Improving Your House’s Fire Safety
By Glenn Nader
Wildfire sees your home as just another fuel source. The defensible space you construct around your home will keep your house safe from all but the most disastrous wildfires. However, if the wildfire does break through your first line of defense, an ignition might occur on your home’s exterior. The kind of construction and kind of materials used along with annual maintenance can improve your home’s ability to survive fire.
Fire needs three elements to burn:
Heat (to preheat material to point it produces gases that combust.)
Believe it or not, wood does not actually burn. When it is preheated to a certain temperature, it gives off gases that combust. Fire can be exposed to your home in 3 different ways:
Embers that can move up to a mile ahead of the storm by being carried by wind and can enter unscreened vents and catch your house on fire. They can also catch combustible roofs, pine needles, and leaves on fire. Cracks in siding and connection points can also be a place for embers to start a fire. Embers larger than 1/8 are required to provide enough heat to preheat materials for combustion to occur.
Radiant heat is the heat given off by combustion by adjacent fuel sources. Radiant heat from the fire around your house can heat the surface of combustible building materials to a point that the volatile gases are given off and start combustion.
Flames touching the house (i.e. impingement) The contact of the fire flame will preheat the combustible building materials of your home. Depending on the extent (time and intensity) of the exposure and each materials boiling point at which they start to produce combustible gases will determine if your house catches on fire.
Studies in Australia (Ramsay) have characterized the three kinds of fire exposure to the homes as follows:
Pre fire front - Half hour of embers before the fire front encounters the home.
Fire front – embers, radiant heat and flame impingement for 5 – 10 minutes depending on the fuel load around the home. This is only 1-2 minutes in a grass fire.
Post fire front – embers and burning debris continue to be exposed to the home for 3.5 to 4 hours, depending on the wind, fire and fuel conditions. Many buildings are ignited and destroyed during this period and it is an important period for building survival.
This clearly indicates that embers are the biggest exposure to the home and should be the first thing that a home owner addresses to reduce the risk of fire.
Four concepts of reducing your home chances of burning:
- Reduce the amount of heat the structure will be exposed to through vegetation management and construction design.
- Limit the time the structure is exposed to heat through vegetation management and construction design.
- Use fire-resistant building materials.
- Check all construction for cracks or areas that embers can enter. Calk or fill those areas.
Existing Home Modification to Improve Fire Safety
Vents (Attic, Eave, Sub floor or Foundation)
Vents are required by the building code to prevent accumulation of water vapor that prevents wood from rotting. Anecdotal evidence from structure loss surveys after recent California wildfires indicates that use of 1/4-inch mesh size screens in vents has not been effective in eliminating entry of burning embers, but because of concern over moisture-related performance problems, building code officials are cautious about allowing 1/8-inch (or less) mesh screens. Smaller mesh sizes are more prone to plugging from airborne debris and paint-over from spray and/or brush. Firebrands of a size of lager than 1/8 inch moving through the vents can start the house on fire. The vent openings should be screened to prevent large firebrands entering the home with ¼ inch mesh made with corrosion resistant metal to help minimize required maintenance. Both your vents and screens should be constructed of non corrosive metal materials that will not burn or melt when exposed to radiate heat or fire- brands. Make sure that there is no vegetation within 10 feet of the vents. More vegetation clearance will be required near vents that face the downhill side of the house. Vent covers made of wood or steel can be constructed that can be placed over the vents at the onset of a fire in the area. This has the draw back that this must be remembered during all the other concerns during a fire. Eave vents are prohibited in some areas of California, as the flames are focused by the wall into the eave. Turbine vent should be one directional.
Building Material Fire Rating
Various national organizations provide ratings or evaluations for the fire resistivity of materials or building assemblies by testing them. A building assembly is a combination of materials forming a component of a building such as a roof or wall. The ratings are in the following categories:
Combustible or noncombustible
Classes: A (best), B, & C
Time: Twenty-minutes, one-hour, two-hour, and four-hour.
The difference between a noncombustible material and a rated material or assembly is the surface resistance to ignition versus the protection afforded the building behind it. A good example of a noncombustible material is metal roofing and siding. Metal is non-combustible, but an excellent conductor of heat. If the fire remains present long enough, the heat will be conducted through the metal and ignite the material behind it.
RoofDuring a wildfire, firebrands can fall on your roof and can catch leaves and needles on fire. Protect your home by keeping the roof clean and have a roof made of fire-resistant material. There are two components to the roof to that make the roof resistant to fire the surface material and the sub roof construction. There are three levels of classification awarded under the test protocol, A, B and C, with A being the most fire resistant.
Wood shingles are very susceptible to fire and are the major cause of home loss during wildfire. Homeowners with this type of roof have placed sprinklers on the roof to wet the wood before a fire. This solution has the flaw of the power may go out with a fire and without a backup generator, this prevention method may fail. Another prevention action is treated wood shake shingle products have ratings of Class C or better. Over time, the effectiveness of this chemical is reduced by weathering and may leave your roof unprotected. Replacing a roof is very expensive, but installing a Class A roof covering is the ultimate way to protect your home from fire.
Metal roof may not be combustible, but as pointed out earlier it conducts heat, so it is also important to keep leaves and needles off the roof during fire season. May sure that there are no cracks in the roof construction where embers can get under the roof and cause a fire.
Although tile roof may be noncombustible, house with tile roof have burned due to the lack of bird stops being placed at the end of the tile row. Ember can be driven under the tile and catch the home on fire. Even worse is birds can make nest there that provide fuel during a fire. Along with purchasing bird stops and check the roof for broken tile or any other spot that embers can get under the tile.
Asphalt shingle roof
Asphalt shingle roof loose there fire protection as they breakdown over time. Check your roof and make sure that the fibrous material is not exposed. Also check for cracks in construction that will allow an ember to get under the asphalt shingle and start a fire. As always make sure the roof is free of leaves and needles during fire season.
Fiber Cement shingles
These products are noncombustible, but require an underlayment for a class A rating.
Concrete or Slate shingles
Decks can represent a large threat to the home. Embers can land on the deck surface or under the deck and ignite. Also decks built on steep slope allow the flame to be directed under the deck and makes it very vulnerable to ignition. Some ways to minimize the risk of flames getting under the deck is:
Clear all flammable vegetation down slope (insert formula).
Enclose the underside of the deck with non combustible siding.
Consider placing vents for keeping dry if enclosed.
Place a cement wall down slope of the deck to direct the flame away from the deck.
Annually make sure the underside of the deck kept free of flammable material.
The deck material will impact the ability of an ember to ignite and start the deck and possibly the house on fire. Studies have shown that well maintained redwood decks with little or no cracks resists fire as well a solid composition decking. Hollow core decking is more susceptible to burning, as they do not have the mass or thickness to absorb the heat from embers or flame and will give combustible gases that ignite quicker. PVC decking material has a very little resistance to heat and will melt or ignite depending on the thickness. Wood railing also introduces additional fire risks. Consider replacing with steel railing. The best methods to minimize the risk of wood deck surface to fire are to treat the wood surface regularly to prevent cracking and decayed wood.
Both these condition increase the ignitibility of the wood surface. Inspect deck, and replace, repair decay damaged wood. Also constant sweeping of leaves and needles off of the deck will improve it resistance to fire.
For more information on decking material fire resistance go to http://nature.berkely.edu/forestry/structures
Exterior walls are susceptible to a wildfire’s radiant heat and flame impingement. Wall materials that resist heat and flames include cement (Hardie), plaster, stucco and concrete masonry such as stone, brick or block. Even with non combustible sliding you need to check for any cracks that can allow embers to get under the siding.
Wood siding can be safely used in areas where it’s is not compromised by flammable vegetation, woodpiles, stored building materials, etc., are kept clear from the sliding by least 30 feet away on flat slopes. Also check for cracks in the wood sliding assembly construction where flames can work through to the interior of the home wall. Sheets with shiplap joints provide 18 times more time of protection of interior wall from flame penetration than wood siding that is abutted together with plain beveled siding (Dost). Flames that can continuously expose the wall can work through the abutted material.
Vinyl siding melts at relatively low temperatures, exposing what is underneath it to the possibility of ignition. Solid sheathing under the vinyl siding is used since this siding is the first to fail under radiant heating.
Metal siding and stucco will not burn or melt at typical wildfire flame temperatures. Many exterior building materials thought to be fire resistant, however, are compromised by improper installation, poor placement of openings, lack of maintenance or the presence of combustible materials against or adjacent to them. The selection of fire resistant exterior wall coverings should always be underlain with solid sheathing.
Stucco needs to be 7/8 inch thick and a three coat system to provide an hour rating of fire protection.
Log sliding is combustible but due to the large size of the wood it takes large amounts of heat to preheat the logs to the point they combust. Cracks are the major problem with this construction and calking is required on a regular basis.
Brick, stone, block are both permanent and fire proof. Ratings of two hours are the best products to use in regard to fire resistivity.
Windows are one of the weakest parts of a building with regard to fire. They usually fail before the building ignites, leaving an opening for flames and firebrands to enter your home. This applies to both double pane and single pane glass since untempered double pane glass is only slightly more resistant to heat than single pane glass. On the other hand, single or double pane tempered glass windows doors and skylights typically fracture at higher exposures, well above the radiant heat exposures capable of igniting the surrounding wood. Radiant energy from burning vegetation or from an adjacent burning structure can ignite flammable materials inside a structure through the windows (even if the window remains intact). The combination of low e and tempered glass features for windows provides the best possible solution for windows in a wildland fire. The glass will stay intact through out the fire event and it will transfer less radiant energy to combustibles behind it.
Exterior Windows, Glass Doors and Skylights
Windows with improved glass technology will only work as long as the glass remains in place, which is held by the frame so the frame needs to also withstand the fire.
Vinyl frames do not ignite, but can melt. Use of welded and steel reinforced vinyl frames improves the chances of survival.
Exterior window screens can add some limited addition protection and they provide a surface for fire protection foam to adhere to where it does not adhere well to the smooth surface of glass.
Only an additional ten to twenty minutes of protection is necessary for a window to survive a fire. Exterior window covers, such as in-place shutters that only need to be swung into place, can add this time. Wood shutters will ignite under flame impingement, but can assist in blocking radiant heat and protection the glass and home. Metal shutters provide improved protection for the window, as they will not ignite with flame impingement, but they will conduct radiant heat. The disadvantage of shutters is that they require homeowner to close them before the fire comes to the house.
Eaves & SoffitsThe extension of the roof beyond the exterior wall is the eave. This architectural form is particularly prone to ignition because as fire approaches the building the exterior wall deflects the hot air and gasses up into the eave. The longer the eave the more protection of the siding against rot, but the more it allows for the trapping of radiant heat and flame. If the exterior wall is combustible this effect is amplified. The solution is to cover the eave with a soffit with at least 3/4” plywood in low fire hazard areas, noncombustible in moderate and high areas, and one hour rated material in very high hazard areas.
Rain GuttersRain gutter can provide a potential ignition source with leaf and needle accumulation. Once they are on fire they can ignite the allow fire to get under the roof surface and ignite the plywood edge if it edge is exposed to the gutter. Metal gutters hung with a metal overlap on top of the plywood subsurface provide the most protection. Check the gutter construction. Gutters can be regularly cleaned to also decrease the fire danger. There are many manufactured remedies that limit or block leaves and needles that are alternative to constant cleaning during fire season that has the potential of falling off the roof. The simplest is the screens that can be purchased and placed on the top of the gutter. These guard against leaves well, but provide very low protection against needles and smaller debris. The other more expensive option is to replace the gutters will new metal topped gutters that are designed to allow water to drain into the gutter and leaves and needles to slide over the top and off the roof.
Plastic gutters are seen as either a good thing or bad. Some fire fighters report that if leaves are not cleaned out that they are quickly knocked down during a fire. The very light plastic gutters are thought to melt before the leaves and needles can ignite the roof. If the house has a non combustible roof and plastic gutters, a metal flashing protecting the roof subsurface from gutter fire could be applied.
Residential buildings typically use wood doors with glass inserts. The same fire issues related to window glass apply to glass in doors. An unrated wood door is typically one and one-half to two inches thick, and can readily ignite and burn through in only ten minutes, which is much faster than the rest of the structure will burn. Wood doors are available with a class C, twenty-minute rating. These doors are typically used between the garage and the house and are a good solution in moderate fire hazard situations. But, in very high fire hazard situations, they may not be appropriate because the door will burn according to its rated time, and this may be long enough to ignite other exterior building components.
Metal doors, steel and aluminum
Metal doors are non-combustible and available with twenty minute, forty-five minute and one and one half-hour ratings, which makes them the most appropriate solution for very high hazard situations. Glass sizes are restricted in these doors. The surfaces are available with embossing to simulate wood grain and raised panel designs. Just as in energy conservation, a good fire resistive door requires adequate weather stripping so that the seal prevents hot gasses or burning embers from entering the building.
Fireplace chimneysWindblown embers can access your home through your fireplace’s chimney flue. Once inside, these firebrands then collect on flammable objects greatly increasing the chance of combustion. The situation can also be reversed: embers from your own fire can fly out the chimney and start a wildfire right in your own neighborhood.
FoamsGel coatings are made using water adsorbent polymers, providing an insulating layer, and can be sprayed on the exterior surfaces of your home prior to evacuating. Limited research has shown that these coatings lose effectiveness with time since application, and so usefulness in part depends on delay between evacuation and wildfire exposures.
There are several spray on foam products that can be applied to the home in advance of the fire and decrease the ignitability of the building material by increasing the amount of heat required to create ignition by as much as 18 times (Grand). The products effectiveness of protection will lessen with time, depending on amount of sunlight and humidity. Thus, the closer the application occurs in time to the point of fire exposure the more effective it will be. Some of the foams can only be stored in your house waiting for fire for a set period of time and remain effective.
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