Decks and the Space Under Them: Wildfire Prep

Why are decks a wildfire weak point?

The Colorado State Forest Service's Home Ignition Zone guide explains it: many deck designs let embers accumulate in the gaps between boards and at the joists below, and embers can fall through decks and ignite flammable material beneath. However the fuel under a deck ignites, the burning material can readily ignite the deck and threaten the home. CSFS also notes that horizontal surfaces such as wooden decks are at greater risk from embers than vertical ones.

A deck is also attached to the house, so it moves the edge of the home outward. The IBHS Wildfire Prepared Home checklist measures the 5-foot noncombustible zone from the outer edge of an attached combustible deck rather than from the wall, and says that for elevated structures such as decks and stairs, that zone extends beneath the structure. Our zone 0 guide covers the zone itself.

What should be cleared from under a deck?

Everything. NFPA's home ignition zone guidance says to remove anything stored underneath decks or porches. MSU Extension's WUI landscaping MontGuide says not to let dry leaves and needles accumulate under decks, because such accumulations create a receptive fuel bed for embers. CSFS says not to use the space under decks for storage. The IBHS checklist says to remove all stored items, giving firewood, tools and equipment as examples.

Under a typical western Montana deck that means firewood, scrap lumber, lawn furniture, kayaks and canoes, propane cylinders, gas cans, plastic bins and the drift of pine needles and leaves in the corners. Then the ground itself: grass and weeds growing under the deck should be removed, and the surface should be gravel or bare mineral soil rather than bark or landscape fabric. Our guides to firewood storage and propane and fuel cover where those items should go instead.

Should I screen in the space under my deck?

For low decks, IBHS says yes. Its checklist says decks 4 feet high or less, measured at any point from the walking surface to the ground, should have the perimeter enclosed with 1/8-inch corrosion-resistant metal mesh, installed on the outside of any lattice. NFPA's guidance likewise says to screen or box in areas below patios and decks with wire mesh to keep debris and combustibles from accumulating, and CSFS says to screen or wall in stilt foundations and decks with 1/8-inch metal mesh.

The point is to keep embers and blown debris out of a space that is hard to clean. A screened deck still needs the ground under it cleared before the mesh goes on, and the mesh should be checked each year for gaps where animals have pushed through or where needles have built up against it. Higher decks are usually left open so the space can be seen and cleaned.

What about lattice skirting and deck posts?

IBHS asks for at least 6 inches of noncombustible material at the base of combustible posts, supporting walls and stairs, and for combustible lattice, wood or vinyl, either removing the bottom 6 inches or replacing it with noncombustible material. The base of a post or lattice panel is where needles and debris collect against it, which is why the requirement focuses there. Metal post bases, concrete piers or a strip of metal flashing all meet it.

The same 6-inch rule applies to the wall of the house above an attached deck: IBHS wants at least 6 inches of noncombustible material on any combustible siding above the deck surface. On many Montana homes the siding runs right down to the decking, which is a job for a contractor who does siding or flashing; it is not work we do.

What should be on top of the deck in fire season?

Less. The IBHS checklist says to keep deck surfaces clear of leaves, pine needles and debris, and not to keep planter boxes, climbing vines or woody plants on the deck, while allowing up to ten small noncombustible pots under 36 inches tall that can be moved indoors in a Red Flag Warning. On combustible decks it says to replace combustible furniture with metal, aluminum or cast iron frames, and to remove combustible items including rugs, wood planters, storage containers and firewood. Cushions and small movable items can stay but should go indoors during Red Flag Warnings or extended absences.

Sweep the gaps between boards as well as the surface. Needles wedge into the gaps and along the ledger board against the house, which is exactly where embers lodge. A leaf blower run along the gaps and the house edge every few weeks in late summer is more effective than an occasional broom.

Do trees over the deck matter?

Yes. Branches overhanging a deck drop needles onto it and, in a fire, bring flame closer to the house. The IBHS checklist removes trees, limbs and branches from above the 5-foot zone, which extends under and around an attached deck, and NFPA says to plan trees so the mature canopy stays at least 10 feet from the structure. Pruning back overhanging limbs and removing small trees growing up through or beside the deck is tree work; our guides to how high to limb trees and ladder fuels explain how to do it without harming the tree.

What about a hot tub on the deck?

The IBHS checklist sets rules for hot tubs within 30 feet: at least 10 feet from the home and other large combustible items, and not under combustible overhead structures such as pergolas or covered porches. On a combustible wood or composite deck, it asks for the decking beneath the hot tub to be replaced with noncombustible material extending at least 2 feet around it; on concrete, pavers or soil, it asks for a 5-foot noncombustible zone around the tub. The cover and cabinet are the combustible parts, so a tub set on a paver pad out from the house is the simpler arrangement.

Should I replace my wood deck?

Not necessarily. IBHS's base Wildfire Prepared Home level accepts an existing combustible deck if it is kept clear, its underside is cleared and enclosed where low, and the 6-inch bases are in place. The Plus level asks for a noncombustible deck within 30 feet of the home, either new construction from materials such as lightweight concrete, tile, stone, pavers, brick or aluminum, or a retrofit with a solid noncombustible walking surface and noncombustible railings within 5 feet of the home.

For a deck at or near ground level, a paver or stone patio is a noncombustible alternative, and IBHS exempts fully noncombustible, uncovered patios from needing their own 5-foot zone. Our guide to patios, paths and rock as firebreaks covers that option. Composite decking is not the same as noncombustible; IBHS does not treat it as such.

How often does under-deck prep need repeating?

At least once a year, and more often under pines. CSFS's Home Ignition Zone checklist marks clearing the roof, deck and gutters, and raking needles and debris from 5 feet around the foundation and deck, as tasks to address as needed, more than once a year. Late summer, before the driest weeks, and again after needle drop in fall are the useful times. Snow on decks is its own topic; see our guide to clearing snow off decks.

Most deck prep is clearing work that fits naturally into a seasonal cleanup. This guide is part of our Montana Wildfire & Defensible Space Guide. HJ Property Care & Tree Service does seasonal cleanups that include the ground under and around decks, gravel installation, and the tree pruning and fire mitigation around them, across Missoula County and the Bitterroot Valley. We do not build or rebuild decks or install siding and flashing. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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