Pressure-treated fire retardant wood is used in applications where public safety is critical, so the International Building Code sets specific requirements for what is acceptable for fire-retardant treated wood products. To meet these requirements, fire retardant treatments must undergo a number of rigorous tests and certifications to be accepted under the building codes.
These standardized tests are intended to confirm the fire retardant capabilities, performance in high temperatures and humidities, and strength adjustments in the lumber or panel design values following pressure treating. These tests are done by qualified, certified laboratories, with periodic retesting and inspection by accredited quality control agencies.
Testing Required by Code
Section 2303.2 defines the standards of quality for approved fire-retardant treated wood products. It cites the specific ASTM tests and other certifications that must be achieved to be accepted under the code. The following tests are referenced for fire retardant wood used in interior applications in the code:
ASTM E 84 or UL 723 - Flame Spread
This test measures the surface burning characteristics of a material under fire conditions and assigns a flame spread rating. The code requires the project have "a listed flame spread index of 25 or less and show no evidence of significant progressive combustion when the test is continued for an additional 20-minute period." Smoke emissions also are measured and ratings assigned on a similar scale.
ASTM D 3201 - Hygroscopicity
This assesses the potential of the material to absorb moisture at specific temperatures and relative humidity levels encountered in service. The tests are run at 90 percent relative humidity, but many run tests at higher humidities and temperatures to test the products in a "worst case" scenario.
ASTM D 5664, D 5616 - Strength
The pressure treating and drying process impacts the strength values and adjustments should be made when used in engineered applications. The tests for lumber (D 5664) and plywood (D 5616) are conducted to define the specific design value adjustments that must be made for each specific fire retardant formulations. Specification materials for each preservative will list lumber adjustments for compression parallel to grain (Fc), tension parallel to grain (Ft), horizontal shear (Fv), bending (MOE, E) and extreme fiber stress in bending (Fb), as well as adjustments in allowable loads and spans for plywood.
Exterior Exposure Tests
Fire retardant wood that will be used outside must meet more rigorous testing intended to replicate the demands of exposure to the elements. These tests simulate extensive exposure to weather, including rain and sunlight:
ASTM D 2898 - Accelerated Weathering
Sometimes referred to as the "800-inch rain test," this test has two methods to simulate weathering. Method A subjects the materials to 12 one-week condition cycles of 96 hours of water exposure and 72 hours of drying at 140 degrees F, the equvalent of 800 inches of rainfall over 12 weeks. Method B subjects materials to 1,000 hours of 24-hour exposures featuring four hours of wetting, four hours of drying and eight hours of resting, with drying time at 150 degrees F with continuous UV exposure.
Other Standards, Certifications
There are other important standards and certifications that fire retardant formulations may meet, depending on the manufacturing. These include nationwide standards as well as certifications within local jurisdictions:
AWPA U1, Specification H, UCFA
Fire retardants are proprietary formulations which are not specifically referenced within the American Wood Protection Association Book of Standards. AWPA does define the general requirements for fire retardants that are infused into wood in Commodity Specification H under standard U1. These requirements cite the code-required testing and guidance for design value adjustments for fire retardant-treated wood products.
Underwriters Laboratories provide an FR-S rating for fire retardant lumber and plywood, which indicates the wood has a flame spread and smoke developed rating of 25 or less as determined by the ASTM E 84 test. All the fire retardants used in pressure treating have the FR-S rating and can use the UL mark.
ICC ES Evaluation Report
Because fire retardants used in pressure treating are proprietary, they are approved for use under building codes through an International Code Council Evaluation Service report, referred to as an ICC-ES Report or an ESR. The ESRs, which each have a unique number, indicate the acceptance criteria used to evaluate the product, how it should be used and other information. Every preservative manufacturer provides a copy of their respective fire retardant ESR on their website.
The National Fire Protection Association published NFPA 703, which is the standard for fire retardant-treated wood and fire retardant coatings for building materials. It references the ASTM standards shown above.
This specification for the U.S. military defines the requirements for fire retardant lumber and plywood, covering both interior and exterior uses.