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Fire Weather in San Mateo County

Like topography, weather conditions have a large influence on fire behavior. Weather conditions significantly impact the rate, intensity, and direction in which fires burn. Temperature, humidity and especially wind are the more important weather variables used to predict fire behavior.

San Mateo County has warm, dry summers with a fire season that typically extends from May to October. Cool, moist winters are characteristic of the fog belt area, with spring rainfall that accelerates light, flashy fuel growth.

Summer temperatures are often quite warm, with some days over 100 degrees. It is common for coastal fog to form during summer evenings and mornings, while the steep ridge to the west that blocks the fog from moving inland. According to weather data at Woodside Fire Protection District Station 7 from 1960 to 1990, the average maximum temperatures in June through September exceed 80 degrees daily. Hot days associated with stagnant air and high pressures aloft pose times of extreme fire danger. During this time, periods of continuous high temperatures and low relative humidities dry fuels to a point where the National Fire Danger Rating System (NFDRS) Burn Index rmay exceed 81, indicating that a vegetation fire starting at that time would exhibit extreme resistance to control.

"Diablo," or "Santa-Ana" winds occur infrequently, but can influence fire behavior greatly. These offshore wind patterns create extremely low humidities and enhance the possibility of ignition and extreme fire behavior. Once a fire escapes initial attack, teh fire itself can generate its own wind, creating strong updrafts that entrains additional wind into the burning environment.

Exterior Siding

Fire Resistant Siding Materials

Your home’s exterior walls and siding are most susceptible to radiant heat and open flame. Typically, the corners of your home are the weakest part of the structure. This is due to high surface to volume ratios. It is recommended that Class A or B rated siding materials be used.

Siding products can be classified into three basic categories. These include combustible, noncombustible, and something called ignition resistant. Although most people would assume that, in terms of fire resistance, an ignition-resistant mate- rial would fall somewhere between a noncombus- tible and combustible material, only the California Building Code actually defines this term. Materials that are classified as ignition resistant have to pass a standard test. Some products rated as noncombustible have been defined as such in the building code. Other materials can be rated as noncombustible by passing a standard fire test. The most common combustible siding products include solid wood and other composite wood products (plywood, oriented strand board, and other compressed wood fiber products) and siding made from vinyl or other plastics. Wood that has been treated with an exterior fire-retardant chemical, and that has passed a standard flame spread test after undergoing a specified weathering procedure consisting of wetting and drying cycles, is called ignition resistant. Common noncombustible materials include three-coat stucco, metal siding, and fiber cement siding.

An ignition-resistant material should not be confused with ignition-resistant construction. The latter takes into consideration all components and materials on the outside of the house, as well as the vegetation management plan used in the defensible space zone around the home. In California, combustible materials can be used in ignition-resistant construction.

There are two ways that combustible siding can make your home vulnerable to wildfire. First, the siding can ignite, with flame spreading up the wall, providing a flame contact exposure to other components on the wall, including windows, eave (soffit), and vents. Under this scenario, the combustible siding transfers the problem to these other components. If the windows, eave, and vents do not fail, the home will experience considerable damage but may not be destroyed. Under this scenario, however, total loss of the home is far more likely.

The second way that combustible siding can make your home vulnerable to wildfire is when fire penetrates through the siding (most likely at a horizontal or vertical lap joint between individual pieces of the siding material), into the stud cavity, and then into the living space of the home. Under this scenario, the home will probably be lost, particularly if no one is present to put out the fire. When thinking about all the ways your home is vulnerable to wildfire exposures, a number of other components are more important (your roof, vents, windows, deck, and vegetation), but ultimately your siding can become the weak link. The importance of maintaining near-home vegetation and minimizing near-home storage of combustible materials has already been discussed, but when combustible siding is also used, these items become even more important to manage. Siding is also an important factor in home-to-home ignittion. If another building is adjacent to the house, consider noncombustible siding.

Testing at the University of California Fire Research Laboratory has shown that, when ignited, compressed wood particle or fiber siding products and wood shingle siding will result in more rapid vertical flame spread up the wall, and more rapid penetration through the lap joints and into the stud cavity. Similar results have been observed with vinyl siding. In both of these cases, underlying wood or gypsum sheathing would improve the resistance to flame penetration at a lap joint.

Combustible trim is also vulnerable to ember attack and potential flame contact exposure, particularly at internal corners. In these locations the trim can either be applied on top of siding that extends all the way to the corner or adja- cent to siding when trim is applied directly to the studs. When applied directly to studs, the trim-to- siding gaps should be protected and maintained.

These caulked joints are likely to fail if subjected to flames. Vegetation is commonly planted in these corners, which are also susceptible to accumu- lating windblown vegetative debris. During wildfires, this area would also be susceptible to the accumulation of windblown embers. Once ignited, the proximity of adjacent trim pieces makes it easier for flames to propagate vertically up the wall. Therefore, internal corners would be a good location for a noncombustible or ignition-resistant material.

Siding Patterns and Assembly

Testing of siding products conducted at the UC Fire Research Laboratory showed that the siding joint is the most vulnerable part of a combustible siding assembly. Flame penetration through the siding occurred more readily at less complicated lap joints. For example, a ship lap or tongue-and-groove lap joint provided greater protection against flame penetration into the stud cavity compared with a plain bevel joint. With sustained flame expo- sure, even noncombustible siding materials will eventually fail. For example, failure occurred in a noncombustible lap siding product after approximately 22 minutes of a flame contact exposure. Failure in these materials occurred either as a result of increased heat transfer through the material, with subsequent ignition of a stud, or flame penetration at cracks that developed. Use of structural sheathing, located underneath the siding, provides enhanced protection against flame penetration at lap joints. Because of seis- mic (earthquake) design issues, sheathing is commonly used, particularly in California.

As previously discussed, prolonged expo- sure from the wildfire itself is unlikely, since the duration of the main flaming front at a given building is relatively short (5 to 10 minutes at the most), and even combustible siding products can protect against penetration for these time periods. Prolonged radiant or flame contact exposure from abundant near-home vegetation, or other build- ings in proximity, could more easily result in joint penetration.

In summary, although siding is an important component in terms of providing protection for the building from the weather, and also in terms of the aesthetics of the building, in most cases it is less important from a wildfire exposure perspective compared with other components and assemblies. With proper selection and maintenance of near- home vegetation, most siding products will be able to resist typical wildfire exposures. However, viewing the side of the building as a system, use of noncombustible or ignition-resistant materials will reduce the potential fire exposure on windows and the eave area of a building.

Fire-Retardant Treatments and Coatings

Exterior rated, fire-retardant-treated (FRT) wood products are pressure-impregnated with chemi- cals that improve the fire performance character- istics. These wood products undergo a processing step whereby these chemicals are fixed or otherwise contained to prevent or minimize leaching of the chemical. The process is similar to that used in preservative-treated lumber, which contains chemicals to resist attack from fungi and other wood-destroying organisms. In order to qualify as an exterior-use product, the treated wood must undergo accelerated or natural weathering cycles and pass subsequent fire tests to provide evidence of a prescribed level of performance over the intended service life of the product. There are practical and scientific reasons why a natural weathering cycle equivalent to the intended service life is not required prior to the product being accepted for use.

Instead of leaching losses from wetting, the more likely reduction in performance will be from in-service weathering (i.e., weathering resulting in the loss of wood fibers from the surface, thereby depleting the fire-retardant chemicals). Weathering will be variable, depending on exposure (i.e., the north, east, south, and west sides of your home), but it has been shown to be a slow process. The weathering process can be reduced considerably with a periodic application of a penetrating stain or a film-forming coating.

Stains and coatings are used for aesthetic reasons, and also to improve the resistance to certain exposures (e.g., weathering, fungi, and insects). Film-forming intumescent paints and penetrating stains, applied in the field, have not been shown to provide long-term, improved protection from fire exposures. None of these prod- ucts have demonstrated long-term, fire-resistance performance in an exterior environment. Until these products can show adequate performance after a defined weathering procedure, they should not be relied on to improve the fire performance of exterior-use building materials.

A gel coating is a new fire protection alternative that is becoming more available to the homeowner. Gel coatings can be effective, assum- ing they are applied correctly and address points of fire entry as discussed in this publication. Gel coatings are purchased in a concentrate form, and can be applied to the home using a garden hose and an attachment that is supplied by the gel supplier. These coatings lose effectiveness with time (in the order of hours) as a result of water evaporation, but there is no data that evaluates the rate at which this reduction in effectiveness occurs. We do not think it is warranted to rely on a coating in lieu of managing vegetation or improving building materials or design.

From Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations: ANR Publication 8393 15

Fire Resistant Decks

The vulnerability of decks to wildfire will depend on the decking board material, any combustible materials stored under the deck or kept on the deck, and the topography and amount and condition of vegetation leading to the deck. Even though noncombustible decking products are available (e.g., metal decking and lightweight concrete), many decking products are combustible. Untreated wood and wood treated with fire retardant, as well as plastic and wood-plastic composite products, are all combustible and therefore vulnerable to ember and other wildfire exposures. However, the deck can be designed to minimize exposure to embers. During fire season an important job will be to keep your deck free of easily ignited materials such as leaves and needles that have accumulated in the gaps between deck boards and at the interface between the deck and the siding of the house. Metal flashing can be applied to the lower 18 inches of the wall to protect combustible siding from the embers that may accumulate during a wildfire. It is important that the metal flashing be tucked in behind the lap-joint where it terminates. Decayed wood (rotted wood) is more readily ignited, so periodically inspect for decayed members and replace them.

In California, deck boards on new homes now must meet a minimum performance requirement (based on energy release when burning) to be used in wildfire-prone areas. Flame spread information is usually also available for decking products. A listing of most of the decking products that comply with the California requirements is maintained in a free online database called the Building Material Listing (BML), formerly known as the WUI Product Handbook, and is maintained on the Office of the State Fire Marshal (OSFM) Web site.

Some people are surprised to find that untreated wood decking is included in the Building Material Listing (BML). The testing conducted on combustible decking products has shown that most products are not highly combustible by themselves. Typically, other fuel sources contrib- ute to larger deck fires (debris or combustible material stored under or on top of the deck, or a deck located on a slope containing a lot of combustible vegetation). The take-home message is to avoid storing other combustible materials on and under your deck. If your home is located on a slope, and your deck is overhanging it, your defen- sible space should be increased to avoid a flame contact exposure during a wildfire.

Deck Enclosures

Regardless of what type of deck board is used, ignition can occur from an accumulation of combustible material (grass, leaves, needles) under the deck. Embers can readily ignite this debris during a fire. Enclosing the underside of the deck is one method to reduce the risk of ignition. There are two ways to enclose your deck. You can enclose it by applying sheathing or siding around the perimeter (a vertical enclosure), or by attach- ing sheathing/panel materials to the underside of the structural support members (a horizontal enclosure). The more careful you are about not storing combustible materials near your home (and under your deck), maintaining near-home vegetation, and cleaning up windblown debris, the less important enclosing your deck becomes.

The closer the deck is to the ground, the harder it is to use as a storage area, but it also becomes more difficult to clean out debris that will accu- mulate. If you have a solid surface deck (e.g., one with a lightweight concrete walking surface), it is probably already enclosed horizontally. If you are going to store combustible materials under your deck, then vertically enclosing it with a noncombustible or ignition-resistant material would make sense. If you do this, make sure you avoid any potential moisture-related degrada- tion issues by providing adequate venting or take other moisture control actions.

Deck enclosure will not guard against embers falling on top of the deck, so be aware of combustible material that you have on your deck. Firewood should be moved off of the deck during wildfire season. Combustible materials commonly found on decks (e.g., brooms, umbrellas, patio furniture, door mats, etc.) should be moved off the deck if possible, or moved as far from the building as possible during wildfires and spaced to avoid clustering items.

Windows

During a wildfire an open window is the most vulnerable to flames or embers. Closed windows fail if the glass breaks or if the frame material ignites and burns through into the home. If the glass breaks, embers can easily enter through the opening and ignite materials in the home. Glass breaks as a result of temperature differences (and resulting stresses) that develop between the glass that you can see (i.e., look through) and the glass that is protected by the framing material when a window is exposed to the heat of a fire. The stresses cause small cracks that occur at the edge of the glass to grow. Since larger pieces of glass (in larger windows) have more edge (and therefore more small cracks), larger windows are more vulnerable than smaller ones.

A discussion of windows is complicated because they can be made from many materials. Different kinds of glass can be used (e.g., annealed, tempered, and laminated), as well as different framing materials (e.g., wood, vinyl, aluminum, vinyl- and aluminum-clad wood, and fiberglass). Depending on the type of window, other components (often combustible) can be used inside the frame (e.g., in the pulley system of a single or double-hung window) and different materials are also used to hold the glass in the frame. Because the window is set in the wall, its performance can depend on whether or not the siding ignites. As is the case with siding, glass breakage or frame ignition will depend on the severity of the radiant heat level (both the amount and duration) and whether direct flame contact from burning vegetation or other near-home materials occurs.

There has been conflicting information regarding the relative importance of glass versus frame material when windows are subjected to wildfire exposures. For example, some builder’s and homeowner’s guides (e.g., Slack 2000; FEMA 2008) do not recommend using vinyl- and wood-framed windows. This recommendation, in part, comes from research by Mowrer (1998), where he reported that certain vinyl-framed windows deform under radiant exposures much lower than that required to break the glass. The window fails when the deformed frame allows the glass unit to fall out completely or allows a gap to develop between glass and frame. This problem can occur with windows that have horizontal or vertical separators in the middle of the window (i.e., a single- or double-hung window or a slider window). Research has shown that as long as the horizontal or vertical separator member has an internal reinforcement bar, the deformation will not take place (Quarles and Beall 2001). Vinyl windows that are certified to comply with the AAMA/WDMA/CSA 101/I.S.2/A440 Standard/ Specification possess this feature, so look for labeled products indicating this certification. For most vinyl window manufacturers, inclusion of the reinforcement bar has always been standard practice because of the need to meet other performance requirements (e.g., the forced entry requirement and certain structural code resistance requirements such as wind load resistance).

McArthur (1991) tested wood- and aluminum- framed windows and reported that the glass was the most vulnerable component. These results generally agree with unpublished research conducted at the University of California Fire Research Laboratory. In these studies a variety of frame and glass types were studied. During one test the beading material that holds the glass to the frame failed, allowing the glass to fall out prior to breaking, but otherwise these results support the recommendation to use the more resistant tempered glass, and select frame material based on other factors (aesthetics, cost, etc.). Therefore, to improve the performance of their windows under wildfire exposures, homeowners should upgrade to a multipaned unit (two or three panes), using tempered glass (the California building code requires that at least one pane in a multipaned unit be tempered). Tempered glass is about four times stronger, and also much more resistant to thermal exposures, than the normal annealed glass commonly found in most windows.

Research conducted in Australia has shown that a reflective film on the exterior surface of the outside glass provides effective protection against radiant exposures (Bowditch et al. 2006). These coatings are most often used to reduce energy costs of a building and there are a number of commercially available products. If you are considering use of a reflective coating to improve the performance of your windows during a wild- fire, consult with an energy professional to evalu- ate other impacts of the coating. Bowditch et al. (2006) also reported that laminated glass did not perform any better than annealed glass (i.e., the glass typically used in windows).

Low-E coatings have sometimes been discussed as a means of enhancing the wildfire performance of windows. Low-E coatings are always on one of the inner surfaces of a dual-pane unit. Mathematical modeling has predicted that a low-E coating placed on the inner surface of the outside glass would improve the performance of a window exposed to a wildfire (Cuzzillo and Pagni 1998). This has not been confirmed by laboratory testing, but agrees with reports that manufacturers will place low-E coatings on this surface in order to minimize the potential for thermal stress (Carmody et al. 1996).

Ignition of window curtains from a radiant exposure, prior to glass breakage, is unlikely as long as annealed or tempered glass is used. Babrauskas (2003) summarized research investigating glass breakage and ignition of materials under radiant heat. He reported that glass breakage occurred at radiant exposures between 10 kilowatts per square meter for single-pane annealed glass (approximately 25 kilowatts per square meter for double pane) and 45 kilowatts per square meter for tempered glass. Cotton and polyester materials (which curtains might be made of ) were igniting at about 40 kilowatts per square meter, but he also reported that glass reduces the amount of radia- tion transmission by half. With these types of glass, breakage is far more likely to occur before curtains ignite. It is still a good idea to remove easily ignit- able things away from the window in case of glass breakage and ember entry. If you have more expensive types of glass (e.g., ceramic, borosilicate, or dual-pane/intumescent-filled units), then igni- tion of interior objects would be far more likely because they are much more effective at resisting breakage under elevated radiant heat exposures. However, windows with any of these kinds of glass are very expensive and beyond the budget of most homeowners. (Automatic shutters would be more affordable and would provide comparable protec- tion from radiant exposures.) More importantly, if you expect these kinds of fire exposures, then certain near-home vegetation management projects should be a top priority on your To Do list.

Window Screens

Research has shown that window screens improved the performance of glass exposed to radiant heat (McArthur 1991). Bronze, fiberglass (with polyvinyl chloride coating), and alumi- num screens all improved glass performance by increasing the time needed for edge cracks to develop. Results from McArthur’s study showed that bronze screens were most effective and aluminum the least effective. Unpublished data from testing conducted at the University of Cali- fornia Fire Research Laboratory showed that screens do not provide any added protection from a flame contact exposure, such as that from burning vegetation located under the window. Window shutters would be effective against both radiant and flame contact exposures.

Windblown embers will still be able to pass through screens. If the glass in the window has failed, and the screening is still in place, the wind blowing against the screen and into the building will result in the ember being reduced in size until it can pass through. Since window screening is usually fine mesh (approximately 1⁄16-inch opening), embers that pass through will have minimal energy and will not be likely to ignite internal furnishings. If the glass and screening have both failed (i.e., large gaps in both), entering embers will be able to ignite combustible materials in the home.

From Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations: ANR Publication 8393 13