written 23 December 2018
published 29 December 2018
In early December, Cal-Fire and the Mendocino Fire-Safe Council presented a seminar at the Mendocino College, sharing the following information.
A fire front can pass over a building in 15 minutes leaving it intact, but it is still vulnerable to four different paths of ignition. Burning material can land on the roof, igniting flammable roofing. Flames from trees, shrubs or adjacent buildings, can make contact with flammable house siding. Prolonged exposure to radiant heat from burning material will ignite flammable material, even without direct flame contact. The most widespread impact is from burning embers, small pieces of flaming material lofted into the air, spreading fire far in advance of the actual fire front, which can swirl into nooks and crevices in any building they encounter.
Exposure to these four fire paths can be reduced by selecting proper construction materials and preparing the area around the house. A Class A roof is the most fire-resistant rating, and defines materials that are not flammable, such as concrete or clay roof tiles, fire rated fiberglass asphalt shingles, and metal roofing. These same kinds of materials can be used for siding. Consideration of radiant heat exposure is necessary, as metal and concrete can transmit heat, igniting the underlying structure, even though the siding will not burn.
Thermopane windows are more fire resistant than single pane. Open windows or skylights provides a path for fire to enter the house. Close windows and remove sheer curtains during a fire, but heavier drapes can reduce radiant heat transfer into the room.
Protection against embers is probably the cheapest change to make, but requires extensive consideration. High winds can drive a fire, and the fires can then generate more winds. The resulting turbulence blows embers around, before, during, and after the front of the fire has passed. One of the most vulnerable places on a house are the attic vents, which allow embers access to combustible insulation and very dry framing material. Most vents have 1/4" mesh, if any, but new fire rules require 1/8" mesh. This simple replacement can save your house.
However, embers can also blow into cracks between rafter tails and the roof decking, between exposed ends of roof tiles, or into the leaf debris in the roof gutters. Proper design, caulking, and regular gutter cleaning is worth the effort.
A wooden fence attached to the house is another ignition point. Dry debris collecting at the base of the fence can be ignited by blowing embers, which then ignites the fence and then the house siding. Combustible material such as firewood or construction lumber should not be stored under a deck.
Landscape modification of eliminating trees and shrubs within 10' of the house can reduce flame contact and radiant heat exposure. Removing lower tree branches can prevent ground fires from moving into the upper part of the trees. Remove any limbs extending over the house.
The Cal Fire defensible space suggestions of 30' clearance around the house and 100' brush reduction, don't apply in suburban settings, where the fence line might be 10' from the house, and the neighbor's house is another 10' beyond that. Small changes in landscape and construction can make a difference, but a community scale level of defense is required as well.
In 2004, a shaded fuel break was created along part of the western edge of the Ukiah city limits. This area is 100'-200' wide, with ground brush and litter removed, trees thinned to prevent crown to crown fire transmission, and the lower limbs of the remaining trees removed to prevent fire from spreading up from the ground. This park like environment allows access for fire equipment, and the reduction of fuel makes defending the area easier. The break has not been maintained, but this year the City and County have committed to restoring this break, widening it in places, and extending it from the Boonville road to Hensley Creek. In the recent Paradise fire, an existing shaded break made it possible to prevent the fire from expanding north.
Preparation can increase the odds of saving our homes in the kinds of fires we are now experiencing.
