“Class A” is one of the most common lines in North American specs

“Class A flame spread” appears regularly in North American grating specifications. It comes from the ASTM E84 surface burning test, and building codes turn the resulting index into Class A/B/C. Understanding what E84 measures — and what it does not — is how you judge whether a Class A report applies to a project.

What the standard is

• Current edition: ASTM E84-26a, published 1 April 2026, superseding E84-26;
• Method: 25-ft (about 7.6 m) horizontal tunnel test (Steiner tunnel), 10-minute exposure;
• Specimen: nominal 24 ft × 20 in (7.32 m × 508 mm), mounted in the ceiling position with the exposed face down;
• Calibration: flame traverse of a select-grade red oak specimen in about 5.5 minutes;
• Results: flame spread index (FSI) and smoke developed index (SDI).

Reading the indexes

Both are relative values: inorganic cement board is the usual zero reference and red oak the 100 reference. Building codes generally classify by FSI:

Class	Flame spread index
Class A	0–25
Class B	26–75
Class C	76–200

SDI is reported separately, and codes set their own smoke limits. There is no fixed conversion between FSI and SDI; one cannot be derived from the other.

Three things E84 does not tell you

• It is not a fire-resistance rating. E84 measures surface flame spread and smoke density, not how long a assembly stays intact in a fire; fire resistance is tested by methods such as ASTM E119.
• It is not a real fire scenario. The tunnel is a controlled relative comparison and does not directly predict heat release, toxicity or fire growth in a real building.
• It does not declare a material non-combustible. E84 does not itself classify non-combustibility; methods such as ASTM E136 do that.

The standard also notes that melting, dripping or delaminating materials may produce low FSIs that are not directly comparable, and that supporting materials behind the specimen can lower the FSI. Reports must therefore record specimen thickness, mounting and backing conditions.

What it means for FRP grating

North American projects often require grating to reach Class A (FSI ≤ 25). Flame-retardant formulation, resin system, fillers and thickness all affect the result; the same formulation can give different FSIs at different thicknesses or mounting conditions. A project report should meet these conditions:

• The specimen matches the supplied formulation, thickness and colour;
• The mounting (ceiling position, backing, supports) matches the installation;
• The report is current and issued by an accredited laboratory;
• Both FSI and SDI are reported, not just “Class A”.

Relationship to GB 8624 and EN 13501-1

All three systems assess reaction to fire, but their methods, specimen sizes and classification logic differ. An E84 FSI cannot be converted into a GB 8624 Class B1 or an EN 13501-1 B/C/D class. Export projects should be tested and declared separately for the target market.

Edition history

E84-26a was published on 1 April 2026 and supersedes E84-26. The “a” suffix denotes a revision within the same year; purchasing documents should cite “E84-26a” so the laboratory does not use an older edition.

Where to find it

The official entry is in the Niteck FRP grating standards library. For flame-retardant product data, see specifications; for tunnel and platform applications, see load capacity for those uses.

This article explains the method and does not replace the standard text; the current edition governs all test conditions and criteria.