According to a publication by the Washington State Department of Natural Resources, "Living with Fire, A Guide for the Homeowner,"
"Defensible space is the area between a house and an oncoming wildfire where the vegetation has been modified to reduce the wildfire threat and to provide an opportunity for firefighters to effectively defend the house. Sometimes, a defensible space is simply a homeowner's properly maintained backyard."
When evaluating the risk of wildfire, firefighting professionals look at three components:
1. Weather- Hot temperatures can lead to dry plants easily catching fire and wind can spread the fire quickly over a large area.
2. Topography- Steep slopes spreads fires more quickly than flat land.
3. Fuel- In the case of wildfires, fuel is living vegetation (trees, shrubs, grass, wildflowers) and dead plant material (dead trees, dried grass, fallen branches, pine needles, etc.). Houses can also add fuel with the materials such as untreated wood shakes and shingles.
The component that can be controlled the easiest is the fuel around the house.
The aim of defensible space is to change the vegetation around the home by:
• Increasing the moisture content of plants.
• Decreasing the amount of flammable vegetation.
• Shortening plant height.
• Altering the arrangement of plants.
In order to do this, firefighting professionals rely on the three R's:
1. Removal- Taking away trees and shrubs that are dead or are most volatile in a fire such as junipers.
2. Reduction- Removing dead wood from trees and shrubs, low tree branches, and mowing dead grass.
3. Replacement- Replacing flammable plants for plants that are not as flammable.
30 foot and 100 foot zones. Create two zones where you clear flammable material.
Zone 1- This zone is 30 feet from your house. In it you only have small amounts of flammable vegetation, no dead vegetation or flammable debris such as twigs, branches, dried grass. Plants in this zone are well maintained and irrigated. Also, be sure that your roof does not have dry debris such as needles or branches. Any tree branches near your house should be at least 10 feet away from your chimney.
Zone 2- This zone is between 30 and 100 feet from your house. Where you can thin trees so that they are at least 10 feet apart on flat land. Between a 20-40% slope, trees should be 20 feet apart. If there is a 40% or steeper slope, thin trees to at least 30 feet apart. Shrubs should be spaced double the size of the shrub on flat land, 4 times the size on 20-40% slopes and 6 times the size on 40% or greater slopes. For example, on flat land if shrubbery is 6 feet in diameter, keep them 12 feet apart.
Ladder effect. Fire tends to climb like up a ladder. So if shrubs that are next to trees catch fire, the fire will jump to the trees. Firefighters recommend that there be a vertical separation of three times the height of shrubs to the lower branches of adjacent trees. For example, if shrubbery next to trees is 3 feet tall, prune away the branches of the trees to 9 feet high.
Comment: Raising concerns about wildfire danger is highly unusual. Normally, we have wet Junes until July 4. Summer weather starts on July 5 and only lasts until the end of September. But, last year we saw a dry June, followed by a hot and dry June this year. Long range forecasts apparently say that this summer will continue to be hot and dry. At least one local fire official has said that forecasts predict another hot and dry summer for 2016 with relief taking as long a 2017 to arrive.
Our forest land on the west slopes of the Cascades have traditionally been moist enough to allow for lush forests and undergrowth. With this long term dry spell, homeowners and communities need to assess whether or not they need to re-adjust their landscaping to protect their homes. Some communities may have little that they can or need to do. Others may need to clean away flammable material to ensure that buildings are protected. If you have any questions about fire safety around your home, contact your local fire district.
Department of Natural Resources: