Proposed Standards: Require mobile homes to meet the same standards of fire safety as conventionally built houses, and, in addition, be provided with (a) full skirting; (b) a means of quickly enclosing, or detaching, carports and porch canopies during fire emergency situations; and (c) nonflammable, or at least fire-retardant, porches and sun decks, if any such areas are provided at all (carpeting of such areas should be prohibited).

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