The heat generated by a fire rises along slopes and accelerates the movement of fire toward higher ground, thus making houses located on or at the top of steep slopes the most vulnerable (Figure 3—48). Houses located on high points but set back from the slope are less exposed because they are not in the direct path of rising heat. The first step in reducing fire hazard is to determine likely paths for the movement of wildfire toward the site. Sustainable residential sites located in these regions should be designed to minimize the potential threat of fire damage. Water can be stored in an above-ground tank or a cistern below the ground (also see “Conserve Water” in this chapter). Another factor that should be studied is the type and density of vegetation that surrounds a site. A swimming pool is another source of water for firefighting and thus a good reason to have one in the landscape beyond the obvious recreational benefits. Off-site areas that have this type of vegetation pose a threat to a residential site and should factor in when designing the site for wildfire protection. and off-site vegetation. ZDKE3 ’
Zones 1 and 2 should prevent a fire from moving from the landscape to the house. Three subzones are recommended within the defensible space (Figure 3-50). Zone 1 should extend 5 to 10 feet from the exterior wall of the house and should be treated as a break between the house and the surrounding landscape. Each year, thousands of acres of land are burned and millions of dollars in property damage result from wildfires. zone г.’ •’
Three subzones within the defensible space for fire. The size and makeup of the defensible space should be based on topography, wind direction,
A house should be located back from the edge of a slope to be out of a fire’s path moving uphill. Thickly massed living vegetation along with the remains of dead vegetation that has accumulated over years of time provides a rich fuel source for a fire and a continuous path of movement. This zone may contain only native plants as long as they are thinned to prevent fire from moving between tree canopies or from the ground upward via a continuous vegetation mass. ground and the treetops. Zone 1 should be the wettest area of the site and thus irrigated if necessary (also see “Conserve Water”). Figure 3-47
Cross-section through a rain garden. Again, the
Recommended sizes of a defensible space for fire vary with steepness of a site. need for irrigation is greatest near the house and less so as distance from the house increases. The general rule of thumb is that the defensible space should extend 150 feet in all directions from the house on a site that is relatively flat or sloped up to a 20 percent gradient (Figure 3-49). As the steepness of slope increases, so should the size of the defensible space, especially in the downhill direction from the house. Keeping plant materials watered and healthy minimizes their ability to catch fire. Dry plants, of course, readily act as tinder for a fire and promote its spread through the landscape. Wildfires have a tendency to move with the prevailing summer wind and uphill from a valley or canyon bottom to the tops of slopes and ridges.