Weather, along with topography and fuels, determines how fires burn. Temperature, humidity, and above all wind drive the rate, intensity, and direction of fire spread, and San Mateo County's weather pattern concentrates its worst fire conditions into late summer and fall.

San Mateo County's Fire Weather Pattern

The county has warm, dry summers, with a fire season that historically runs from late spring through the first significant fall rains, and with damaging fires possible outside that window in dry years. Wet winters and spring rainfall accelerate the growth of light, flashy fuels (grasses) that cure as summer advances. Coastal fog moderates the west side of the county through summer evenings and mornings, while the Skyline ridge blocks much of that moisture from reaching inland slopes. Inland valleys see summer days over 100 degrees, and extended stretches of heat, stagnant air, and low humidity dry fuels to the point where a new start resists control.

Offshore Wind Events: The Danger Days

The Bay Area's most dangerous fire weather comes with offshore ("Diablo") wind events, the Northern California counterpart to Southern California's Santa Ana winds. These dry, gusty northeast winds reverse the normal onshore flow, drop humidity to single digits, and push fire downslope and across barriers at speeds suppression cannot match. The 1991 Tunnel Fire in the Oakland hills, the 2017 North Bay fires, and the 2025 Los Angeles fires (on Santa Ana winds) were all offshore wind events. A fire that escapes initial attack in these conditions also generates its own wind, entraining air into the fire environment and accelerating unpredictably.

Learn More

• National Weather Service Bay Area forecast office
• Live fuel moisture: how our vegetation cures into fuel
• How topography influences wildfire risk
• PG&E Public Safety Power Shutoff information

Siding is rarely the first thing to fail in a wildfire, but it can be the weak link. Burning siding delivers flame to windows, eaves, and vents above it, and in home-to-home fire spread the siding facing a neighboring building often decides the outcome.

Combustible, Noncombustible, and Ignition-Resistant

Siding falls into three categories. Combustible products include solid wood, plywood, oriented strand board and other compressed wood-fiber products, and vinyl and other plastics. Common noncombustible products include three-coat stucco, metal, and fiber cement. Ignition-resistant is a specific term defined in the California Building Code: wood treated with an exterior-rated fire retardant that passes a flame spread test after a prescribed weathering procedure qualifies, as do other materials passing the standard test.

Do not confuse an ignition-resistant material with ignition-resistant construction. The latter describes the whole building as a system, all exterior components plus the defensible space around it, and California allows some combustible materials within ignition-resistant construction.

How Siding Fails

Combustible siding creates two failure paths. First, ignited siding spreads flame up the wall and attacks the windows, eaves, and vents above; if those hold, you get heavy damage rather than a loss, but if any fails, the house is likely gone. Second, fire can penetrate through the siding itself, usually at a lap joint between pieces, into the stud cavity and then the living space. With nobody home to intervene, that is usually a total loss.

UC Fire Research Laboratory testing found that compressed wood-fiber products, wood shingle siding, and vinyl all produce fast vertical flame spread and faster penetration at lap joints. Joint design matters: shiplap and tongue-and-groove joints resist penetration better than plain bevel joints. Structural sheathing under the siding, already common in California for seismic reasons, adds substantial protection at the joints. Even noncombustible siding fails under long enough flame contact (one noncombustible lap product failed at about 22 minutes), but the fire front of a wildfire typically exposes a building for only 5 to 10 minutes. Prolonged exposures come from burning vegetation and neighboring buildings, which is why near-home fuels and structure separation dominate the siding question.

Corners and Trim

Internal corners are the weak points of a wall: they have high surface-to-volume ratios, they collect windblown debris and embers, vegetation is commonly planted in them, and caulked trim joints fail under flame. Corners are a smart location for noncombustible or ignition-resistant material even when the rest of the wall is wood, and corner plantings should go.

Fire-Retardant Treatments and Coatings

• Pressure-impregnated, exterior-rated FRT wood is the real thing: chemicals fixed in the wood, weathering-tested, and listed for exterior use. Its performance declines slowly as surface fibers weather away, and periodic penetrating stain or film coating slows that loss.
• Field-applied intumescent paints and stains have not demonstrated long-term fire performance outdoors and should not be relied on to improve exterior fire resistance.
• Gel coatings applied before evacuating can work if applied correctly, but they lose effectiveness within hours as water evaporates. Do not rely on a last-minute coating in place of vegetation management and better materials.

Adapted from Quarles et al., Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations (UC ANR Publication 8393, PDF). Approved WUI siding products are listed in the OSFM Building Materials Listing.

Decks burn homes three ways: embers ignite debris in the board gaps and at the house wall, embers ignite what is stored underneath, and a burning deck delivers direct flame to the siding and windows above it. The deck material matters less than what accumulates on, under, and around it.

Deck Materials

Noncombustible decking exists (metal decking, lightweight concrete), but most decking is combustible: untreated wood, fire-retardant-treated wood, plastic, and wood-plastic composites all burn. In California, deck boards on new homes in wildfire-prone areas must meet a minimum performance requirement based on heat release. Products that comply are listed in the Office of the State Fire Marshal's Building Materials Listing (formerly the WUI Products Handbook).

People are sometimes surprised that untreated wood decking appears in that listing. Testing shows most deck boards are not highly combustible by themselves. Large deck fires almost always involve other fuel: debris in the gaps, combustibles stored under the deck, or vegetation on a slope below it. That is the take-home message. Manage what the deck collects and shelters, not just what it is made of.

Maintain the Deck as an Ember Zone

• Keep the gaps between deck boards and the deck-to-siding intersection free of leaves and needles through fire season.
• Apply metal flashing to the lower 18 inches of the wall where the deck meets the house, tucked behind the siding lap joint, to protect combustible siding from ember accumulations.
• Inspect for decayed boards and replace them. Rotted wood ignites far more easily than sound wood.
• Move firewood off the deck during fire season, and move brooms, door mats, umbrellas, and furniture off or well away from the building when fire threatens. Avoid clustering combustible items.
• If the deck overhangs a slope with vegetation below, extend your defensible space downslope to prevent flame contact from below.

Enclosing the Underside

Embers readily ignite debris and stored materials under a deck. You can enclose a deck vertically, with sheathing or siding around the perimeter, or horizontally, with panels attached to the underside of the joists. The more disciplined you are about storing nothing under the deck and cleaning up windblown debris, the less enclosure matters. If you will store combustible materials under the deck, enclose it vertically with noncombustible or ignition-resistant material, and provide venting or other moisture control so the enclosure does not create a decay problem. Low decks are hard to use for storage but also hard to clean under; solid-surface decks are usually already enclosed horizontally.

Adapted from Quarles et al., Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations (UC ANR Publication 8393, PDF). See also Fire Safe Marin: Harden Your Home.

Windows fail in a wildfire two ways: the glass breaks from heat, or the frame ignites or deforms. Either way, the opening lets embers into your home, where they ignite furnishings and burn the house from the inside. Upgrading glass is one of the highest-value hardening investments you can make.

How Windows Fail

An open window is the worst case, so closing windows if you have time before evacuating matters. Closed windows fail when radiant heat creates temperature differences between the exposed glass and the glass shielded by the frame. Those stresses grow the tiny cracks that exist at every glass edge until the pane breaks. Larger windows have more edge and more cracks, which makes them more vulnerable than small ones, and single-pane annealed glass is the most fragile common configuration.

Glass Matters More Than Frame

Older guidance often warned against vinyl and wood frames. The research is more specific. Testing found glass is usually the most vulnerable component, and frame material can be selected on cost and aesthetics once the glass is right (McArthur 1991; UC Fire Research Laboratory). The vinyl-frame problem occurs in windows with unreinforced horizontal or vertical separators, which can deform under radiant heat and let the glass unit fall out. Vinyl windows certified to the AAMA/WDMA/CSA 101/I.S.2/A440 standard include internal reinforcement bars that prevent this, so look for that certification label when buying vinyl windows.

Coatings, Films, and Expensive Glass

• Exterior reflective films have been shown to protect glass against radiant heat in Australian testing. They are sold mainly for energy savings; if you consider one, consult an energy professional about the other effects.
• Laminated glass performed no better than ordinary annealed glass in the same testing.
• Low-E coatings, applied to the inner surface of the outside pane of a dual-pane unit, are predicted by modeling to improve fire performance, though this has not been confirmed in laboratory tests.
• Ceramic, borosilicate, and intumescent-filled glazing resist breakage well but cost more than most budgets allow. Automatic or operable shutters provide comparable protection against radiant heat for less, and they also protect against flame contact.

If your site could plausibly deliver those extreme exposures to a window, the better investment is upstream: fix the defensible space that would produce the exposure.

Window Screens

Screens help. Bronze, coated-fiberglass, and aluminum screens all delay glass edge-cracking under radiant heat, with bronze the most effective and aluminum the least. Screens do not protect against direct flame contact, such as burning vegetation under the window, which is one more reason nothing combustible belongs under a window. Fine window screening (about 1/16 inch) also strips energy from embers passing through it: if the glass fails but the screen holds, the embers that get in are small and unlikely to ignite furnishings. Keep easily ignited materials (curtains, papers, furniture) away from windows facing vegetation during fire season.

Adapted from Quarles et al., Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations (UC ANR Publication 8393, PDF). See also CAL FIRE: Hardening Your Home.