Probably the best approach to fire hazard classification or zoning from the standpoints of both technically
correct theory and practical application by local planning department staffs is that developed in 1973 by the
California Department of Forestry. It rates fire hazard
based on three weighted values each of fuels, weather,
and topography (table 2).
In light of more recent information (Deeming and others 1977) the limits and factor weights may be revised, and whether a "low" or "base level" fire hazard classification should be recognized at the lowest levels of all factors in the table should be explored. Still the factors used and the approach to combining and analyzing them are sound. Technically it might be more correct to use up to 20 fuel types and five slope classes as recognized by the National Fire Danger Rating System (Deeming and others 1977) but this would unnecessarily complicate analysis and mapping. In any event, this system or a modification of it can. identify those areas which present varying degrees of fire hazard, including those so hazardous that no construction or development should be allowed. It can also indicate which areas can be made reasonably safe through the application of various levels of design, construction, and maintenance standards (Orange County Bd. Sup. 1976, Task Force on California's Wildland Fire Probl. 1972, San Bernardino County Bd. Sup. 1974, Helm and others 1973).
Any such system should provide for raising to the next higher level of hazard those limited areas that are in saddles above canyons on the windward side of ridges or at the mouths of canyons on the lee side of main ridges (Los Angeles County Fire Dep. 1970).
Proposed Standards: Establish fire hazard severity zones on the basis of a graded classification system using fuel loading, fire weather, and slope as primary factors. Recognize and map at least two, but preferably three, levels of hazard. Provide for classification in the next higher category of especially hazardous situations, such as saddles on ridges above windward canyons and mouths of lee canyons.
Proposed Standards: Require that fire protection elements of general and specific plans cover both basic structural protection and protection from wildland fire, that protective measures enumerated in these plans correspond to the level of fire hazard severity found to exist in the area covered by each plan, and that the fire department participate in reviewing plans.
In California, general plans including safety elements are mandated to counties and cities by State law (California Government Code Sect. 65300 et seq.). The standards set by the State to which these plans and elements must be prepared, especially those having to do with wildland fire, have been very weak. Consequently, an almost total lack of uniformity exists from one local jurisdiction to another. The governmental units that are larger, most heavily damaged in the past, and more progressive have adopted enabling ordinances containing rather comprehensive requirements relating to fire safety. In contrast, some of the smaller units and those that have not perceived or understood the severity of the problem have treated it in one sentence or one paragraph (Task Force on California's Wildland Fire Probl. 1972).
One important element of the General Plan is the Land-Use Element. It establishes the areas that can be devoted to different uses (e.g., residential, commercial, manufacturing, agriculture, open space). This element has great potential for enhancing structural fire safety in wildland areas if wildland fire hazard severity classes are accounted for during its development. So far this potential has rarely been considered, and no case has been found where the Land-Use Element of the General Plan was actually implemented in this manner. 3
The Uniform Building Code (UBC) published by the International Conference of Building Officials has been adopted by many counties and cities. Others have no such code or ordinance. A major component of the UBC is the establishment of Fire Zones 1, 11, 111. These zones set standards by requiring decreasing degrees of fire safety to be built into structures of various types of occupancy and prohibit certain occupancies in Zones I and II. Zone 111 generally encompasses residential areas having the lowest (base level) fire danger and requires minimal fire safety construction. In all zones, the standards are designed to provide protection from fires starting within the building or in another one nearby (3 to 20 feet). The problem of fires approaching through large expanses of vegetation is not addressed anywhere in the UBC (Task Force on California's Wildland Fire Probl. 1972, Int. Conf. Build. Off. 1976).
A few jurisdictions that have adopted the UBC by reference in an ordinance have amended it in various ways to attempt to cover the wildland fire problem. Some have added fire zones and incorporated certain restrictions and requirements for structures located in them. Others have declared fire hazardous wildland areas to be in Fire Zone II. In most cases the only effective remedial measure is the requirement of fire-resistive roofing (Build. News, Inc. 1977, County Sup. Assoc. Calif. 1966, San Bernardino County Bd. Sup. 1974).
Proposed Standards: Recognize varying degrees of wildland fire hazard in local ordinances. Establish construction and spacing standards commensurate with each degree, and make such standards applicable to residences and their appurtenant structures as well as to other types of occupancy.
The Uniform Building Code (UBC), Uniform Fire Code (UFC), and most subdivision codes are based on two assumptions: an internal ignition and a fire department response time of less than 15 minutes. These are relatively safe assumptions in most cities and some fire districts, but are valid only in very limited areas where fire protection is provided by a county, State, or Federal agency. They are almost totally irrelevant in most mountain and wildland areas where only seasonal resource fire protection is usually available (Alger 1971).
In addition to being subject to external ignition and up to 2-hour response times, structures built on sloping ground are affected by the same fire behavior phenomenon discussed earlier in relation to the wildland fuel. The slope creates an effect similar to that of wind and causes fire to spread faster uphill than downhill or on the level. Buildings situated on slopes should be required, therefore, to be spaced farther apart than similar buildings built on essentially level ground. On excessively steep slopes (more than 55 percent), all wood frame construction probably should be prohibited 3a(Los Angeles County Fire Dep. 1970, Oreg. St. Dep. For. 1978b, County Sup. Assoc. Calif. 1966, San Bernadino County Bd. Sup. 1974, Helm and others 1973).
In recent years a new concept in residential development has been adopted in some areas. This is the cluster development in which the population density limitations of general land-use plans are met by placing individual homes or multifamily residences (e.g., duplexes, apartments, condominiums) in small groups on smaller lots than would otherwise be required. The land area thus saved is reserved for community open space. This practice is esthetically pleasing to most people, and can also improve the fire safety in rural or wildland areas as long as certain minimum spacing standards are followed. The space between-buildings, for instance, should not be less than half that required between residences in conventional subdivisions in similar areas. And the native vegetation should be modified at least to fuelbreak standards of width and hazard reduction in the spaces between clusters (Task Force on California's Wildland Fire Probl. 1972, Colo. St. For. Serv. 1977).
Because of the close spacing commonly employed in mobile home parks, those situated in wildland fire hazardous areas are particularly susceptible to destruction or serious damage from conflagrations. Required spacing between mobile homes in such parks should be no less than that allowed between buildings in a cluster development in a similar fire hazard classification zone (Los Angeles County Fire Dep. 1970).
Most of the references on structure density and spacing base their recommendations primarily on slope. Only rarely are the other primary factors contributing to wildland conflagrations (i. e., fuels and weather) mentioned, mostly in work done in the late 1970's. The most logical and defensible standard on which to base structure density and spacing requirements is fire hazard severity classification and mapping (Colo. St. For. Serv. 1977, Helm and others 1973).
Proposed Standards: Establish minimum standards of building spacing and density, as shown below. Provide for the imposition of higher standards or the prohibition of building where local conditions (e.g., excessively steep slopes, ridge saddles, canyon mouths) create critical fire hazards:
Individual Buildings Clusters Prop. Hazard Density Spacing line Between Between class: setback bldgs. clusters (feet) Moderate 3 per 60 30 40 100 acre High 1 per 80 40 40 150 acre Extreme 1 per 100 50 50 200 4-5 acresPrevious Section Back to Table of Contents Next Section