Fibre Reinforced Facade Encyclopedia: Testing and standards
The flame spread index is the number produced by the ASTM E84 tunnel test describing how far and how fast flame travels across the surface of a material, on a scale anchored at zero for inorganic reinforced cement board and 100 for select grade red oak flooring. It is the primary fire hazard property used to regulate interior finishes in the United States and Canada, and it is the figure behind every Class A, Class B or Class C claim in North American product literature.
Zero and 100 are not theoretical limits. They are two real materials burned in the same tunnel: cement board, which does not propagate flame, and red oak, which does. Everything else is scaled between and beyond them, so an index of 25 means a quarter of the flame travel of oak, not a quarter of some absolute maximum.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Source test | ASTM E84, also published as UL 723 | Steiner tunnel; the method is covered in the ASTM E84 entry |
| Scale anchors | 0 = inorganic reinforced cement board; 100 = select grade red oak | Calibrated at the start of each test series |
| Class A (or Class I) | Flame spread index 0 to 25 | Smoke developed index 450 or less |
| Class B (or Class II) | Flame spread index 26 to 75 | Smoke developed index 450 or less |
| Class C (or Class III) | Flame spread index 76 to 200 | Smoke developed index 450 or less |
| Above 200 | Not classified for regulated interior finish use | |
| Code reference | IBC Chapter 8, interior wall and ceiling finish | Class required varies by occupancy and location |
| Test duration | 10 minutes under a 5.3 MW gas burner | Specimen 7.3 m long, mounted in the tunnel ceiling |
| Australian equivalent | None; AS 1530.3 spread of flame index runs 0 to 10 | Different scale, different apparatus, not convertible |
What the number measures
In the Steiner tunnel the specimen forms the ceiling of a horizontal duct and a gas burner is applied at one end for ten minutes while air is drawn along the tunnel. An observer records the position of the flame front against time. The flame spread index is derived from the area under the flame front distance against time curve, normalised so that the red oak calibration gives 100 and the cement board calibration gives zero.
Because the derivation is an area under a curve, two materials can reach the same maximum flame distance and score very differently. A surface that flashes quickly to three metres and stops scores higher than one that creeps to the same point over ten minutes. The index is a measure of flame travel over time, not of peak reach.
What the thresholds mean in code
The IBC regulates interior wall and ceiling finishes by class, with the class required depending on the occupancy group and where in the building the finish sits. The pattern is consistent: the most demanding locations, vertical exit enclosures and exit passageways, require Class A; exit access corridors typically require Class A or B; rooms and enclosed spaces are allowed the lowest class for that occupancy. Sprinkler protection generally permits one class lower than the unsprinklered requirement.
Every class also carries a smoke developed index limit of 450 or less. A material with a flame spread index of 20 and a smoke developed index of 600 is not Class A; it is not classified at all for that use. The two indices are reported together and applied together, which is covered further in the smoke developed index entry.
The traps
- Thermoplastics fall out of the test. A material that melts and drips away from the tunnel ceiling removes itself from the flame and can return a misleadingly low index. ASTM E84 carries a caution about this, and it is one of the reasons the IBC requires NFPA 285 for combustible components in the exterior walls of Type I to IV construction rather than relying on E84 alone.
- The index says nothing about a facade. E84 is a bench scale interior finish test with a specimen mounted horizontally. Vertical flame spread on an external wall with a ventilated cavity is a different phenomenon, tested at full scale by NFPA 285, BS 8414 or AS 5113.
- The result belongs to the tested build-up. Adhesive, substrate, coating and thickness all change the index. A report for a panel bonded to cement board does not cover the same panel on battens.
- Class numbering is duplicated. Class A, B and C in the IBC are the same as Class I, II and III in NFPA 101. Class 0 and Class 1 in the older British BS 476 system are unrelated, and the NCC group numbers are unrelated again.
- There is no conversion to the Australian scale. AS 1530.3 reports a spread of flame index from 0 to 10 from an entirely different apparatus, and the NCC regulates wall and ceiling linings on group number in any case.
Relevance to composite facade panels
For a fibre reinforced polymer panel a flame spread index is the entry level piece of North American evidence, useful for interior finish approvals and as a screening figure, but on its own insufficient for exterior wall use above the height threshold in IBC Chapter 14. ShapeShift’s published reaction to fire evidence for ShapeShell-RT is Australian, British and European: an AS 5637 Group 1 classification from AWTA Product Testing, a pass of the full-scale BS 8414 and AS 5113 facade test at BRE Global, and an EN 13501-1 classification of B-s2,d0 from Intertek Testing Services. ShapeShell-RT is a polymer composite and is not non-combustible.
Standards and references
- ASTM E84 Standard Test Method for Surface Burning Characteristics of Building Materials; UL 723
- IBC Chapter 8 Interior Finishes, interior wall and ceiling finish classification
- NFPA 101 Life Safety Code, Class I, II and III interior finish
- ASTM E136 for non-combustibility; NFPA 285 for exterior wall assemblies
- AS 1530.3 for the unrelated Australian spread of flame index; AS 5637.1 for group numbers
Related terms
Questions specifiers ask
What is a good flame spread index?
Lower is better. Class A covers 0 to 25 and is required in the most demanding interior locations under the IBC. Above 200 a material is not classified for regulated interior finish use.
Does Class A mean non-combustible?
No. Non-combustibility in the United States is decided by the ASTM E136 furnace test. Many Class A materials are combustible; the flame spread index only describes surface flame travel.
Is flame spread index the same as the Australian spread of flame index?
No. The Australian index from AS 1530.3 runs 0 to 10 from a different apparatus, and the NCC regulates wall and ceiling linings on group number instead. There is no conversion.
Why is a smoke developed index always quoted with it?
Because the IBC classes require both. Each class carries a smoke developed index limit of 450 or less, so a low flame spread index with high smoke fails the classification.
Can a flame spread index be used for external cladding?
Not by itself on a taller building. The IBC requires NFPA 285 testing of the wall assembly where combustible components are used in exterior walls of Type I to IV construction above the height threshold in Chapter 14.
ShapeShift publishes the standard, the result and the testing body behind every ShapeShell fire classification. See ShapeShell fire test data
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.