Roofing
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 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:
- 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.
- 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.
Siding
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:
- 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.
- Extend the applicable degree of fire-resistance of all
exterior walls from ground level to roof line.
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