Class A vs Class B Fire Rating for Decks

Q: My ESR shows a Class A listing at 3 inches per foot slope. Can I use it on my 1/4 inch per foot roof deck?

Only if the ESR explicitly covers the lower slope. ASTM E108 / UL 790 is calibrated at a 5 inch per foot test slope; severity of test exposure decreases as slope decreases, so a steep-slope listing is not automatically valid at low slope. Many walking deck ESRs are listed at low slope (1/4 inch per foot to 2 inches per foot) specifically for balcony and roof-deck applications. Read the slope range in the ESR conditions of use before specifying.

How ASTM E108 / UL 790 roof classifications map to California WUI code, walking-deck rules, and ICC-ES ESR documentation

Last updated: April 22, 2026

Do I Need Class A?

Rule of thumb: if your deck is also a roof, specify Class A.

Any walking deck that covers habitable or conditioned space below (balconies over apartments, rooftop terraces, podium decks, decks over garages or retail or storage with occupancy, parking under residential) IS a roof covering. Roof coverings over occupied space default to Class A in California (CBC Chapter 15 + §705A / CWUIC §504.2 + local amendments) and in virtually every jurisdiction where USMS serves. If any part of the deck serves as a roof, specify Class A and stop debating.

The decision questions below apply mainly to ground-level patios, decks over open landscape, and decks over strictly unconditioned / unoccupied space where the deck is not simultaneously acting as a roof.

Fire classifications under ASTM E108 or UL 790 were developed for roof coverings, but the same rating shows up on deck coating product sheets and ICC-ES reports. That creates confusion when an owner or inspector has to decide which rating actually applies to a balcony, roof deck, or walking surface over occupied space. Answer the three questions below to narrow the answer before you read the details.

1. Is the deck a walking surface over occupied or conditioned space?

If YES, the deck is also a roof. Class A is the answer. A walking surface over occupied space is simultaneously a roof (CBC Chapter 15 and §705A / CWUIC §504.2) and a deck (CBC §709A / CWUIC §504.7.3). Both sets of rules apply. You can't pass one by passing the other, and the roof side defaults to Class A over any occupied space.

“Occupied or conditioned” includes: apartments, condos, single-family living space, retail, offices, storage with occupancy, parking, and any habitable or mechanically conditioned space below the deck. If in doubt, treat it as occupied.

2. Is the property in a designated Fire Hazard Severity Zone (FHSZ)?

Check the OSFM FHSZ viewer for your parcel. Mandatory WUI construction is generally triggered in State Responsibility Area (SRA) Moderate, High, or Very High zones and Local Responsibility Area (LRA) Very High zones. In FHSZ, §705A / CWUIC §504.2 requires Class A roof covering regardless of whether the deck covers occupied space. Local jurisdictions may extend further.

3. Does your local jurisdiction enforce WUI code (or apply its own amendments)?

Cities and counties adopt California Building Code with local amendments. Los Angeles, for example, enforces WUI requirements as adopted with LADBS plan-check conditions. Confirm current amendments at plan-check before specifying. Don't assume state baseline applies; don't assume a local amendment is stricter without verifying.

Quick verdict

Class A requiredAny deck over occupied or conditioned space. The deck is also a roof, and roof coverings over occupied space default to Class A per CBC Chapter 15 + §705A / CWUIC §504.2. The walking surface ALSO has to meet a separate walking-deck path under CBC §709A / CWUIC §504.7.3 / CRC R337.9. Two decisions, both apply. You can't pass one by passing the other.

Class A requiredProperty is in a FHSZ (any deck, any use). §705A / CWUIC §504.2 triggers Class A for the roof side regardless of what's below.

Class B may be acceptableLimited case: ground-level patio or deck over strictly unconditioned / unoccupied space (e.g., open landscape, crawl space only), outside any FHSZ, and the construction type permits Class B or C under CBC Chapter 15 Table 1505.1. Always verify with the AHJ. Local amendments frequently upgrade the baseline, and a “Class B acceptable” reading is brittle.

Check with AHJMixed triggers, boundary FHSZ parcels, recently reclassified zones (CAL FIRE updated SRA maps effective April 1, 2024 and LRA recommended maps in Feb–Mar 2025), or multifamily projects straddling SB 721 / SB 326 compliance scope. Default to Class A when in doubt. It's the safer spec and virtually never rejected at plan-check.

Why Two Ratings Exist

The fire performance of an exterior assembly is tested two different ways, and the code requires both on decks over occupied space. Understanding the split is the difference between specifying a code-compliant system and buying a product that looks compliant on its data sheet.

Roof-covering fire tests (E108 / UL 790)

ASTM E108 and UL 790 are parallel standards that evaluate roof coverings under simulated exterior fire. They produce Class A, B, or C ratings based on how an assembly resists spread of flame, burning brand exposure, and intermittent flame exposure from above. This is the classic “Class A roof” test, developed in the mid-20th century for roof shingles, membranes, and tiles. It is the rating referenced by CBC Chapter 15, CBC §705A, and California Wildland-Urban Interface Code (CWUIC) §504.2.

Ember-resistant construction tests (E2886, E2632, E2726)

Modern California WUI research established that the dominant ignition pathway in wildfires is ember intrusion, not radiant heat or direct flame. A separate family of ASTM tests addresses that: E2886 (vents), E2632 (under-deck fire exposure), E2726 (ignition under top-down ember exposure), and E3118 (deck/railing ember exposure). These are not interchangeable with E108 / UL 790 Class A. A roof-covering Class A does not mean the assembly passes the deck ember tests, and passing the ember tests does not mean the assembly passes as a roof covering.

A walking surface over occupied space is both a roof and a deck. Full code compliance requires documented test results under both regimes: E108 or UL 790 for the roof side, and a compliance path from CBC §709A / CWUIC §504.7.3 for the walking side (which typically means E2632 + E2726).

Inside the E108 / UL 790 Test

ASTM E108 and UL 790 share test protocols. An assembly is built on a standardized test deck and subjected to a sequence of controlled fire exposures. Pass criteria set the boundary between Class A (most severe), B (moderate), and C (light). The three tests most often cited in ICC-ES reports are spread of flame, burning brand, and intermittent flame.

Test	Class A	Class B	Class C
Spread of flame (max distance)	≤ 6 ft	≤ 8 ft	≤ 13 ft
Burning brand (size / weight)	~2,000 g, 12" × 12" × 2.25"	Two 500 g brands, 6" × 6" × 2.25"	Twenty ~9.25 g pieces
Intermittent flame (cycles)	15 cycles	8 cycles	3 cycles
Burner temperature	1,400°F ± 50	1,400°F ± 50	1,300°F ± 50
Wind simulation	12 mph across all three classes

Numeric values above reflect ASTM E108 (current revision) as cited by published industry references. Always verify against the live ASTM E108-20a standard for contract-accurate numbers: store.astm.org/e0108-20a.html.

Slope calibration gotcha

E108 / UL 790 is calibrated at a test slope of 5 inches per horizontal foot. Severity of test exposure decreases as slope decreases below 5"/ft, which means a steep-slope Class A listing is not automatically valid at low slope or dead-level applications. The ICC-ES report or listing must explicitly cover the slope range you're building at. A low-slope roof deck (the most common balcony / roof-deck geometry) needs an assembly listed at low slope specifically. Don't let a steep-slope listing substitute.

Class A vs B vs C: Side by Side

The descriptive names (“severe,” “moderate,” “light”) map to the test exposures above. Practical assembly examples vary by manufacturer, so the table below sketches the general pattern. Always defer to the specific ICC-ES report for any given product.

Class	Effective against	Typical roof covering examples	Typical walking deck assembly pattern
A	Severe fire test exposure	Clay or concrete tile, slate, metal roofing over fire-rated deck, listed Class A asphalt shingles, rated membranes over noncombustible deck	Non-floating, multi-layer: stapled metal lath over plywood + cementitious underlayment + fiberglass mat + acrylic base / texture / top coats
B	Moderate fire test exposure	Some listed asphalt shingles, certain modified bitumen assemblies	Non-floating liquid-applied: cementitious texture layer + fiberglass mat + acrylic base / texture / top coats (no metal lath)
C	Light fire test exposure	Basic asphalt shingles, wood shakes (treated), untreated roll roofing (varies by listing)	Simpler liquid-applied coatings without cementitious underlayment

On walking deck assemblies, “non-floating” means the waterproofing and walking surface are bonded to the substrate as a monolithic assembly. “Floating” means loose-laid or edge-fastened with a separate walking surface (pavers, tile, raised decking) on top. See the Deck Waterproofing Guide for the full taxonomy.

When Code Requires Each Class

Two separate code tracks set the minimum class. The CBC Chapter 15 baseline applies to every project as a function of construction type. The WUI overlay adds Class A as a floor anywhere in a designated fire hazard zone regardless of construction type. On top of both, any deck that also serves as a roof over occupied space defaults to Class A in practice. The Chapter 15 baseline is rarely the ceiling for this assembly type.

Baseline: CBC Chapter 15 Table 1505.1

CBC §1505.1 requires roof assemblies to be tested to ASTM E108 or UL 790, with minimum class set by construction type.

Construction type	Minimum roof class
Type IA, IB, IIA, IIIA, IV	Class B
Type IIB, IIIB, VA, VB	Class C

Chapter 15 alone permits Class B or C for some construction types. In practice, for roof decks over occupied space, the effective spec floor is Class A, driven by the WUI overlay (most multifamily property in California sits in or adjacent to a Fire Hazard Severity Zone), local amendments (LA and most municipalities of size), architect/insurer spec, and plan-check conservatism. Treat the Chapter 15 baseline as a lower bound, not a target.

WUI overlay: §705A (2022 cycle) or CWUIC §504.2 (2025 cycle)

For projects in a Fire Hazard Severity Zone, the WUI provisions require Class A. In the 2022 California Building Code, this lives in CBC Chapter 7A §705A.2: “Roof assemblies in the Fire Hazard Severity Zones shall be Class A rating when tested in accordance with ASTM E108 or UL 790.”

For the 2025 code cycle (effective January 1, 2026), the California Building Standards Commission moved the WUI provisions out of CBC Chapter 7A and into the new California Wildland-Urban Interface Code (CWUIC), Title 24 Part 7. The roof-class requirement is now at CWUIC §504.2; the deck walking-surface requirement is at CWUIC §504.7.3. (Specific 2025 section numbering should be re-verified at the UpCodes 2025 CWUIC source. These sections were still being finalized during the adoption window.)

Projects permitted under the 2022 cycle continue to cite CBC §705A / §709A; projects permitted under the 2025 cycle cite CWUIC §504.2 / §504.7.3. Both cycles will be in play through 2026 on overlapping project timelines.

Residential WUI: CRC R337

The California Residential Code mirrors the WUI provisions at R337 (Materials and Construction Methods for Exterior Wildfire Exposure). R337.5 mirrors CBC §705A (Class A roof in FHSZ) and R337.9 mirrors §709A (deck walking surfaces). For single-family homes (CRC Group R-3), these are the applicable citations. Note that R337 is not R327. R327 is a separate residential chapter on fall-prevention / aging-in-place and has nothing to do with WUI.

By Occupancy Type

The code citations change depending on the occupancy group. This matters for plan-check submittals. Getting the citation right the first time avoids back-and-forth.

Occupancy	Applicable code path	Roof class trigger	Deck walking-surface trigger
R-2 (multi-family, apartments, condos)	CBC Ch 15 + Ch 7A (2022) / CWUIC (2025)	Class B baseline; Class A in FHSZ via §705A / CWUIC §504.2	§709A / CWUIC §504.7.3 compliance path (e.g., E2632 + E2726)
R-3 (single-family residential)	CRC R337 (Residential WUI)	R337.5 → Class A in FHSZ	R337.9 compliance path (mirrors §709A)
B (office), M (retail), A (assembly)	CBC Ch 15 + Ch 7A (2022) / CWUIC (2025)	Class B/C by construction type; Class A in FHSZ	§709A / CWUIC §504.7.3 if walking surface exists
Parking structure (S-2 open)	CBC Ch 15 + Ch 4 / Ch 7A (if in FHSZ)	Class B/C by construction type; Class A in FHSZ	Podium decks over occupied space trigger §709A / CWUIC §504.7.3

The Walking-Deck Twist

This is the most commonly missed detail on plan-check submittals: a Class A roof covering does not, on its own, satisfy the walking-deck fire rules in CBC §709A, CWUIC §504.7.3, or CRC R337.9.

§709A.3 lists several compliance paths for the walking surface material of decks, porches, balconies, and stairs. The most frequently specified path is:

Material tested to ASTM E2632 (under-deck fire exposure) and ASTM E2726 (ignition under top-down ember exposure)
Ignition-resistant material per §704A.3
Material passing SFM Standard 12-7A-4 or 12-7A-4A
Exterior fire-retardant-treated wood
Noncombustible material

“Listed to ASTM E108 or UL 790 as a Class A roof covering” is not on that list. The two test regimes measure different hazards: top-of-roof flame exposure for E108 / UL 790, versus under-deck exposure and top-down ember ignition for E2632 / E2726.

What this means in practice

A walking surface over occupied space in a FHSZ needs an assembly with documented test results under both regimes: E108 or UL 790 (for the roof-class requirement) and E2632 + E2726 (for the deck walking-surface requirement). The cleanest way to document this is a single ICC-ES evaluation report that references both test methods against the same assembly. Otherwise you need two stacked listings and a plan-check reviewer willing to read them together.

Reading an ICC-ES ESR's Fire Classification Section (Contractor Checklist)

ICC-ES Evaluation Reports (ESRs) are the authoritative document for plan check. Every ESR covering a walking deck system is evaluated under ICC-ES AC39 (Acceptance Criteria for Walking Decks), which specifies the fire, wind, waterproofing, and durability data an ESR must document. When you download an ESR, walk through this checklist before citing it on a submittal.

The five items every ESR Fire Classification section must answer

Which test methods were used: ASTM E108 or UL 790 for roof class; ASTM E2632 and E2726 for deck walking-surface ember performance. A compliant walking deck over occupied space typically cites all three.
Which class was achieved: A, B, or C, stated explicitly and tied to the test method (don't let marketing "Class A" stand in for a formal listing).
Which assembly was tested: the full component stack from substrate up, including specific layer chemistries, coverage rates in gal/sq or mils, and fabric/mesh products. A substituted layer can invalidate the listing.
Substrate limitations: minimum plywood thickness, concrete prep, insulation compatibility. A system tested over 5/8" plywood is not automatically valid over 1/2" plywood or metal deck.
Slope limitations: steep-slope or low-slope listings are not interchangeable (see §3 above). Confirm the slope range tested covers your project geometry.

Worked example: ESR-3672 (Deck Flex)

A single ESR can document multiple rated assemblies. ICC-ES ESR-3672 is a useful example because it covers two Deck Flex systems under one report. That illustrates why ESR reading requires attention to the specific assembly, not just the product brand.

Assembly	Fire class over plywood	System architecture	Installed thickness
Deck Flex W.M.	Class A	Non-floating: stapled metal lath + cementitious underlayment + fiberglass mat + base / texture / top coats (7 components)	~1/4"
Deck Flex W.F.	Class B	Non-floating: cementitious texture layer + fiberglass mat + base / texture / top coats (6 components, no metal lath)	~3/16"

Same manufacturer, same ESR, two different fire classifications. A plan-check reviewer looking for a Class A listing will not accept a W.F. submittal even though both products are under ESR-3672. The class is a property of the specific assembly, not the brand. Pull the ESR directly from icc-es.org/report-listing/esr-3672/ and cite the specific W.M. or W.F. assembly on your submittal.

Submittal packet checklist

ESR PDF (current revision), full document, not a marketing excerpt
ICC-ES public listing page URL for the assembly
Specific assembly identifier (e.g., "W.M. Class A per ESR-3672") with layer stack matching the ESR
Test method citations: E108 / UL 790 for roof class; E2632 / E2726 for deck walking surface where applicable
Substrate description (plywood thickness, concrete prep) matching the ESR conditions of use
Slope declaration matching the ESR slope range
Coverage rate per layer matching the ESR
Applicator qualification documentation if the ESR requires a manufacturer-approved installer

Need Class A documentation for your WUI project?

ICC-ES ESR-referenced Class A non-floating walking deck systems. Full assembly documentation for plan-check. Typical turnaround: one business day.

or call 714-248-6555 · email partners@usmadesupply.com

California-Specific Overlays

California layers several overlapping regulatory frameworks on top of the base CBC fire requirements. These overlays decide when the Class A roof / walking-deck combination becomes mandatory regardless of construction type.

CBC Chapter 7A → CWUIC Chapter 5 (code cycle transition)

The 2022 CBC places WUI provisions in Chapter 7A (sections 704A–710A). Effective January 1, 2026 (2025 code cycle), Chapter 7A is retired and the provisions move into the new California Wildland-Urban Interface Code (CWUIC), Title 24 Part 7. Roof-class requirement moves from §705A to CWUIC §504.2; deck walking-surface requirement moves from §709A to CWUIC §504.7.3. Both cycles coexist in practice: projects permitted under 2022 cite the §705A / §709A sections; projects permitted under 2025 cite the CWUIC sections. Confirm code cycle at plan-check intake.

CAL FIRE Building Materials Listing (BML)

The Office of the State Fire Marshal (OSFM) maintains the Building Materials Listing program, which publishes materials tested to California SFM standards (including 12-7A-4 / 12-7A-4A for walking decks). When a manufacturer cites CAL FIRE BML in addition to ICC-ES, they are documenting compliance against the state-specific SFM standards. Some jurisdictions accept either; some require both. Check with the AHJ.

LA City amendments

Los Angeles enforces CBC / CRC / CWUIC as adopted, with LADBS plan-check conditions. Confirm current LADBS amendments at plan-check intake rather than assuming state baseline. LA's FHSZ Supplemental Checklist has been revised more than once and specific documentation requirements (including ESR citations and assembly layer stacks) evolve.

Updated FHSZ maps (2024–2025)

OSFM adopted updated State Responsibility Area FHSZ maps effective April 1, 2024, and released recommended Local Responsibility Area maps Feb–Mar 2025. A parcel that was not in an FHSZ under prior maps may now be included, which pulls it into the Class A roof and §709A / CWUIC §504.7.3 deck requirements. Re-check osfm.fire.ca.gov for any project where you previously relied on older mapping.

PRC §4291 defensible space (separate from construction rating)

Public Resources Code §4291 sets vegetation clearance requirements around structures in the SRA. Defensible space is enforced separately from construction rating. It does not substitute for Class A roof / deck walking-surface compliance, but failure to maintain it can invalidate the benefit of a compliant assembly in a wildfire scenario. See the Defensible Space & Exterior Hardening guide for the details.

Reference links (primary sources)

Frequently Asked Questions

Is a Class A roof covering the same as a Class A walking deck?

No. Class A refers to performance under ASTM E108 or UL 790, a test regime developed for roof coverings. A walking deck over occupied space must also satisfy the separate walking-deck fire rules in CBC §709A (or CWUIC §504.7.3 under the 2025 cycle, or CRC R337.9 for single-family), which typically means testing to ASTM E2632 and ASTM E2726. Full code compliance on a roof deck requires both sets of test results against the same assembly.

Can I upgrade a Class B deck to Class A by adding a topcoat?

Generally no. Fire classification is a property of the complete tested assembly, not a coating on top of something else. Adding a layer not evaluated under the original ESR invalidates the listing rather than improving it. If you need Class A, you install a system specifically listed as Class A. That usually means stripping the existing assembly to substrate and rebuilding the full layer stack per the ESR.

Does a Class A rating cost more than Class B?

Yes, generally. Class A walking deck assemblies carry additional layers (typically stapled metal lath plus a thicker cementitious underlayment) that a Class B liquid-applied assembly doesn't include. Labor is heavier and the installed thickness and weight increase. It is a one-time spec decision with insurance and code implications, so the comparison isn't material cost per square foot alone; it is total lifecycle cost including plan-check approval and long-term warranty.

Who signs off that the installed assembly matches the ESR?

The building inspector during framing and final inspection. Some ESRs require a manufacturer-approved applicator as a condition of the listing; in those cases the manufacturer issues a project-specific completion letter confirming the assembly was installed per the ESR. Keep the ESR, the installer's qualification documentation, and the manufacturer completion letter in the project file. An insurance claim or re-sale inspection years later may need them.

Does a HOA-mandated roof warranty cover the fire rating?

A manufacturer product warranty covers material defects; it does not certify that your installed assembly achieved a particular fire class. The class is documented by the ESR, not by the warranty. If your HOA governing document references fire rating, you satisfy it with the ESR citation on file, not with the warranty certificate. Keep both.

My ESR shows a Class A listing at 3"/ft slope. Can I use it on my 1/4"/ft roof deck?

Only if the ESR explicitly covers the lower slope. E108 / UL 790 is calibrated at 5"/ft test slope; severity of test exposure decreases as slope decreases, so a steep-slope listing is not automatically valid at low slope. Many walking deck ESRs are listed at low slope (1/4"/ft to 2"/ft) specifically for balcony and roof-deck applications. Read the slope range in the ESR conditions of use before specifying.

If my property is outside every FHSZ, can I use Class C?

Potentially, subject to CBC Chapter 15 Table 1505.1 (Class C is allowed for Type IIB / IIIB / VA / VB construction) and any local amendments. For a walking deck over occupied space, the walking-surface fire rules may still apply independently. Check §709A or your jurisdiction's equivalent. Most commercial and multi-family projects end up at Class B minimum either way.

Where do I look up the ESR for a given product?

Start at icc-es.org/reports-directory and search by manufacturer or ESR number. The public listing page links to the current ESR PDF. Download the PDF directly from ICC-ES rather than from a manufacturer marketing page. Revisions update on ICC-ES first and outdated PDFs still circulate elsewhere.