Fire Resistant Decks

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.