ASTM E84 Fire Ratings for FRP Grating: Class 1, Class 2 and Smoke
ASTM E84 is the most cited fire test in FRP grating specifications, and the most incompletely cited. Here is what it actually is.
How the test works
A 7.6 m (25 ft) specimen forms the ceiling of a tunnel furnace. An ignition source runs at one end with controlled airflow; instruments record how far flames travel relative to two calibration materials — red oak flooring (index 100) and inorganic cement board (index 0). Two numbers come out:
- Flame Spread Index (FSI) — 0–100+ scale against the oak/cement calibration
- Smoke Developed Index (SDI) — light obscuration on the same 0–100 scale, often reported 0–450+
The classes
| Class (aka) | FSI requirement | Typical grating systems |
|---|---|---|
| Class A / Class 1 | FSI ≤ 25 | Fire-retardant vinyl ester; phenolic |
| Class B / Class 2 | FSI 26–75 | FR-additive ISO polyester |
| Class C / Class 3 | FSI 76–200 | Standard polyester |
The smoke gap — and why it matters
“Class 1” constrains flame spread only. Vinyl ester FR panels reach FSI ≤ 25 with SDI commonly 450–750; phenolic reaches the same FSI with SDI of 10–50. In occupied escape routes, tunnels and offshore, that smoke difference is decisive — which is why serious specs carry an explicit SDI limit (≤ 450 is common; offshore often much lower).
What to demand on the certificate
- FSI and SDI, for the delivered product's resin and thickness
- Test lab identity and report number — verify it exists
- Batch-level certification for project-critical orders, or third-party witness testing where the spec allows
One more honesty note: E84 measures surface flame spread, not structural fire endurance. FRP grating is not a fire-rated floor structure; it is a walkway with code-governed surface burning behavior. Specs that ask E84 to answer structural questions are misusing the test.