Exterior Siding

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.