The flame spread index (FSI) is a number that describes how quickly fire travels across the surface of a building material. It’s measured on a scale where 0 represents fiber-cement board (essentially no flame spread) and 100 represents red oak wood. The lower the number, the slower fire moves across the material’s surface. Building codes use this number to determine which materials can be used on walls, ceilings, and other interior surfaces.
How the Test Works
The flame spread index comes from a standardized test called the Steiner Tunnel test, formally known as ASTM E84. A sample of the material, roughly 24 inches wide and 24 feet long, is placed inside a long, enclosed tunnel. A propane flame is applied to one end of the sample for 10 minutes while observers track how far and how fast the flame front travels along the surface.
The test is calibrated using two reference materials. Fiber-cement board anchors the bottom of the scale at 0, meaning it produces no measurable flame spread. Select grade red oak, conditioned to a moisture content between 6% and 8%, anchors the scale at 100. During calibration, the flame front on the red oak sample is expected to reach the far end of the tunnel in about five and a half minutes. Every other material is scored relative to these two benchmarks, so the FSI is a comparative measurement rather than an absolute physical property.
It’s worth noting that scores can exceed 100. Materials that burn more aggressively than red oak simply receive a higher number. There is no upper cap.
Class A, B, and C Ratings
Building codes group materials into three classes based on their flame spread index. You’ll sometimes see these labeled with Roman numerals (I, II, III) instead of letters, but they mean the same thing.
- Class A (Class I): FSI of 0 to 25. These materials resist flame spread the most and are required in high-risk or high-occupancy areas like exit corridors, stairwells, and large assembly spaces.
- Class B (Class II): FSI of 26 to 75. A moderate rating, typically acceptable for hallways and rooms with lower occupancy demands.
- Class C (Class III): FSI of 76 to 200. These materials are permitted in less critical locations, though local codes vary on where exactly they’re allowed.
All three classes also carry a maximum smoke developed index (SDI) of 450. The SDI is measured during the same tunnel test and reflects how much smoke a material produces as it burns. A material that technically qualifies as Class A based on flame spread but produces too much smoke would still fail the requirement.
Flame Spread Index vs. Flame Spread Rating
These two terms are used interchangeably in practice. “Flame spread index” refers to the specific numerical score from the ASTM E84 test, while “flame spread rating” sometimes refers more loosely to the Class A, B, or C designation. In official documentation and product spec sheets, you’ll see both phrases pointing to the same test result. There’s no meaningful technical distinction between them.
FSI Values for Common Materials
The numbers can vary depending on thickness, moisture content, and surface treatment, but published test results give a useful sense of where familiar materials fall on the scale. Solid Douglas fir, a common structural and finish wood, typically scores around 70 to 80, placing it in the Class B range. Douglas fir plywood varies significantly with thickness: a thin quarter-inch panel scores about 85 (Class C), while a thicker 23/32-inch panel drops to around 35 (Class B). Thicker panels generally perform better because they take longer to heat through.
Fiber-cement board, used as the test’s zero-point calibrant, is effectively noncombustible. Standard gypsum board (drywall) also scores very low, well within Class A territory. At the other end of the spectrum, untreated materials like certain softwoods or some synthetic panels can score above 200, pushing them beyond Class C and making them unsuitable for interior finish in most code-compliant construction.
Where Building Codes Apply FSI
Building codes primarily use the flame spread index to regulate interior finish materials, meaning the surfaces you see on walls and ceilings. The logic is straightforward: in a fire, wall and ceiling surfaces are what allow flames to spread rapidly through a room or down a corridor, cutting off escape routes.
Codes are strictest in areas where rapid evacuation matters most. Exit stairways and passageways typically require Class A materials. Corridors serving those exits often require Class A or B. Other occupied rooms may allow Class C. The specific requirements depend on the building’s use, its size, and whether it has a sprinkler system. Sprinklered buildings sometimes receive a one-class reduction in requirements, meaning a space that would otherwise need Class A materials might accept Class B.
Foam plastic insulation gets particular scrutiny. Most building codes require foam insulation to have an FSI no greater than 75 and an SDI no greater than 450. When foam is used in exterior walls, the threshold tightens to an FSI of 25 or less, reflecting the risk that exterior fires can spread rapidly up a building’s facade.
Limitations of the Test
The ASTM E84 test measures surface burning behavior under one specific set of conditions: a controlled propane flame in a horizontal tunnel. It doesn’t simulate every real-world fire scenario. A material’s behavior in an actual building fire depends on factors the tunnel test can’t capture, including orientation (vertical surfaces burn differently than horizontal ones), ventilation, and proximity to other combustible materials.
The test also only evaluates surface flame spread. It doesn’t measure a material’s tendency to ignite, its heat release rate, or its structural performance during a fire. Two materials with identical FSI scores can behave very differently once fully involved in flames. For that reason, building codes treat the flame spread index as one piece of a larger fire safety picture, alongside requirements for fire-resistance ratings, sprinkler systems, and compartmentalization.