Understanding the critical difference between flame spread ratings and fire resistance ratings can save your project—and potentially lives.
In the design and construction industry, we often hear the terms "flame spread rating" and "fire rating" used interchangeably. However, these are two completely different safety measures that serve distinct purposes in building design. Confusing them can lead to costly mistakes, code violations, and most importantly, compromised safety.
Flame Spread Rating measures how quickly fire spreads across the surface of a material once ignited. Think of it as the material's ability to resist becoming fuel for a fire.
Fire Resistance Rating measures how long an entire assembly (walls, floors, doors) can withstand fire exposure while maintaining its structural integrity and preventing fire passage.
Flame spread ratings are determined using the ASTM E84 test method, which measures how flames travel across a material's surface in a controlled tunnel test. The results are classified into three categories:
Class A (0-25): Excellent flame spread resistance (like cement board)
Class B (26-75): Good flame spread resistance
Class C (76-200): Moderate flame spread resistance
Based on extensive testing data from the American Wood Council, most wood products fall into Class B or C categories:
Baltic Birch Plywood (3/4"): Typically achieves Class C rating (FSI around 120)
Douglas Fir Lumber: Class B rating (FSI of 70)
Western Red Cedar: Class B rating (FSI of 45)
Most OSB and MDF: Class C ratings (FSI 100-155)
For māk studio's vertical slat wall systems, this means our Baltic birch products typically qualify for Class C applications, making them suitable for most commercial spaces including offices, retail, and reception areas—especially in sprinklered buildings.
Fire resistance ratings are measured in hours (1-hour, 2-hour, 3-hour, etc.) and indicate how long a complete assembly can:
Maintain structural integrity
Prevent fire passage
Limit temperature rise on the unexposed side
These ratings apply to entire systems—not individual materials. A 1-hour fire-rated wall assembly might include gypsum board, insulation, framing, and specific installation methods working together as a complete system.
The International Building Code (IBC) sets different flame spread requirements based on where materials are installed and whether the building has sprinklers
Fire resistance ratings are required for:
Structural elements (beams, columns)
Fire-rated walls and partitions
Floor/ceiling assemblies
Fire doors and windows
Penetrations and joints
While Baltic birch plywood naturally achieves Class C ratings (FSI ~120), there are two primary methods to achieve Class A performance for the most stringent requirements like exit stairways and non-sprinklered corridors.
Fire retardant coatings can upgrade wood products to Class A performance through several mechanisms:
Intumescence: The coating expands when heated, creating an insulating char layer
Flame Retardant Additives: Chemical compounds that interrupt the combustion process
Thermal Insulation: Reduces heat transfer to the underlying wood substrate
Smoke Reduction: Limits toxic smoke production during fire exposure
Flamecheck FIF Clear Coat-X: Water-based polyurethane achieving Class A at 100 sq ft/gallon coverage rate
Firefree Class A: Intumescent coating meeting ASTM E84 standards including 30-minute extended test
BanFire Intumescent Paint: Applied at 325-400 sq ft/gallon for Class A performance
Flame-Off Clear Spray: UL Classified on wood with ASTM E84/UL 723 Class A rating
For the most demanding applications—exit stairways, elevator interiors, and high-traffic areas where coatings cannot achieve Class A—factory fire-retardant MDF provides guaranteed performance.
Certified Class A Rating: FSI of 15, Smoke Developed of 5 (Guardian Fire Testing Laboratories)
ASTM E84 Compliant: Meets both US Class 1/A and Canadian Class 1/A standards
No Added Formaldehyde (NAF): CARB NAF exempt, EPA TSCA Title VI compliant
Sustainable: Made from pre-consumer recycled wood fiber
Applications: Elevator interiors, casework, furniture, decorative panels where Class A is required
Available Thicknesses: 1/4" to 1-1/8" (all thicknesses maintain Class A rating)
Fire Retardant Coatings: Coverage rates typically 100-400 sq ft/gallon, 2-4 hour dry time, some jurisdictions require certified applicators
Fire Rated MDF: Factory-applied fire retardancy, consistent performance, ideal for most stringent applications
Cost Considerations: Coatings add labor and material costs; Medite FR has higher material cost but eliminates field application
Testing: Always test coating applications on sample pieces to verify technique and appearance
A 1-hour fire-rated wall assembly doesn't automatically mean the finish materials meet flame spread requirements. You need both.
Flame spread ratings can change when materials are combined. A wood veneer over different substrates may perform differently than the veneer alone.
Materials less than 0.036 inches thick applied directly to surfaces are exempt from flame spread testing, but thicker applications require proper ratings.
Many designers don't realize that Class C wood products can be upgraded to Class A performance with proper fire retardant coatings or by switching to factory fire-retardant materials like Medite FR MDF.
Office Spaces (Sprinklered): Our Class C Baltic birch systems typically comply for most interiors applications in the U.S
Corridors: Fire-retardant coating can upgrade to Class B or A ratings as required
Exit Stairs: Class A fire retardant treatment required for non-sprinklered buildings
Most Stringent Applications: Consider Medite FR MDF substrate for guaranteed Class A performance
High-Traffic Areas: Factory-applied solutions (Medite FR) provide consistent performance
Request ASTM E84 test reports from manufacturers
Verify ratings match your specific product configuration
Confirm installation methods don't void ratings
For fire retardant coatings, document application rates and methods
For Medite FR, obtain Guardian certification documents
Maintain documentation for code officials
Understanding the difference between flame spread and fire resistance ratings isn't just about code compliance—it's about creating safe, beautiful spaces. Here's your quick reference:
Flame Spread = Surface Performance: How quickly fire spreads across your finish material
Fire Resistance = Assembly Performance: How long your wall/floor system resists fire
You Need Both: Most projects require compliance with both types of ratings
Context Matters: Building type, occupancy, and sprinkler systems affect requirements
Upgrades Available: Fire retardant coatings and factory fire-retardant materials can achieve Class A performance
When specifying wood systems for your next project:
Identify your building's occupancy classification
Determine flame spread requirements by location
Verify fire resistance requirements for assemblies
Request proper documentation from manufacturers
Consider fire-retardant treatments when needed
Test coating applications on sample pieces
Evaluate factory fire-retardant materials for stringent applications
Need help navigating fire safety requirements for your wood wall systems? Our team at māk studio works with architects and designers daily to ensure code compliance while achieving stunning design goals. We can coordinate fire retardant treatments, source Medite FR MDF, and provide complete ASTM E84 documentation for your projects.
⚠️ Important Disclaimer: While this article provides general guidance based on model codes and industry standards, the Local Authority Having Jurisdiction (AHJ) is the ultimate authority for your specific project. Always consult with local building officials, fire marshals, and code enforcement agencies before finalizing material specifications. Local amendments, interpretations, and specific project conditions may require different approaches than those outlined in this article.
This article is based on the American Wood Council's "Flame Spread Performance of Wood Products Used for Interior Finish" (DCA1-2019), current International Building Code requirements, manufacturer data from Flamecheck International, Firefree Coatings, Roseburg Forest Products (Medite FR), and other certified fire retardant coating providers. Always consult with local authorities having jurisdiction for specific project requirements.