Idaho · Fire rating guide

Fire-resistant roofing, Idaho

A roof's fire rating belongs to the whole assembly (covering, underlayment and deck), and Class A is the highest of the three assembly ratings. In Idaho, the roof is where windblown embers land first, and many homes that burn catch from embers. This guide covers what the ratings mean, how roofing materials compare, what to ask your insurer and how to protect the roof edge, from the people who replace roofs in Idaho every week.

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Fire rating

Class A Fire Rated Available, as a complete assembly

Two layers of GAF VersaShield® over 15/32-inch plywood, per ASTM E108 / UL 790.

The metal

Aluminum does not ignite

Embers that land on the roof have nothing to burn.

Boise Foothills

A Class A roof in the WUI

Boise's fire code asks for a Class A roof assembly or covering on ignition-resistant construction (Boise City Code 5-12-53).

How much wildfire risk does an Idaho roof face?

Idaho's Ready, Set, Go! guide, published by the Idaho Department of Lands, puts it plainly: "Your roof is the most vulnerable part of your home because it can easily catch fire from windblown embers." In southwest Idaho the risk is highest where homes meet the Boise Foothills, rangeland and the river corridors, and in the mountain towns to the north. Embers can travel well ahead of the fire itself.

Boise writes it into its fire code. The city has added a Wildland-Urban Interface chapter to the International Fire Code: the fire code official maps the wildland-urban interface areas (generally the foothills, plus valley and desert areas with their own class of construction) and decides which lots are in them. Ignition-resistant construction there calls for a Class A roof assembly or covering, and a roof that is replaced, or has 50 percent or more of its covering replaced within 12 months, has to meet the same rule. Ada Fire Adapted publishes the Boise and Ada County maps.

How a house catches fire from outside

Many houses lost to wildfire are never touched by a wall of flame. They catch from embers: burning bark, leaves and debris carried by the wind ahead of the fire.

Embers settle where debris collects: in gutters and roof valleys, against vents, in gaps at the eaves and on decks. A roof covering that burns, or a roof full of dry needles, gives them fuel. That is why the roof, and everything around its edges, is the first thing to get right.

What Class A, B and C actually mean

Roof fire ratings in the U.S. come from ASTM E108, also published as UL 790. The tests expose a roof sample to fire from outside the building: a flame spreading across the surface, burning wood brands dropped on it, and repeated bursts of flame. The rating belongs to the assembly that was tested, meaning the covering, the underlayment and the deck together. Change one layer and the rating may not hold.

Class A

Severe fire exposure

Maximum flame spread: 6 ft
Burning brand test: One 12 × 12 in brand, about 4.4 lb
Intermittent flame: 15 cycles

The highest of the three ratings.

Class B

Moderate fire exposure

Maximum flame spread: 8 ft
Burning brand test: Two 6 × 6 in brands, about 1.1 lb each
Intermittent flame: 8 cycles

Better than C; not the top rating.

Class C

Light fire exposure

Maximum flame spread: 13 ft
Burning brand test: Twenty 1.5 × 1.5 in brands, about 0.3 oz each
Intermittent flame: 3 cycles

The lowest rating a roof covering can carry.

Roofing materials compared for fire

Typical classifications as tested. Any one product can differ, so ask for the product's fire listing and the assembly it was tested on.

Typical fire classification of common roofing materials
Material	Typical fire classification	What to know
Untreated cedar shakes and shingles	Commonly unrated	Wood burns, and the gaps between shakes collect needles and leaves. Many older cedar roofs in Idaho are untreated.
Fire-retardant-treated cedar	Class B or C; some Class A as a tested assembly	The treatment can weather out over the years, and hot, dry summers are hard on it. Check the product's listing and age.
Asphalt shingles (fiberglass)	Commonly Class A as a tested assembly	Fiberglass-mat shingles usually reach Class A as an assembly. The roof still has to be kept clear of needles and leaves, which can ignite on top of it.
Clay and concrete tile	Commonly Class A as a tested assembly	Non-combustible, but heavy, and the gaps under the tile need closures at the eaves and ridge.
Natural slate	Commonly Class A as a tested assembly	Non-combustible and heavy; the roof structure has to be built for the weight.
Synthetic and rubber composite	Varies by product	Some products reach Class A only with a specific underlayment. Read the listing, not the brochure.
Interlock® aluminum	Class A Fire Rated Available, as a complete assembly	Two layers of GAF VersaShield® over 15/32-inch plywood, per ASTM E108 / UL 790. Aluminum itself does not ignite.

The Interlock® roof, built as a Class A assembly

Interlock® roofs are heavy-gauge aluminum, made in North America by Interlock Roofing at its plant in Delta, BC, and installed in Idaho by our own company. Class A Fire Rated Available, as a complete assembly: two layers of GAF VersaShield® fire-resistant underlayment over 15/32-inch plywood, per ASTM E108 / UL 790.

The panels weigh roughly 0.4 to 0.7 lb per square foot depending on profile, a fraction of tile or slate. Every profile (Slate, Cedar Shingle, Shake, Mediterranean Tile and Standing Seam) needs a minimum 3/12 roof slope.

If your house backs onto forest, or your insurer has asked about the roof, tell us at the estimate. We price the Class A assembly and walk the roof edge with you: gutters, vents, eaves and anywhere debris collects.

Product engineering and test reports are the manufacturer's to explain: interlockroofing.com ↗

Beyond the roof: the home ignition zone

The National Fire Protection Association (NFPA) divides the ground around a house into three zones, and says to start with the one closest to the house. Most of it is ordinary yard work.

Immediate zone · 0 to 5 ft

The house and the ground within 5 feet of it. NFPA says to make this zone non-combustible and to start here: clear leaves, needles and debris from the roof and gutters, and fix loose or missing shingles where embers could get in.

Intermediate zone · 5 to 30 ft

Keep plants, mulch and grass managed and spaced, and move firewood, propane and anything that burns away from the house.

Extended zone · 30 to 100 ft

Thin and prune trees and clear dead material, so a ground fire stays on the ground and away from the house.

Roof and roof-edge checklist

Choose a roof built as a Class A assembly. Ask your roofer what the assembly is, not just the panel.
Keep the roof and gutters clear of pine needles, leaves and windblown debris, especially in valleys and behind chimneys.
Screen eave and attic vents with 1/8-inch metal mesh, as NFPA recommends, to keep embers out of the attic.
Close gaps at the roof edges and ridge where debris and embers collect.
Don't store firewood, propane or anything combustible against the house or under a deck.

Your local fire district can tell you about burn bans and defensible-space advice where you live. In Boise and Ada County, Ada Fire Adapted publishes the wildland-urban interface maps and local contacts.

Questions to ask your insurer

Each insurer sets its own rates and rules, so we can't tell you what a roof will do to your premium. Ask your agent, and get the answer in writing:

Does the policy ask for the roof's fire classification, its material or its age?
Would a new roof with a documented Class A assembly change the premium or the coverage?
What paperwork do you want after the roof is replaced?

We give you the product documents and the assembly details for your file.

Fire-resistant roofing: questions, answered

What is the most fire-resistant roofing material?

Non-combustible coverings such as metal, clay or concrete tile and natural slate, installed as a tested assembly. Under ASTM E108 (UL 790) the fire classification belongs to the whole assembly (covering, underlayment and deck), so the same covering can rate differently on a different build-up. On our roofs: Class A Fire Rated Available, as a complete assembly, with two layers of GAF VersaShield® over 15/32-inch plywood.

Is a metal roof fire resistant?

Aluminum does not ignite, so embers landing on it have nothing to burn. The fire rating, though, is for the assembly: Interlock® roofs have Class A Fire Rated Available, as a complete assembly, with two layers of GAF VersaShield® over 15/32-inch plywood, per ASTM E108.

What is a Class A fire-rated roof assembly?

A roof assembly (covering, underlayment and deck) that passed the most severe of the three fire tests in ASTM E108 (UL 790): flame spread of no more than 6 feet, a burning 12 × 12 inch brand of about 4.4 lb without the deck igniting, and 15 cycles of intermittent flame. The B and C assembly classes use smaller brands and allow more spread.

Does Boise require a Class A roof assembly?

In the city's wildland-urban interface areas, yes. Boise City Code 5-12-53 adds a Wildland-Urban Interface chapter to the fire code: ignition-resistant construction calls for a Class A roof assembly or covering, and a replacement roof, or replacing 50 percent or more of the roof covering within 12 months, has to meet it. The fire code official keeps the maps and decides which lots are in. An Interlock® roof is Class A Fire Rated Available, as a complete assembly.

Are asphalt shingles fire resistant?

Most fiberglass asphalt shingles reach the top fire class when tested as an assembly over the specified deck. The weak point is usually what sits on and around them: needles, leaves and windblown debris in valleys and gutters, open vents and gaps at the eaves.

Are cedar shakes a fire risk?

Untreated cedar shakes are commonly unrated: wood burns, and the gaps between shakes collect debris. Treated shakes do better, but the treatment can weather out.

Does a metal roof make my house fireproof?

No roof does, and no honest roofer will say so. A fire-rated roof assembly and a covering that does not ignite take away the largest target on the house. Clear gutters, screened vents and a non-combustible strip around the walls do the rest.

Does my house need a fire-rated roof assembly?

It depends on the building and where it is, and local rules differ, so check with your city or county building department before you start. We can price the Class A Fire Rated Available assembly on any Interlock® roof.

Will a fire-resistant roof lower my insurance?

Each insurer sets its own rates and rules, so ask yours directly and get the answer in writing. Useful questions: does the policy ask for the roof's fire classification, does the roof's age or material change the premium, and what documents do they want after a replacement. A new roof with a documented fire-rated assembly is something you can show them.

Can you install the fire-rated assembly on my house?

Yes, in Idaho. Tell us at the estimate that you want the Class A Fire Rated Available assembly priced, and we'll walk the roof edge with you: gutters, vents and eaves. Call (208) 842-4762 or book a free consultation.

Sources

Figures are as published by each source on the date noted. Page reviewed October 2026.

Get the Class A assembly priced.

A written price after your roof consultation. Interlock Metal Roofing (PNW) Inc. · 3295 W Elder St, Suite 209, Boise, ID 83705