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 .148The 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: Use vent screens of corrosion resistant wire mesh with a mesh size of one-quarter inch.
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:
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:
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:
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).
Proposed Standards:
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).
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).
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).