Fire Ratings & Flame Spread
The Surface-Burning Test Everyone References
ASTM E84 (also adopted as NFPA 255 and UL 723, the Steiner tunnel test) measures two numbers as a material burns in a tunnel: the flame spread index (FSI) and the smoke developed index (SDI).
| Class | Flame Spread Index (FSI) | Smoke Developed (SDI) | Typical Use |
|---|---|---|---|
| Class A (I) | 0 – 25 | 0 – 450 | Exit corridors, tunnels, high-occupancy, offshore fire zones |
| Class B (II) | 26 – 75 | 0 – 450 | Many industrial platforms, general occupancy |
| Class C (III) | 76 – 200 | 0 – 450 | Lower-risk, less code-driven areas |
Class boundaries are relative to asbestos-cement board (0) and red oak (100). Confirm the class your local code demands before ordering.
Phenolic vs. Fire-Retardant Polyester
| Property | Fire-Retardant Polyester / Vinyl Ester | Phenolic |
|---|---|---|
| Flame spread | Meets Class B; Class A with FR formulation | Inherently low flame spread, Class A |
| Smoke density | Moderate smoke when burning | Very low smoke, low toxicity |
| Smoke toxicity | Higher (combustible resin) | Low — preferred for escape routes |
| Surface finish / cost | Good surface, lower cost | Rougher surface, higher cost |
| Best for | General industrial fire-code service | Tunnels, offshore, enclosed spaces, escape routes |
Beyond the Rating
FM Approval
For some offshore and industrial insurance-driven applications, Factory Mutual (fmGlobal) approval of the grating system may be required — specify FM-approved resin and construction.
Submit Evidence
Ask for the actual ASTM E84 test report (not a marketing claim). It states FSI, SDI, product identity and the testing laboratory.
How the E84 Tunnel Test Works
ASTM E84 (the Steiner tunnel test, also adopted as NFPA 255 and UL 723) exposes a 7.3 m material sample in a horizontal tunnel to a controlled flame source and measures two outputs as the flame front advances: the flame spread index (FSI), a relative scale where asbestos-cement board scores 0 and red oak scores 100, and the smoke developed index (SDI), capped at 450 for classification purposes. The result is a material property of the tested panel — resin, glass content and surface all contribute — so a Class A certificate applies to the tested configuration, and changes in thickness or resin should be verified, not assumed.
Resin System vs. Class
The class a panel achieves depends on the whole formulation, not the resin family name. Fire-retardant polyester and vinyl ester grades reach Class B as standard and Class A with FR formulations; phenolic is inherently low flame-spread and low-smoke, which is why tunnel codes and escape routes specify it. The practical decision sequence: read your code's class requirement, confirm the panel grade achieves that class in the tested configuration, and check that the smoke-developed value also meets the project limit — smoke is often the binding constraint in enclosed routes.
Choosing the Right Grade
| Service | Typical Requirement | Recommended Grade |
|---|---|---|
| General industrial platforms | Class B or C per local code | Fire-retardant polyester / vinyl ester |
| Exit corridors, high-occupancy, tunnels | Class A, low smoke | Phenolic |
| Offshore escape routes | Class A/B, low smoke, non-sparking | FR vinyl ester or phenolic |
| Indoor non-code areas | No specific class | Standard polyester |
Confirm with the AHJ (authority having jurisdiction) or project code which class applies to your walkway, mezzanine or stair — then confirm the tested class of the exact panel grade on your order. ZeAllgrate provides E84 test reports for its FR and phenolic grades on request.
Fire-Retardant Additives and Real-World Behaviour
Fire-retardant formulations add halogenated or antimony-based chemistry that suppresses flame spread, but they can reduce some mechanical properties and lighten UV tolerance — the reason FR grades are not automatically the right choice everywhere. Where fire class is not required, standard polyester gives the best balance of cost, strength and weathering. Where it is required, specify the tested class and accept the small performance trade-off. The engineering point: fire rating is one axis of selection, applied after the chemical and load axes, never instead of them.
Fire-retardant formulations add halogenated or antimony-based chemistry that suppresses flame spread, but they can reduce some mechanical properties and lighten UV tolerance — the reason FR grades are not automatically the right choice everywhere. Where fire class is not required, standard polyester gives the best balance of cost, strength and weathering. Where it is required, specify the tested class and accept the small performance trade-off. The engineering point: fire rating is one axis of selection, applied after the chemical and load axes, never instead of them.Need a Class A / Phenolic Quote?
Tell us the code and occupancy — we match the resin and supply the E84 test report.
Source: ZeAllgrate (Fiberglass Grating Manufacturers Council / ACMA) design guidelines; ASTM International; ISO 14122; OSHA 29 CFR 1910. Indicative data for specification guidance only — final design verified by ZeAllgrate engineering against certified load/span charts.