FIRE PERFORMANCE STANDARDS
Composite Decking Fire Rating: Class A and WUI Code
ASTM E84 is a test method, not a certification. The composite decking fire rating ASTM E84 produces is a flame spread index and a smoke developed index, and the building code turns the flame spread index into a class — Class A from 0 to 25, Class B from 26 to 75, Class C from 76 to 200.
A Class A rating does not satisfy a wildland-urban interface code, and a WUI listing does not require Class A. They are separate tests, and a fire-zone project usually needs the second one.
SCOPE OF THE TEST
What ASTM E84 Actually Measures
The standard’s own title says what it is: Standard Test Method for Surface Burning Characteristics of Building Materials.
01
The Steiner Tunnel
The test runs in the Steiner tunnel — a 7.62 m (25 ft) horizontal tunnel lined with refractory brick, with a gas burner at one end. The specimen is mounted to the underside of the tunnel’s removable top.
02
The Same Method, Several Names
ASTM E84 is the same method published as NFPA 255, UL 723, and CAN/ULC S102. A report quoting any of those designations describes the same tunnel test, not a different one.
03
Comparative, Not Absolute
Results are comparative, not absolute. Two references set the scale: inorganic reinforced cement board at 0, and select red oak flooring at 100. Values are rounded to the nearest multiple of 5.
04
The Test Assigns No Class
The test produces indices; the building code turns those indices into a class. A laboratory report never states “Class A” as its own finding, and a supplier that presents it that way has added a step the standard does not contain.
05
Surface Burning, Not Fire Resistance
It measures surface burning only, not fire resistance. It also evaluates a horizontal specimen, and was never intended as authority for vertical materials. Reading it as a fire-resistance rating is a scope error.
FIRE RATING REFERENCE
Flame Spread, Smoke Developed, and the Class Table
The test produces two indices. Flame spread is how far and how quickly flame travels along the surface. Smoke developed is derived from how much a light beam across the exhaust duct is attenuated.
HOW TO READ THIS TABLE
The code turns the flame spread index into a class. Smoke developed has its own limit within each class, and a product can carry a Class A flame spread index and still be questioned on smoke.
CLASSIFICATION REFERENCE
Flame Spread Class and Smoke Limits
Three Classes
| Code Class | Flame Spread Index | Smoke Developed Index |
|---|---|---|
| Class A (I) | 0–25 | 0–450 |
| Class B (II) | 26–75 | 0–450 |
| Class C (III) | 76–200 | 0–450 |
01
Above 200 There Is No Class
A flame spread index above 200 falls outside the classification table entirely. The code recognizes three classes, and a product that exceeds the Class C range carries none of them.
02
Class A Flame Spread, Questionable Smoke
A product can carry a Class A flame spread index and still be questioned on smoke. The class table allows a smoke developed index up to 450, but some applications impose a tighter limit.
BEYOND CLASS A
When a Project Requires More Than Class A
Some applications require more than Class A. Materials in ducts and air plenums commonly have to meet a 25/50 requirement — flame spread no greater than 25 and smoke developed no greater than 50.
| Requirement | Limits |
|---|---|
| Class A | Flame spread 0–25, smoke 0–450 |
| 25/50 requirement | Flame spread ≤ 25, smoke ≤ 50 |
| Where it applies | Ducts and air plenums |
THREE TIERS
Class A
Flame spread 0–25 with smoke up to 450.
Class B and C
Flame spread up to 75, then up to 200.
25/50 Requirement
Flame spread ≤ 25 and smoke ≤ 50, applied in ducts and plenums.
Send us the project location and the fire requirement — we will confirm which fire documentation we hold within 12 hours.
WUI CODE REQUIREMENTS
WUI Codes: What a Fire-Zone Deck Actually Requires
A wildland-urban interface code does not ask for a single number. Under the International Wildland-Urban Interface Code, the deck provision for plastic composite boards is a two-part requirement.
WHAT THE CODE ASKS
A Two-Part Requirement
Both are needed. A fire result alone does not qualify a board — see the ASTM D7032 decking requirements for the first step.
What It Requires
ACCEPTED TEST ROUTES
The Fire-Test Routes
Route 4.1 runs weathering per ASTM D7032, then ASTM E1354 at 35 kW/m² radiant heat flux with an electric spark igniter, then ASTM E2632/E2632M to a peak heat release rate no greater than 25 kW/ft² (269 kW/m²). Route 4.2 uses E2632/E2632M alone to the same threshold.
Test Notes
Why the Local Ordinance Decides
A State List Is a Starting Point, Not an Approval
ASTM E2632 is an under-deck test: an assembly built to the manufacturer’s instructions, exposed to a burner for 3 minutes, then evaluated for 40 minutes. Walking surfaces may alternatively comply with E2632 plus an ASTM E84 Class B flame spread, with a protection condition on the wall above — this route accepts Class B, the clearest illustration that Class A and a WUI approval are not the same claim. Two published examples show how far a local authority can narrow a state list: Austin lists a Class A PVC composite deck as permitted in Zones B and C but not Zone A. Portola Valley removed three of California’s state options, excluding every composite material that would otherwise have qualified. The authority that matters is the one where the project is built. CAL FIRE/OSFM maintains the state listed products handbook under Category 8110 — and it is still the city or county that decides. Request the decking technical data sheet and the fire test documentation set.
VERIFICATION CHECKLIST
How to Check a Fire Rating Claim
This checklist applies to any supplier, and it applies to us. Six questions separate a documentable fire rating from a marketing statement, and none of them require a fire engineering background to ask.
PART 01
Test Method and Values
Ask which test method was used, and to which edition. A report should state both the flame spread index and the smoke developed index — not only a class name. The class is derived by the code, not by the laboratory.
Which test method — ASTM E84, and to which edition?
The flame spread index and the smoke developed
index, not only a class name.
Whether it is a surface burning characteristic or
a fire-resistance rating — different documents
entirely.
A class name alone is not a test result. The two indices are what the report actually contains.
PART 02
Laboratory and Product
Ask which laboratory carried out the test, and on what date. Then confirm the figure covers your profile, at your thickness and finish. A result issued for a different product does not transfer to the one you are buying.
Which laboratory, and on what date.
Whether the figure covers your profile, at your
thickness and finish.
Whether the report is current for the product
as supplied.
A result issued for a different product or a superseded report does not transfer to the board you are buying.
TWO MORE REQUESTS
Ask for the WUI Listing and the Local Authority’s Position
If the project is in a fire zone: is there a WUI listing, in which jurisdiction, under which protocol? Then ask whether the local authority accepts that listing — state lists are narrowed by cities and counties, and the authority that matters is the one where the project is built. A state listing is a starting point, not an approval.
PROPERTY BOUNDARY
Flame Spread Is Not Fire Resistance
E84 measures how flame travels across a surface. That is the whole of it. Fire resistance is a different property: how long an assembly holds back fire. It is tested, reported, and documented separately.
TWO SEPARATE PROPERTIES
Surface behavior vs. assembly performance
Surface Burning
Assembly Rating
Tested Separately
Documented Separately
What E84 Measures
E84 measures how flame travels across a surface. That is the whole of it. The test produces indices for surface burning characteristics, and nothing about how long a structure holds back fire.
Surface OnlyWhat Fire Resistance Measures
Fire resistance is a different property: how long an assembly holds back fire. It is tested on a complete assembly, reported in time ratings, and documented under a separate standard — not in an E84 report.
Assembly PerformanceWhat a Report Does Not Cover
A published evaluation report for a composite deck board makes the boundary explicit — it places use as a component of a fire-resistance-rated assembly outside the report’s scope.
Outside ScopeWhere the Spec Goes Wrong
The practical risk is a specification that says “Class A” where the code requires a fire-resistance rating. A code official asking for the second will not accept the first — the two documents come from different tests.
Different Documents
THE PRACTICAL RISK
“Class A” Is Not a Fire-Resistance Rating
A specification that says “Class A” where the code requires a fire-resistance rating will not satisfy a code official asking for the second. The two are different properties, tested under different standards, reported in different documents. Confirm which one the project actually requires before quoting a figure.
Wrong Document
→ rejected at review
JOIST SPACING FAQ
Frequently Asked Questions
Common questions about WPC decking joist spacing, joist span, diagonal layouts, butt joints, and subframe materials.
The correct spacing depends on the board profile, load case, and installation conditions. Published manufacturer guides commonly reference 400 mm (16 in) centers for residential decks and 300 mm (12 in) centers for commercial decks, stairs, and some diagonal layouts. These are reference figures rather than a universal rule. Always confirm the maximum spacing specified for the actual decking profile before framing.
Joist spacing is the center-to-center distance between adjacent joists and is governed by the decking board. Joist span is the distance a single joist travels between its own supports and is governed by the joist section, material, and structural load. They are two separate calculations — a deck can have correct spacing while still resting on an undersized joist.
Yes. When decking boards cross the joists at an angle, the distance between bearing points along each board increases. The joists therefore need to move closer together compared with a perpendicular installation. The exact adjustment depends on the board profile and installation angle, so use the spacing requirement specified for the selected decking system.
Yes. Where two decking boards meet end to end, both board ends need independent support. Two joists are therefore used at the butt joint. The paired joists should not be fixed hard against each other; the subframe detail must retain the required movement allowance for the selected decking system.
Yes. Timber joists can be used with WPC decking, and many published decking spacing charts assume a timber subframe. WPC and aluminum joists have different stiffness characteristics, however, so do not assume that the same spacing automatically applies to every joist material. Confirm which subframe material the spacing specification was developed for.