Which Decking Works in the Boise Foothills Wildfire Zone?

What is Boise's WUI overlay?

Boise's Wildland Urban Interface Overlay (WUI-O) marks land at higher risk of wildfire. The city sorts it into risk zones shown on the zoning map:

• IR1 areas require Class 1 ignition-resistant construction. They generally cover Foothills fire hazard areas, and as the city annexes land to the north and west, IR1 automatically extends to annexed lands generally north and west of Hill Road.
• IR2 areas require Class 2 ignition-resistant construction. They generally cover valley, desert and other fire hazard areas.
• Other fire risk areas may be identified by the Fire Code Official, including properties next to undeveloped land outside the traditional interface.

The Fire Code Official interprets the boundaries and has final say on which parcels are in. If you live in the Boise Foothills, along the upper East End, in Harris Ranch and Barber Valley or near Hill Road in Northwest Boise, check the zoning map before you choose materials.

Does adding a deck trigger WUI rules?

The overlay applies to all new structures and additions built after its effective date, and to renovations that increase a structure's gross floor area by more than 49 percent. It requires compliance with the city's wildland-urban interface fire code. The city's overlay page lists building appendages and projections, underfloor areas, defensible space and firewood storage among the code topics that apply.

Boise's #406 checklist for additions and accessory structures is more direct. When a project site is in the WUI, the plans must show additional fire-resistive construction elements, and the checklist's list includes "decks constructed of appropriate materials" and underfloor areas enclosed or protected. So a new deck in the overlay should expect review of its materials.

What one Treasure Valley WUI checklist actually requires

Eagle publishes subdivision-specific WUI checklists, and its Avimor checklist shows how a local jurisdiction applies the 2018 International Wildland-Urban Interface Code. Under "appendages and projections," the category that covers attached decks, the applicant checks one of two options: approved noncombustible materials, or fire-retardant-treated wood identified for exterior use. The same checklist requires underfloor areas enclosed to the ground with fire-resistive exterior walls, unless the underside of exposed floors and structural members is protected as one-hour or heavy timber or fire-retardant-treated construction. Vents are limited to 144 square inches each with noncombustible mesh no larger than 1/4 inch.

Avimor and Hidden Springs are both Firewise communities, and reporting on Avimor describes fiber-cement exterior walls, fire-resistant roofs and rock mulch as standard. Boise's IR1 rules are not identical to Avimor's checklist, but the checklist is a good preview of the questions an official will ask.

Which decking materials carry wildfire listings?

The IRC already sets a floor for composites. Section R507.2.2.2 requires plastic composite deck boards, stair treads, guards and handrails to have a flame spread index of 200 or less under ASTM E84, so every code-compliant composite meets a basic flammability test. Wildfire listings are a separate, tougher test. The best-known is California's State Fire Marshal program, which lists decking tested under SFM Standard 12-7A-4 for wildland-urban interface use. Idaho doesn't run its own program, so ask Boise Fire whether it accepts the California listing for your zone.

Two details from the TimberTech listing matter in practice. A listing applies to the tested assembly, including joist size, spacing and board gaps, so install it the way it was tested. And a listing can expire, as the Trex copy did, so check the date on the document your supplier hands you. For more on board choices, see our composite vs PVC decking guide and composite decking service.

Which ember-resistant details matter most?

Firewise guidance puts most of its attention on embers, which can travel well ahead of a fire and land in debris on and under a deck. The details that cut ember risk are cheap compared with the decking itself, and most of them help even outside the overlay:

1. Clear the space under the deck. No firewood, lumber, cushions, propane cylinders or dead weeds. Boise's WUI considerations specifically address firewood and combustible storage.
2. Lay a band of gravel or rock under and around the deck instead of bark mulch.
3. Enclose or protect the underfloor area. Solid noncombustible skirting, or metal mesh with small openings, keeps embers and windblown debris out. Avimor's checklist caps vent mesh at 1/4 inch; many wildfire programs recommend finer mesh.
4. Keep board gaps clear of needles and leaves, especially where the deck meets the house wall.
5. Flash the deck-to-wall joint with metal so embers caught at the ledger line can't reach wood framing.
6. Keep cushions and planters off the deck in red-flag weeks, and maintain defensible space around the deck edge.

Framing, fasteners and the house connection

Boards get the attention, but framing and connections matter in a WUI review too. Eagle's Avimor checklist applies its noncombustible-or-fire-retardant-treated rule to the whole appendage, not just the walking surface, and the IWUIC's alternative path protects the underside as one-hour or heavy timber construction. Ask the Boise fire code official the same question about framing: does the deck structure need to be fire-retardant-treated, noncombustible, or protected from below, or does enclosing the underfloor area satisfy the requirement?

Steel and aluminum deck framing systems are one answer where noncombustible framing is wanted. They carry their own span tables and evaluation reports rather than the IRC's wood tables, so the permit drawings reference the manufacturer's documentation. Fire-retardant-treated lumber is the other common answer; it must be labeled for exterior use, and cut ends and fasteners follow the treater's instructions.

The joint where the deck meets the house is the most vulnerable line. Embers collect in the corner, debris piles against the ledger, and a gap between the first board and the wall becomes a slot for burning material. A metal flashing that laps up the wall and over the ledger, a noncombustible strip along the house side of the deck, and keeping that corner swept are inexpensive defenses.

Questions to settle before you order materials

1. Which overlay zone is the parcel in: IR1, IR2, an identified fire risk area, or none?
2. Does the fire code official treat the new deck as an addition subject to the WUI rules?
3. Which deck surface materials are accepted, and is a California State Fire Marshal listing acceptable documentation?
4. Does the framing need to be noncombustible or fire-retardant-treated, or is underfloor protection enough?
5. What underfloor treatment is required: full enclosure, protected underside, or mesh skirting, and what mesh size?
6. Do the HOA or community wildfire plan add requirements beyond the city's?
7. Is the listing on the chosen board current, and does the planned joist spacing and gap match the tested assembly?

Write the answers on the drawings. A plans examiner who sees the zone, the material listing and the underfloor detail on the sheet has fewer questions.

How we handle WUI decks

Before we price a Foothills deck, we look up the lot's overlay and zone, then confirm with the fire code official which deck materials and underfloor treatment the permit will need. We show those details on the #406 drawings, buy boards with a current listing that matches the approved assembly, and build the gravel band and skirting into the scope rather than leaving them as homeowner tasks.

Shortcuts to avoid

• Buying a board because the brochure says "fire resistant" without checking a current listing or the official's acceptance.
• Installing a listed board on a different joist spacing, gap or substrate than the tested assembly.
• Leaving the underside open as storage. It is the easiest place for embers to start a fire.
• Using fire-retardant lumber that isn't labeled for exterior use.
• Assuming a replacement deck is exempt because the house predates the overlay. Ask; new decks and additions are covered.
• Ordering materials before the fire code official has reviewed the plan.

Related resources

Foothills decks usually stack other requirements on top of WUI review: the Hillside overlay, view-deck heights and HOA design rules. Our Boise deck permit guide covers the hillside and historic layers, the HOA approval guide covers committees, and multi-level deck ideas covers sloped lots. When you're ready, request an estimate.