Fire Ratings Explained for Walking Decks: Class A vs Class B
Understanding ASTM E84 fire testing and what Class A and Class B ratings mean for deck waterproofing systems.
What Are Fire Ratings?
Fire ratings classify how building materials react when exposed to fire. For deck waterproofing systems, the most common classifications are Class A and Class B, based on testing per ASTM E84. All walking roof decks must meet Class A requirements, like any compliant roofing products.
These ratings tell you two things: how quickly flame spreads across the surface and how much smoke the material produces when burning. Building codes use these ratings to determine which products can be used in different applications.
ASTM E84 Testing
ASTM E84 is the standard test method for measuring surface burning characteristics. The test exposes a material sample to controlled flame in a 25-foot tunnel and measures two values:
Flame Spread Index (FSI)
Measures how quickly fire spreads across the material surface. Lower numbers mean slower spread. The scale uses red oak flooring (FSI = 100) and cement board (FSI = 0) as reference points.
Smoke Developed Index (SDI)
Measures the amount of smoke produced during burning. Lower numbers mean less smoke, which is critical for safe evacuation during a fire.
Class A vs Class B
Class A (Most Restrictive)
Flame Spread Index: 0-25
Smoke Developed Index: 0-450
Highest fire resistance rating for surface materials. Required for multi-family buildings, commercial applications, and projects with strict code requirements.
Class B (Moderate)
Flame Spread Index: 26-75
Smoke Developed Index: 0-450
Moderate fire resistance. Acceptable for many residential and light commercial applications where Class A is not specifically required.
When Each Rating is Required
Class A Typically Required For:
- Multi-family residential buildings (condos, apartments)
- Commercial properties and public buildings
- Projects in wildfire-prone areas
- Building codes requiring roof covering Class A rating
- Fire-resistance-rated assemblies
Class B Acceptable For:
- Single-family residential projects
- Light commercial applications
- Projects where building codes don't specify Class A
- Interior applications with less stringent requirements
Always verify requirements with your local building department. Code requirements vary by jurisdiction, building type, and occupancy.
Common Misconceptions
Class A Doesn't Mean Fireproof
No surface material is fireproof. Class A means the material resists flame spread and limits smoke production better than Class B or C materials, but it will still burn under sufficient heat.
The Entire System Must Be Tested
Fire ratings apply to the complete system as tested, not individual components. Using different substrates, coatings, or installation methods than what was tested invalidates the rating.
Fire Rating vs Fire Resistance
Fire rating (Class A/B) measures surface burning characteristics. Fire resistance rating (1-hour, 2-hour) measures how long an assembly prevents fire spread through a floor or wall. These are different tests with different purposes.
Reading Fire Test Reports
When reviewing product documentation, look for:
- ASTM E84 test report showing actual FSI and SDI values
- Classification (Class A or Class B) based on those values
- Exact system configuration tested (substrate, coatings, thicknesses)
- ICC-ES evaluation report confirming the fire rating for code compliance
- Limitations on use (substrate types, application methods, slope restrictions)
Example Fire Rated Walking Deck Systems
Deck Flex has two complete Walking Deck and Roof Covering Systems that are ICC-ES listed (ESR-3672) with documented fire ratings:
Deck Flex W.M. Walking Deck & Roof Covering System
Class A fire rating over plywood
FSI: 0-25, SDI: 0-450
Learn more about Deck Flex W.M. →Deck Flex W.F. Walking Deck & Roof Covering System
Class B fire rating over plywood
FSI: 26-75, SDI: 0-450
Learn more about Deck Flex W.F. →Frequently Asked Questions
What is the difference between Class A and Class B fire ratings?
Class A has a Flame Spread Index of 0-25 and is the highest fire resistance rating for surface materials, required for multi-family buildings and commercial applications. Class B has a Flame Spread Index of 26-75 and is acceptable for many residential and commercial applications where Class A is not specifically required e.g. decks that do not serve as roofs and are not above a living area. Both have a Smoke Developed Index of 0-450. All roof decks must meet Class A requirements per IBC and IRC codes just like any standard roofing products.
What is ASTM E84 testing?
ASTM E84 is the standard test method for measuring surface burning characteristics. The test exposes a material sample to controlled flame in a 25-foot tunnel and measures two values: Flame Spread Index (FSI) which measures how quickly fire spreads across the material surface, and Smoke Developed Index (SDI) which measures the amount of smoke produced during burning.
When is a Class A fire rating required?
Class A fire ratings are typically required for multi-family residential buildings (condos, apartments), commercial properties and public buildings, projects in wildfire-prone areas, building codes requiring roof covering Class A rating, and fire-resistance-rated assemblies. Always verify requirements with your local building department as code requirements vary by jurisdiction, building type, and occupancy.
Does Class A mean fireproof?
No, Class A does not mean fireproof. No surface material is fireproof. Class A means the material resists flame spread and limits smoke production better than Class B or C materials, but it will still burn under sufficient heat. It measures surface burning characteristics, not complete fire resistance.
What is the difference between fire rating and fire resistance rating?
Fire rating (Class A/B) measures surface burning characteristics using ASTM E84 testing. Fire resistance rating (1-hour, 2-hour) measures how long an assembly prevents fire spread through a floor or wall. These are different tests with different purposes. Fire ratings apply to surface materials while fire resistance ratings apply to structural assemblies.
Can I use different components than what was fire tested?
No, fire ratings apply to the complete system as tested, not individual components. Using different substrates, coatings, or installation methods than what was tested invalidates the rating. Always follow the manufacturer's published specifications that correspond to their fire test reports and ICC-ES evaluation.
Related Resources
Questions About Fire Ratings?
Our technical team can help you understand which fire rating is required for your project and provide documentation for code officials.