Meta description: Compare Class A vs. Class B fire ratings for building materials and roofing. Learn what the ratings measure, how they differ, and what to check before choosing a product.
“Class A” and “Class B” fire ratings can refer to different tests, depending on the product. For interior wall and ceiling finishes, the classes generally describe flame spread and smoke development under ASTM E84 or UL 723. For roof coverings, they refer to performance under a separate fire-exposure test. Knowing which rating applies is essential: these labels are not interchangeable, and neither alone tells how long a wall or roof will resist fire.
What a Class A or Class B Fire Rating Means
For many interior finish materials, Class A and Class B are categories based on two measurements from a standardized surface-burning test: the flame spread index (FSI) and the smoke-developed index (SDI). ASTM E84, commonly called the Steiner tunnel test, and the closely related UL 723 test expose a sample to controlled flame and airflow in a test chamber.
The test compares the material’s surface-burning behavior with reference materials. A lower FSI generally indicates that flame traveled less rapidly across the tested surface. SDI estimates the amount of smoke produced relative to the test’s reference. The test does not measure every aspect of fire performance, and results apply to the tested product and installation conditions—not automatically to every material in the same product line.
Under commonly used U.S. model-code classifications for interior wall and ceiling finishes, the ranges are:
| Class | Flame Spread Index (FSI) | Smoke-Developed Index (SDI) |
|---|---|---|
| Class A (Class I) | 0–25 | 450 or less |
| Class B (Class II) | 26–75 | 450 or less |
| Class C (Class III) | 76–200 | 450 or less |
These are commonly cited thresholds in building-code provisions, including the International Building Code (IBC). The applicable code edition, local amendments, product category, and testing documentation matter. A product’s published rating should be checked against the exact requirements for the project.
Class A vs. Class B Fire Rating at a Glance
The central difference is the permitted flame-spread range. Class A has the lowest FSI range, while Class B allows a higher FSI. Both classifications generally require an SDI of 450 or less under the familiar interior-finish framework. A Class A rating therefore indicates lower measured flame spread in that test, but it does not mean a material is noncombustible, fireproof, or guaranteed to prevent a fire from spreading.
A Class B finish may still be suitable where building codes permit it. Requirements can depend on occupancy, the location of the finish, the building’s construction, and whether the material is used in a corridor, exit enclosure, room, or other regulated area. The building official and project specifications determine what is acceptable.
How the ASTM E84 and UL 723 Tests Work
In an ASTM E84 or UL 723 test, a sample is mounted in a horizontal tunnel and exposed to a controlled flame. Observers and instruments record flame travel and smoke development over the test period. The results are converted into indices and used to classify the surface-burning characteristics of the specimen.
The test is useful for comparing products under a consistent procedure, but it has important limits:
- It is a surface-burning test. It does not establish how long a complete wall, floor, ceiling, or roof assembly will maintain its structural integrity or prevent fire from passing through.
- It does not reproduce every real fire. Actual conditions vary with ventilation, fuel, room geometry, installation, and other materials nearby.
- It evaluates a specific specimen. Thickness, substrate, facing, adhesive, mounting method, and other test details can affect the result.
- It is not a toxicity rating. SDI is a smoke-development index, not a measurement of smoke toxicity or the hazards of every combustion product.
Some products may receive an E84 or UL 723 classification through an approved extended application or other code-recognized procedure rather than a test of the exact final configuration. The product listing and evaluation report should explain the approved uses and limitations.
Class A vs. Class B Roof Fire Ratings
Roof-covering classifications use a different system. In the United States, roof fire ratings commonly refer to tests such as UL 790 or ASTM E108, which evaluate roof coverings or roof assemblies under specified external fire exposures. Their Class A, B, and C labels describe levels of resistance to that exposure—not the same FSI ranges used for interior finishes.
Broadly, Class A provides the highest level of roof-covering fire protection in this classification system, Class B an intermediate level, and Class C a lower level. A roof’s result may depend on the tested combination of covering, underlayment, deck, and installation details. A label on a shingle or membrane alone may not establish that any roof assembly containing it has the same classification.
A Class B roof rating cannot be compared numerically with a Class B interior-finish rating. The tests measure different behavior, use different procedures, and serve different code purposes. When a specification says “Class B,” it should identify the product type and the applicable test standard.
Fire Spread Rating Is Not the Same as Fire-Resistance Rating
One common source of confusion is the difference between a surface-burning classification and a fire-resistance rating. An ASTM E84 Class A or B result does not mean the material or assembly can withstand fire for a particular number of minutes or hours.
Fire-resistance ratings, such as one-hour or two-hour ratings, apply to tested or listed assemblies that meet defined criteria for a period of time. A rated wall assembly may include specific layers, fasteners, framing, joints, and penetrations. Its rating is not determined simply by adding the individual ratings of its components. Openings, ducts, pipes, and other penetrations may require listed firestop systems to preserve the assembly’s performance.
Other terms also describe different properties. “Noncombustible” is a code-defined classification, while “fire-retardant-treated” describes treatment intended to reduce the contribution of certain wood products to fire. Neither phrase should be treated as a synonym for Class A, and neither automatically establishes a fire-resistance duration.
Where Class A and Class B Ratings Matter
For interior finishes, the required classification depends on the adopted building code and the particular location and occupancy. Codes may impose stricter requirements in certain spaces, such as exit enclosures or corridors, than in other areas. Rules can also differ for textiles, foam plastics, decorative materials, or specialized products, which may be subject to additional tests or provisions.
For roof coverings, the required class can depend on the building’s location, construction, occupancy, local wildfire conditions, and code requirements. Local rules may specify a minimum roof classification or require an approved assembly. Insurance requirements and project specifications may also call for particular documentation, but they do not replace the applicable building code.
Because local jurisdictions adopt and amend model codes on different schedules, a general online summary cannot determine the requirements for every project. The authority having jurisdiction—often the local building department—can confirm which code edition and provisions apply.
How to Verify a Product’s Fire Rating
Before selecting or approving a product, confirm that the rating is tied to the correct test and the intended installation. Useful documentation may include a test report, certification-directory listing, code evaluation report, or manufacturer’s technical data sheet. The documentation should identify the tested product, standard, classification, and conditions of use.
- Identify the product and application. Determine whether the rating is for an interior finish, roof covering, insulation, door, wall assembly, or another product category.
- Check the test standard. Look for ASTM E84 or UL 723 for relevant surface-burning classifications; look for UL 790 or ASTM E108 when verifying a roof-covering fire classification.
- Confirm the full classification. For interior finishes, check both FSI and SDI where required. For roofing, verify the class of the tested assembly and the specific installation conditions.
- Match the installation to the listing. Confirm substrate, thickness, attachment, underlayment, orientation, joints, and other limitations. Substituting components can invalidate an assumed rating.
- Check code and local requirements. Ask the building department, design professional, or code consultant whether the classification meets the project’s applicable provisions.
Marketing language such as “fire rated” or “fire resistant” is not enough to establish compliance. A reliable claim should be supported by identifiable test or listing documentation that matches the intended use.
Common Misunderstandings About Class A and Class B
“Class A means the material cannot burn.”
Not necessarily. For an interior finish, Class A describes a result within a surface-burning test. A Class A product may still be combustible. The classification does not mean the material cannot ignite under other conditions or contribute to a fire.
“Class A means a one-hour fire rating.”
No. Class A is not a time rating. A one-hour fire-resistance rating applies to a tested assembly under a separate test and code framework. Product labels should be checked for the exact property being claimed.
“Class B is unsafe or prohibited.”
That conclusion cannot be drawn from the class alone. Class B represents a different measured performance range, and it may be permitted for certain uses. The project’s code requirements and conditions determine whether it is acceptable.
“A Class A roof and Class A wall finish are equivalent.”
They are not. Roof-covering classifications and interior surface-burning classifications come from different tests. Always state the product type and test standard when comparing fire ratings.
Choosing Between Class A and Class B
When both classifications are allowed, the choice may depend on the location, product availability, design goals, cost, and other performance requirements. A lower FSI can be a useful selection criterion for interior finishes, but it should not replace consideration of the complete fire-protection strategy, including detection, sprinklers, compartmentation, exits, and rated assemblies.
For roofing, the appropriate class should be established for the roof assembly and local code context. A product that carries a strong rating in one installation may not retain that classification when paired with a different deck or underlayment. Reviewing the full listing before ordering can prevent costly substitutions or inspection problems.
In practical terms: Class A is the more restrictive surface-burning class for interior finishes, while Class B permits a higher flame-spread index. For roofs, Class A is the highest of the common external fire-exposure classes, but it is based on a separate test system. The safest comparison starts by identifying the product, test standard, complete assembly, and local code requirement.