Rebuilding after a wildfire without Hardie: which sidings actually carry a Class A rating
Owners rebuilding older houses in fire zones often want wide painted boards like the originals, without James Hardie products. Noncombustible and fire-retardant-treated options exist, but a Class A label alone may not satisfy a wildland-urban interface code.
Class A is a surface-burning result. Under ASTM E84 a material qualifies with a flame spread index of 0 to 25 and a smoke developed index of 450 or less, supplier Wood Source explains. The standard tunnel test runs 10 minutes; ASTM E2768 extends it to 30. Colorado's Wildfire Resiliency Code accepts noncombustible material, ignition-resistant material or fire-retardant-treated wood on exterior walls, and Wood Source notes that Class A alone does not meet it.
California moved its wildfire rules out of Chapter 7A into a standalone Wildland-Urban Interface Code; the switch covers permit applications filed from Jan. 1, 2026, according to a code guide from Northern California contractor Sierra Siding. The siding must be noncombustible, ignition-resistant and marked for outdoor use, or fire-retardant-treated wood that qualifies. Ignition-resistant means a 30-minute E84 run with flame spread held to 25 or less. A wall can also pass as an assembly tested to SFM 12-7A-1 or ASTM E2707, which apply direct flame for 10 minutes, and some products need a State Fire Marshal building materials listing.
Fiber cement from other makers comes closest to a wide board. Allura sells lap siding up to 12 inches wide in 12-foot lengths, calls it noncombustible with a Class A rating, and holds CAL FIRE WUI listing 8170-2180:0501. For real wood, Humboldt Sawmill sells Class A fire-retardant-treated redwood siding, pressure-treated with Hoover's Exterior Fire-X and carrying State Fire Marshal listings, but only in 1x6 and 1x8 profiles. Fire Safe Marin lists three-coat stucco and metal siding as noncombustible and warns that field-applied intumescent paints and stains have not shown lasting protection outdoors.
The wall assembly matters as much as the board. Fire Safe Marin cites University of California fire lab tests showing the lap joint is the weak point of combustible siding: shiplap and tongue-and-groove joints held back flames better than plain bevel joints, and sheathing behind the siding helped. One noncombustible lap product failed after about 22 minutes of direct flame, while a wildfire's main flame front lasts 5 to 10 minutes at most at a given house.
Why it matters: Before ordering, get the product's E84 or E2768 report, its exterior-use label and any state fire marshal listing, and confirm with the building department which compliance path the wall needs. Treated redwood from Humboldt tops out at a 1x8 profile.
Sources
- Fire-Resistant SidingFire Safe Marin, Jun 1, 2021
- What is Class A Siding?Wood Source, May 23
- California Siding Code & Fire-Zone Rules 2026Sierra Siding, Nov 3
- Fiber Cement Siding | Fiber Cement BoardAllura USA, Nov 3
- Fiber Cement Lap SidingAllura USA, Nov 3
- Class A Fire-Retardant RedwoodHumboldt Sawmill Company, Nov 3