Article of the Month - April, 2008

Landscaping for Fire Prevention I: Home Ignition Zones and Defensible Space.  by Yvonne C. Barkley and Chris Schnepf

Towns and cities are spreading into the surrounding countryside as more and more people move out to the woods. If you live on forested property, or are considering building there, you are part of the growing wildland-urban interface – where the urban environment meets the wild.

In 2007, nearly every fire in the West involved structures, and many of those structures were homes on the wildland/urban interface. The season starts early in the west, with spring wildfires being common in New Mexico and Arizona until early July, when they can expect the summer monsoon season to begin. As the temperatures rise and the precipitation rates drop, the rest of the west begins to dry out. Fire season in Idaho usually gets going about the second week of July and can last well into October in the lower elevations. Statistics for the 2007 fire year report over 85,000 wildland fires with more than 9.3 million acres burned - that’s almost 14,000 square miles, an area four times the size of Yellowstone National Park. An estimated 5,200 building were destroyed, ranking second to the 2003 fire season, when 5,700 buildings were lost.

Things were seen last year that had never been seen before. Things happened that were not supposed to happen. Any many feel that last year’s fire season was not abnormal, but the new normal. It may be muddy outside right now, or you may even still have snow on the ground, but if you live in the Inland West it is never too soon to think about the next fire season. Residents of the forested areas of Idaho should view wildfire the same way residents of the Midwest view tornadoes – an expected natural phenomenon that can devastate property and life – and prepare accordingly.

The Home Ignition Zone. The most effective ways to increase the protection of your home from wildfire is to reduce the ignition potential of your home and surrounding landscape.

Home ignition zones are those areas of your home and outbuildings that are the most susceptible to ignition. There are three sources of ignition. Heat can be moved from one piece of fuel to another by conduction. Larger fuels must usually be in contact with, or close to each other for conduction to transfer enough heat for kindling to occur. A home with a well defined and well maintained defensible space will be nicely protected from surface fires that are moving via conduction.

A second type of ignition occurs when radiation contributes to the fire spread by preheating and drying small fuels to their ignition points. Radiation can occur with both surface and crown fires, and can ignite structures from up to 100 feet if conditions are extreme.

A third phenomenon, convection, preheats surface and vertical fuels, and contributes greatly to fire spread by transporting embers which can ignite spot fires ahead of the main burn. Most homes ignite by convection from burning brands and embers that fall onto the roof of a house, a deck, or an outbuilding. New research has shown that in many fires this is the number one source of home ignitions.

Your Home. In Figure 1 you will see three zones. Zone 1 includes your home. Steps have been taken to decrease and/or eliminate the ignition potential of your home. Particular attention is paid to nonflammable roofing materials, enclosing decks, soffits and overhangs, and removing debris from roofs and gutters.

Figure 1. Home Ignition Zones.

Zone 1.                                          Zone 2.                                 Zone 3.

If you have not built yet, try to place structures on flat ground. Fires burn uphill more rapidly than down or across a flat. Avoid draws, as they can serve as a chimney, creating more intense fires that spread rapidly with uphill drafts.

Use fire resistant materials to build or to replace flammable materials. Tile, metal or asphalt roofing materials and brick or stucco walls are the most fire resistant. Untreated or poorly treated wood shake shingles are the least fire resistant and in many cases, offer almost certain ignition of your home.

Enclose the undersides of decks and porches, eaves, soffits, and fascia with a nonflammable material - cement products work well. Screens should be constructed from noncombustible materials and vents should be screened to prevent firebrands or other flammable objects larger than ¼” from entering the space. Vents should also be boxed in with noncombustible materials to reduce their size. PVC and vinyl products should be avoided as they melt or burn at low temperatures.

Double pane windows and protective shutters or fire-resistant window treatments (blinds, drapes) will help insulate the inside of your house from the heat of a fire, lessening the chance of the interior becoming overheated and igniting.

Make sure your chimneys are screened and construct covers for all attic and basement openings. Pay attention to places where combustible materials meet each other – a vine covered wooden fence leading up to your wooden stairs can act like a trail of gunpowder leading to a barrel of explosives. Also remove shrubs and combustible fencing from around propane tanks and woodpiles. Park boats, campers, and other flammable recreational vehicles in an enclosed area or away from your home.

If possible, have power lines buried to protect them from fire and prevent them from igniting vegetation. If burying the lines is not possible contact your power company to make sure trees and branches are trimmed 10 to 15 feet away from lines.

Your Landscape. In Zone 2, approximately 100 feet around the home is surrounded by a greenbelt of well-watered and maintained plant materials. Perennials, groundcovers, and annuals are planted in groupings with individual trees and shrubs. These islands of vegetation are surrounded by rock or brick retaining walls and well-watered turf. This zone requires removing undergrowth and dry debris on the ground every three to five years and pruning trees. Firewood is stored on a gravel pad and is also surrounded by well-watered turf.

To prepare and maintain Zone 2:

• Remove highly flammable brush from around each home for at least 100 feet.

• Remove shrubs at the base of structures.

• Keep tree branches at least 15 feet away from chimneys and stovepipes, utility lines and roofs.

• Store firewood 30 to 100 feet from any structure and create a defensible space around the pile.

• Remove dead shrubs and trees.

• Thin shrubs growing 100 feet and further from each house into individual plants or remove them entirely.

• Eliminate “ladder fuel” configurations in vegetation (ladder fuel refers to the plant growth structured like the rungs of a ladder – leaves, grasses, small shrubs, large shrubs, and trees). Removing these “rungs” helps prevent destructive crown fires.

• Reduce the chance of surface fire climbing into tree crowns by pruning the base of the crown 6 to 15 feet from the ground.

• Reduce lateral movement of fire between crowns by cutting branches that span between crowns to 10 feet, or more, apart.

• Prune all dead branches.

Beyond 100 feet. Zone 3 (beyond 100 feet) is composed of native plants that are thinned. If possible, highly flammable vegetation is removed and replaced with less fire-prone species. However, you do not have to be as aggressive in reducing fuels in these areas as you would immediately next to your house. Leaving green branches or lopping them into small pieces will help recycle nutrients to your trees. Coarse woody debris (wood larger than 3 inches in diameter), has relatively low fire risk – leaving a small amount of it, laying flat on the ground benefits forest growth and wildlife. To avoid bark beetle problems, avoid cutting pines larger than 3 inches in diameter in the winter and spring (or destroy or peel the bark of stemwood pieces that size). For areas with steep slopes or windswept exposures greater defense distances will be needed.

Life in the wildland/urban interface is enjoyable, but not without danger. In western forest environments, wildfire should be anticipated and planned for, not unexpected and dismissed. You have a choice – if you take precautions to protect your property it will increase your chances of escaping serious damage from wildfire.

For additional information and informative figures and photos, email the UI Extension Forestry office and request a copy of Protecting and Landscaping Homes on the Wildland Urban Interface. by Yvonne C. Barkley, Chris Schnepf, and Jack Cohen. Station Bulletin No. 67, January, 2005. Idaho Forest, Wildlife and Range Experiment Station, Moscow, ID.