In fire safety, time is everything. The faster a fire spreads, the less time people have to escape, and the harder it becomes to control the situation. That’s where flame spread ratings come into play.
For building product manufacturers, architects, and contractors, flame spread classifications are more than just numbers — they’re key to meeting codes and ensuring material safety.
In this article, we’ll break down how ASTM E84 testing determines Class A, B, and C fire ratings (and what those classifications actually mean) as well as how these ratings apply in real-world construction.
What Is a Flame Spread Rating?
Flame spread rating indicate how quickly flames will travel along the surface of a given material.
It’s one of the key fire rating classifications used in both residential and commercial construction, helping code officials, designers, and inspectors assess the relative fire hazard of different building materials.
Flame spread ratings are based on the Flame Spread Index (FSI), a numerical value derived from a standardized test that ranks how far and fast flames can move across a material's surface. Materials with lower FSI values are less likely to play a significant part in spreading fires, which typically makes them a safer option.
Knowing the entire picture of how flame spread works and how it’s classified helps you stay code-compliant and make smarter material decisions.
Understanding ASTM E84: The Standard Test Method
One of the most commonly used fire spread test method is ASTM E84, also known as the Steiner Tunnel Test. Recognized by code bodies like the International Building Code (IBC), ASTM E84 is often required for code approval in construction.
During testing, a 25-foot-long tunnel is lined with fire-resistant materials, and the material sample is mounted to the ceiling of the tunnel. A gas flame is then applied to one end of the sample and observed for a duration of ten minutes.
The test evaluates how far flames travel and the amount of smoke produced by comparing the material's performance to fiber-cement board (index 0) and red oak (index 100), which serve as tunnel flame spread calibrants.
Key Measurements of ASTM E84
ASTM E84 has two important measurements that will come from the test results: Flame Spread Index (FSI) and Smoke Developed Index (SDI).
FSI quantifies the rate and extent of flame spread, while SDI measures the amount of smoke produced. These values then help to classify a material’s surface burning characteristics, which determine where it can be used within a building based on the fire rating class.
Class A, B, and C Classifications: What They Mean
There are three classifications, based on specified ranges of flame spread and smoke developed indices, that typically come out of the ASTM E84 test.
Class A identifies the best performing materials, falling within the range of 0 to 25 FSI. Class B is mid-range, with 26 to 75 FSI, and Class C is the lowest rating of 76 to 200 FSI. All classes have a code-specified smoke developed index (SDI) limit of 450.
Class A Fire Rating (FSI 0–25)
Materials in this category offer the highest level of fire protection thanks to their slower flame spread. Class A products are often required in areas where fire risk is highest or where people need extra time to evacuate.
Common Class A Materials
- Gypsum wallboard
- Brick and masonry
- Cement board
- Fire retardant treated wood products
- Certain insulation products
Class A fire-rated materials are often required in egress routes, elevator shafts, stairwells, and exterior walls of multi-family or high-occupancy buildings where there are more people to consider.
Class B Fire Rating (FSI 26–75)
Class B materials provide moderate resistance to flame spread and are typically used in settings with controlled access or moderate traffic.
Common Class B Materials
- Certain types of untreated wood products
- Phenolic foam boards
- Polyurethane insulation with fire retardants
- Treated acoustical ceiling tiles
These fire-rated materials are often found in offices, hotels, and restaurants where regulations are still strict, but Class A materials aren’t mandated.
Class C Fire Rating (FSI 76–200)
Class C fire rating classification is for materials with the fastest flame spread. While not suitable for high-risk zones, they may be allowed in detached structures or areas with lower occupant loads and fire risk.
Common Class C Materials
- Certain types of untreated wood products
- Particleboard
- Certain plastics and composite panels
- Foam panels without protective coatings
Class C-rated materials are often seen in storage sheds, garages, or outbuildings where minimal occupancy and local codes allow their use.
Flame Spread Rating Applications
|
Fire Rating Class |
Flame Spread Index (FSI) |
Typical Applications |
|
Class A Fire Rating |
0–25 |
Hospitals, schools, high-rises, egress corridors |
|
Class B Fire Rating |
26–75 |
Offices, conference centers, mid-rise structures |
|
Class C Fire Rating |
76–200 |
Storage areas, warehouses, detached garages |
Why Fire Rating Classification Matters in Building Applications
Selecting the right materials based on their particular flame spread ratings has implications far beyond code compliance because it directly impacts occupant safety. Slower flame spread gives people the opportunity to evacuate under fire and smoky conditions.
These ratings are important for proper insurance liability management. Using incorrect building materials can, in some cases, invalidate an insurance claim if a fire occurs.
Failure to meet the International Building Code (IBC) and other local building codes also has significant ramifications, where permitting can be denied or changes to structures must be made to meet compliance.
For example, using a Class C fire-rated material in a required Class A corridor can jeopardize occupancy approval, even if the building is otherwise compliant.
Common Misconceptions About Flame Spread Ratings
Despite their importance, flame spread ratings are frequently misunderstood. Here are a few common misconceptions around flame spread ratings and classifications.
“Class A means the material is fireproof.”
A Class A fire rating indicates that the material can limit or prevent the spread of flames, but it can still burn under high heat or prolonged exposure. Fireproof implies zero combustion, which is something very few standard construction material ever achieves.
“Flame spread ratings are the same as fire resistance ratings.”
Flame spread relates to surface burning characteristics, while fire resistance relates to how long a wall, floor, or ceiling assembly can prevent the spread of fire and withstand structural failure.
“Lab testing doesn’t reflect real-world conditions.”
While it’s true that ASTM E84 is a controlled lab test and site conditions will vary, lab testing provides a standardized benchmark that building codes and safety professionals rely on to ensure minimum performance thresholds are met across the board.
How NGC Testing Services Can Help
At NGC Testing Services, we specialize in precise and reliable fire testing for a broad range of building materials. Our ASTM E84 testing facility is designed to give you accurate, accredited results fast, so you can meet your deadlines, satisfy code officials, and protect your brand’s reputation.
Understanding the difference between Class A, B, and C fire classifications empowers you to make smarter, safer decisions during material selection. When lives, property, and reputations are at stake, knowledge and compliance aren’t optional — they’re essential.
Let NGC Testing Services be your trusted partner in flame spread testing and fire compliance. Contact us now to discover more about our fire testing services.