How Fire Retardant Treatments Work on Cedar Roofing
Press Release |
November 5, 2026
At the Cedar Shake & Shingle Bureau, we field consistent questions from roofing professionals about fire-rated cedar products, and for good reason. As building codes tighten across fire-prone regions and specifiers demand greater accountability on Class A and Class B assemblies, understanding how fire retardant treatments work on cedar roofing has moved from a niche concern to a core installation competency. The chemistry, the classification system, and the long-term performance expectations all matter to anyone specifying or installing treated cedar today.
How Fire Retardant Chemistry Functions in Wood
Cedar, like all wood, burns through a predictable thermal decomposition process. When exposed to high heat, wood pyrolyzes, breaking down into combustible gases that ignite and sustain flame. Fire retardant treatments interrupt that process at the chemical level.
The primary mechanisms at work are:
- Char layer promotion: Phosphorus-based compounds accelerate char formation on the wood surface. That char acts as an insulating barrier, slowing heat transfer into the substrate and reducing the rate at which combustible gases are generated beneath it.
- Gas-phase flame inhibition: Certain halogen-based compounds release free radical scavengers during combustion, disrupting the chain reactions that sustain an open flame.
- Intumescent action: Some formulations expand when heated, creating a low-density foam layer that physically insulates the wood from direct flame contact.
Most commercial fire retardant treatments for cedar roofing use phosphate or borate-based chemistry, often in combination, to achieve both char promotion and some degree of gas-phase suppression.
Pressure Impregnation vs. Surface Application
The method of application determines both the fire resistance achieved and the product’s durability under field conditions.
Pressure Impregnation
Pressure impregnation, the process used to manufacture fire retardant treated wood (FRTW), forces chemical solution into the wood’s cellular structure under controlled vacuum and pressure cycles. The treatment becomes part of the wood rather than a coating on it, which has critical implications for performance:
- Fire resistance is distributed through the full cross-section of the shake or shingle, not concentrated at the surface
- The treatment is not compromised by mechanical abrasion, weathering, or surface wear during handling and installation
- Products treated through pressure impregnation can achieve Class A or Class B fire ratings as part of a tested assembly
Field-Applied Coatings
Surface-applied fire retardant coatings present a different performance profile. These products are not pressure impregnated. They sit at or near the wood surface and are subject to degradation from UV exposure, moisture cycling, and physical wear. Ratings achieved through surface coatings depend on maintaining that surface layer, which introduces reapplication schedules and field verification requirements that pressure-treated products do not carry.
For new construction in jurisdictions requiring Class A roofing, pressure-impregnated FRTW is the standard specification path.
Find out how long cedar roofing typically lasts.
Understanding Fire Rating Classifications
Fire ratings for roofing are established through ASTM E108 testing, which evaluates performance across four criteria:
- Flame spread: How rapidly fire propagates across the roof surface
- Burning brand: Resistance to ignition from airborne embers
- Intermittent flame: Performance under repeated short-duration flame exposure
- Weathering resistance: Sustained performance of the treatment after cyclical environmental exposure
Class A is the highest rating. Class B is an intermediate designation. Class C provides limited fire resistance. The rating that any given cedar roofing product achieves depends on both the treatment formulation and the full assembly. Underlayment, decking, and fastening all factor into a tested system, and a treatment cannot be evaluated in isolation from the assembly in which it is installed.
Assembly-Specific Testing
This is one of the more consequential technical points for installers. A treated shake or shingle carries its fire rating as part of a specific tested assembly. Substituting underlayment products, altering the battened or solid decking configuration, or changing fastener patterns can take an assembly outside the bounds of its listed test. Code officials increasingly understand this distinction, and installers who do not are exposed to liability on inspections.
Long-Term Performance and Maintenance
Pressure-impregnated FRTW does not require reapplication under normal conditions. The treatment is fixed within the wood structure and remains stable across the service life of the roof when installed according to the assembly listing.
Field-applied treatments introduce ongoing maintenance obligations that pressure-treated products avoid entirely:
- Periodic inspection for coating degradation
- Reapplication intervals that vary by product and climate exposure
- Documentation requirements in jurisdictions that mandate maintained fire ratings
Any product change, including switching to a different treatment brand after initial installation, should be evaluated against the original assembly listing before proceeding.
Working with Certified Products and Assembly Listings
Since 1915, the Cedar Shake & Shingle Bureau’s role has been to give the industry a reliable standard to build against. When specifying fire retardant treated cedar, the starting point is always the product’s listing documentation. ICC-ES evaluation reports and assembly test records establish the conditions under which a rated performance is valid. True craftsmanship in fire-rated cedar installation begins with that documentation and carries through every component of the assembly as tested.
If you are working through a project specification that involves fire-rated cedar roofing and have questions about product standards or assembly requirements, reach out through the contact form on our website. We are here to help trade professionals get it right.
At the Cedar Shake & Shingle Bureau, we field consistent questions from roofing professionals about fire-rated cedar products, and for good reason. As building codes tighten across fire-prone regions and specifiers demand greater accountability on Class A and Class B assemblies, understanding how fire retardant treatments work on cedar roofing has moved from a niche concern to a core installation competency. The chemistry, the classification system, and the long-term performance expectations all matter to anyone specifying or installing treated cedar today.
How Fire Retardant Chemistry Functions in Wood
Cedar, like all wood, burns through a predictable thermal decomposition process. When exposed to high heat, wood pyrolyzes, breaking down into combustible gases that ignite and sustain flame. Fire retardant treatments interrupt that process at the chemical level.
The primary mechanisms at work are:
- Char layer promotion: Phosphorus-based compounds accelerate char formation on the wood surface. That char acts as an insulating barrier, slowing heat transfer into the substrate and reducing the rate at which combustible gases are generated beneath it.
- Gas-phase flame inhibition: Certain halogen-based compounds release free radical scavengers during combustion, disrupting the chain reactions that sustain an open flame.
- Intumescent action: Some formulations expand when heated, creating a low-density foam layer that physically insulates the wood from direct flame contact.
Most commercial fire retardant treatments for cedar roofing use phosphate or borate-based chemistry, often in combination, to achieve both char promotion and some degree of gas-phase suppression.
Pressure Impregnation vs. Surface Application
The method of application determines both the fire resistance achieved and the product’s durability under field conditions.
Pressure Impregnation
Pressure impregnation, the process used to manufacture fire retardant treated wood (FRTW), forces chemical solution into the wood’s cellular structure under controlled vacuum and pressure cycles. The treatment becomes part of the wood rather than a coating on it, which has critical implications for performance:
- Fire resistance is distributed through the full cross-section of the shake or shingle, not concentrated at the surface
- The treatment is not compromised by mechanical abrasion, weathering, or surface wear during handling and installation
- Products treated through pressure impregnation can achieve Class A or Class B fire ratings as part of a tested assembly
Field-Applied Coatings
Surface-applied fire retardant coatings present a different performance profile. These products are not pressure impregnated. They sit at or near the wood surface and are subject to degradation from UV exposure, moisture cycling, and physical wear. Ratings achieved through surface coatings depend on maintaining that surface layer, which introduces reapplication schedules and field verification requirements that pressure-treated products do not carry.
For new construction in jurisdictions requiring Class A roofing, pressure-impregnated FRTW is the standard specification path.
Find out how long cedar roofing typically lasts.
Understanding Fire Rating Classifications
Fire ratings for roofing are established through ASTM E108 testing, which evaluates performance across four criteria:
- Flame spread: How rapidly fire propagates across the roof surface
- Burning brand: Resistance to ignition from airborne embers
- Intermittent flame: Performance under repeated short-duration flame exposure
- Weathering resistance: Sustained performance of the treatment after cyclical environmental exposure
Class A is the highest rating. Class B is an intermediate designation. Class C provides limited fire resistance. The rating that any given cedar roofing product achieves depends on both the treatment formulation and the full assembly. Underlayment, decking, and fastening all factor into a tested system, and a treatment cannot be evaluated in isolation from the assembly in which it is installed.
Assembly-Specific Testing
This is one of the more consequential technical points for installers. A treated shake or shingle carries its fire rating as part of a specific tested assembly. Substituting underlayment products, altering the battened or solid decking configuration, or changing fastener patterns can take an assembly outside the bounds of its listed test. Code officials increasingly understand this distinction, and installers who do not are exposed to liability on inspections.
Long-Term Performance and Maintenance
Pressure-impregnated FRTW does not require reapplication under normal conditions. The treatment is fixed within the wood structure and remains stable across the service life of the roof when installed according to the assembly listing.
Field-applied treatments introduce ongoing maintenance obligations that pressure-treated products avoid entirely:
- Periodic inspection for coating degradation
- Reapplication intervals that vary by product and climate exposure
- Documentation requirements in jurisdictions that mandate maintained fire ratings
Any product change, including switching to a different treatment brand after initial installation, should be evaluated against the original assembly listing before proceeding.
Working with Certified Products and Assembly Listings
Since 1915, the Cedar Shake & Shingle Bureau’s role has been to give the industry a reliable standard to build against. When specifying fire retardant treated cedar, the starting point is always the product’s listing documentation. ICC-ES evaluation reports and assembly test records establish the conditions under which a rated performance is valid. True craftsmanship in fire-rated cedar installation begins with that documentation and carries through every component of the assembly as tested.
If you are working through a project specification that involves fire-rated cedar roofing and have questions about product standards or assembly requirements, reach out through the contact form on our website. We are here to help trade professionals get it right.
