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.
The Key Difference: Surface vs. Assembly
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: The Surface Story
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:
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Class A (0-25): Excellent flame spread resistance (like cement board)
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Class B (26-75): Good flame spread resistance
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Class C (76-200): Moderate flame spread resistance
Real-World Application for Wood Products
Based on extensive testing data from the American Wood Council, most wood products fall into Class B or C categories:
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Baltic Birch Plywood (3/4"): Typically achieves Class C rating (FSI around 120)
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Douglas Fir Lumber: Class B rating (FSI of 70)
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Western Red Cedar: Class B rating (FSI of 45)
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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: The Assembly Protection
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.
Understanding Code Requirements: Where Each Rating Applies
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
Upgrading Wood Products to Class A Performance
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.
Method 1: Fire Retardant Coatings
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
Proven Class A Coating Solutions:
- 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
Method 2: Factory Fire-Retardant MDF (Most Stringent Applications)
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.
Fire Rated MDF:
- 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)
When specifying fire safety upgrades for our wood wall systems, consider these key factors:
- 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
Common Designer Mistakes to Avoid
Mistake #1: Assuming Fire-Rated Means Flame Spread Rated
A 1-hour fire-rated wall assembly doesn't automatically mean the finish materials meet flame spread requirements. You need both.
Mistake #2: Specifying Only the Finish Material
Flame spread ratings can change when materials are combined. A wood veneer over different substrates may perform differently than the veneer alone.
Mistake #3: Ignoring Thickness Requirements
Materials less than 0.036 inches thick applied directly to surfaces are exempt from flame spread testing, but thicker applications require proper ratings.
Mistake #4: Not Considering Fire Retardant Upgrades
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.
Practical Guidance for Specifying Wood Systems
For māk studio Projects:
- 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
Documentation Requirements:
- 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
The Bottom Line for Designers
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
Resources and Next Steps
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.