ASTM E84 Flame Spread Testing
ASTM E84, also published as UL 723, measures surface burning characteristics of building materials in a 25-foot tunnel. Results are reported as a flame spread index and smoke developed index.
ASTM E84, “Standard Test Method for Surface Burning Characteristics of Building Materials,” is the most widely cited fire test for interior finishes and many coatings in North America. It is also published as UL 723 and is closely related to NFPA 255 (withdrawn) and CAN/ULC-S102 in Canada. The test measures how far and how fast flame spreads across the surface of a material and how much smoke it produces. Building codes use the results to classify interior finishes.
The tunnel test
The apparatus is often called the Steiner tunnel. It is a horizontal chamber about 25 ft (7.6 m) long and about 17.5 in wide, with windows along one side.
- The test specimen is mounted on the ceiling of the tunnel, facing down.
- A gas burner at one end applies a controlled flame to the specimen.
- A controlled draft draws flame and combustion gases along the tunnel.
- Observers record the progress of the flame front along the specimen over a 10-minute test.
- A photometer in the exhaust duct measures smoke obscuration.
The results are compared with two reference materials: inorganic reinforced cement board, assigned a flame spread of 0, and red oak flooring, assigned 100.
Results
Flame spread index (FSI)
A number calculated from the distance and time of flame travel. Lower numbers mean slower flame spread.
Smoke developed index (SDI)
A number calculated from the area under the smoke obscuration curve, again relative to the red oak reference.
Classification
Model building codes, such as the International Building Code, group results into classes:
| Class | Flame spread index | Smoke developed index |
|---|---|---|
| A (or I) | 0 to 25 | 450 or less |
| B (or II) | 26 to 75 | 450 or less |
| C (or III) | 76 to 200 | 450 or less |
Required classes depend on occupancy, location in the building and whether sprinklers are present. The authority having jurisdiction decides what is acceptable for a project.
Testing coatings
Coatings are usually tested as applied to a specified substrate, at a specified thickness. Important points:
- Substrate matters. A coating tested on cement board may give a different result from the same coating on wood or foam. The report should state the substrate.
- Thickness matters. Results apply to the tested thickness or range. Heavier films can contain more fuel.
- Systems, not products. A primer, basecoat and topcoat combination may perform differently from any single product.
- Reports and listings. Read the full test report or listing, not just a summary class. Check the date and the laboratory.
Fire-retardant coatings
Some coatings are formulated to reduce flame spread on combustible substrates such as wood. They are distinct from intumescent coatings used to protect structural steel, which are evaluated by fire resistance tests such as ASTM E119 and UL 263. See also fireproofing structural steel.
Coatings over foam plastics
Building codes usually require a thermal barrier or an approved alternative over foam plastic insulation in occupied spaces. Coatings marketed as alternatives to a thermal barrier are evaluated with additional tests such as NFPA 286 or UL 1715, in addition to E84. Only rely on evaluations that cover the specific foam, coating and thickness. See coating EPS foam and spray foam roofing.
Limitations of ASTM E84
- The specimen is mounted on the ceiling, which suits rigid boards. Materials that melt, drip or fall away may be hard to evaluate fairly.
- It measures surface burning only, not fire resistance or structural performance.
- It does not measure toxicity of smoke.
- Results may not predict behavior in real room fires, which is why room-corner tests exist for some uses.
Related tests
- ASTM E119 / UL 263. Fire resistance of building assemblies.
- NFPA 286 and UL 1715. Room-corner fire tests.
- ASTM E108 / UL 790. Fire tests of roof coverings, which classify roof systems including coatings.
- ASTM E162. Radiant panel surface flammability, used in transportation.
- ISO 5660 cone calorimeter. Heat release rate on small samples.
Using E84 data in specifications
- Name the required class and the test standard.
- Require a test report or listing covering the substrate, system and thickness to be used.
- Confirm with the authority having jurisdiction.
- Do not claim a class for a system that has not been tested as such.
Key terms
- Steiner tunnel. The 25-foot horizontal test chamber used in ASTM E84 and UL 723.
- Flame spread index (FSI). A number comparing flame travel against reference materials, with inorganic reinforced cement board at 0 and red oak at 100.
- Smoke-developed index (SDI). A number comparing smoke produced by the sample.
- Class A, B and C. Common code classifications based on FSI ranges, with Class A the lowest flame spread.
- Substrate dependence. Coating results are tied to the substrate tested; a rating on cement board does not automatically transfer to wood.
Frequently asked questions
Does a Class A rating mean a coating is fireproof?
No. It means the tested assembly showed low flame spread and smoke in this test. It says nothing about fire resistance duration.
Can a coating improve a substrate’s E84 class?
Some fire-retardant coatings are designed to do so on specific substrates. Only a test of the coated substrate can show it.
Is UL 723 different from ASTM E84?
They are essentially the same test published by different organizations.
Related entries
- Intumescent coatings
- Fireproofing structural steel
- Fire and explosion hazards
- ASTM standards for coatings
- Coating EPS foam (new)
- Flash point of coatings (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
