What makes wood FRTW
Under IBC 2303.2 (2021 and 2024), fire-retardant-treated wood is wood that is pressure-impregnated, or otherwise treated during manufacture. Pressure treatment is done in closed vessels at 50 psig or more. A surface coating does not count: studs brush-coated with fire-retardant paint after milling are not FRTW, even if the coating reaches a low flame spread index. Reject them.
Naturally durable species have nothing to do with FRTW. That idea belongs to decay resistance for preservative-treated wood.
The flame spread test
IBC 2303.2 requires a listed flame spread index of 25 or less when tested to ASTM E84 or UL 723. That is only half the requirement. The test is then continued for an additional 20 minutes, and the flame front may not progress more than 10.5 ft (3200 mm) beyond the burner centerline.
- Extra period: 20 minutes, so the whole test runs 30 minutes.
- Flame-front travel limit: 10.5 ft past the burner centerline.
- The 10.5 figure is a distance in feet, not a flame spread index.
The trap: a sample with a flame spread index of 20 is not yet FRTW. Treating 25 or less as the whole requirement ignores the extended test. Do not swap the figures either: the added time is 20 minutes and the travel limit is 10.5 ft.
Alternative test standard
IBC 2024 Section 2303.2.1 also allows ASTM E2768 as the test for FRTW. Keep the look-alike standards straight:
- AWPA U1: preservative-treated wood.
- ASTM A153: zinc coating weights for hot-dipped galvanized fasteners.
- ASTM B695: mechanically deposited zinc on certain fasteners.
Moisture content after treatment
IBC 2024 Section 2303.2.9 requires FRTW to be dried to 19% or less for lumber and 15% or less for wood structural panels before use. The trap is reversing them: panels get the tighter 15% limit, lumber gets 19%. Using 19% for both misses the panel limit, and 12% or 10% appear nowhere in the section. Drying is a post-treatment limit; it never substitutes for proper treatment.