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Technical Reference

ASTM Fire Ratings
Explained

Understanding flame spread index, smoke developed index, and what fire classifications mean for California wildfire zone compliance.

NC
Non-Combustible
A
Class A (FSI 0-25)
B
Class B (FSI 26-75)
C
Class C (FSI 76-200)
TEST SAMPLE 25 FT (7.6 M) FLAME SOURCE

ASTM E84 Standard

The Steiner Tunnel Test

The Steiner Tunnel Test (ASTM E84) is the industry standard for measuring surface burning characteristics of building materials. A 24-foot sample is placed in a tunnel and exposed to a controlled flame for 10 minutes.

The test measures two critical factors: how far flames spread across the material surface (Flame Spread Index) and how much smoke is produced (Smoke Developed Index).

25 ft
Tunnel Length
10 min
Test Duration
FSI
Flame Spread Index
SDI
Smoke Developed Index

Flame Spread Index

Understanding the FSI Scale

The Flame Spread Index (FSI) indicates how quickly fire spreads across a material's surface. Lower numbers mean better fire resistance.

0
25
75
200+
NC
FSI = 0
Class A
FSI 0-25
Class B
FSI 26-75
Class C
FSI 76-200

Fire Classifications

Rating Categories Explained

NC

Non-Combustible

ASTM E136

Materials that will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. The highest level of fire resistance.

Flame Spread 0
Smoke Developed 0
Local Acceptance Yes
A

Class A

ASTM E84

The most restrictive rating for combustible materials. Indicates very low flame spread and smoke production. Often required near structures in fire zones.

Flame Spread 0-25
Smoke Developed 0-450
Local Acceptance Check AHJ
B

Class B

ASTM E84

Moderate flame spread. May be acceptable in some jurisdictions, but generally not recommended for higher-risk areas without additional protection.

Flame Spread 26-75
Smoke Developed 0-450
Local Acceptance No
C

Class C

ASTM E84

Higher flame spread rate. May not be accepted in many wildfire zones. Confirm with your local AHJ for allowable materials.

Flame Spread 76-200
Smoke Developed 0-450
Local Acceptance No

Fencing Materials

Fire Ratings by Material

See how common fencing materials compare on the fire resistance scale.

Aluminum
Non-Combustible
Steel
Non-Combustible
Gabion (Stone)
Non-Combustible
Block Wall
Non-Combustible
Ipe Hardwood
Class A (Natural)
FR Composite
Class A
Standard Vinyl
Class B/C
Cedar/Redwood
Class C
Pine/Softwood
Unrated

Common Questions

Fire Ratings FAQ

What's the difference between ASTM E84 and ASTM E136?

Fire ratings depend on the specific product and test reports. Ask for ASTM E84 or E136 documentation and confirm acceptance with your local AHJ.

Does Class A mean a material won't burn?

Fire ratings depend on the specific product and test reports. Ask for ASTM E84 or E136 documentation and confirm acceptance with your local AHJ.

What fire rating is required in my area?

Fire hazard zones and requirements vary by neighborhood and jurisdiction. Check local hazard maps and confirm local requirements with your AHJ. We can help interpret requirements for your property.

Can wood fencing be acceptable in some areas?

Fire hazard zones and requirements vary by neighborhood and jurisdiction. Check local hazard maps and confirm local requirements with your AHJ. We can help interpret requirements for your property.

How do I verify a material's fire rating?

Fire ratings depend on the specific product and test reports. Ask for ASTM E84 or E136 documentation and confirm acceptance with your local AHJ.

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