Fire-Rated Lumber
2026 Fire-Rated Lumber Buying Guide: FRT Wood Explained (2026)
Fire-retardant treated (FRT) lumber is required by building codes in many commercial, multi-family, and wildfire-prone applications. It provides critical fire safety by reducing flame spread and smoke development without resorting to non-combustible materials. This guide explains treatment types, code classifications, and where FRT lumber is required so you can spec the right products and pass inspection.
Key Takeaways
- Fire-retardant treated (FRT) lumber is required by building codes in many commercial, multi-family, and wildfire-prone applications.
- It provides critical fire safety by reducing flame spread and smoke development without resorting to non-combustible materials.
- This guide explains treatment types, code classifications, and where FRT lumber is required so you can spec the right products and pass inspection.
- Flame spread index: Class A (0–25) is required for most FRT applications
- Treatment type: Interior-only vs.
What to Look For
- Flame spread index: Class A (0–25) is required for most FRT applications
- Treatment type: Interior-only vs. exterior-rated FRT
- Base species: Douglas fir and southern yellow pine are most common
- Fastener compatibility: Some FRT treatments are corrosive to standard hardware
- Structural derating: FRT treatment can reduce design values — check the evaluation report
Species Comparison
Southern yellow pine and Douglas fir are the primary species used for FRT lumber due to their treatability and structural properties. SPF (spruce-pine-fir) is available but less common. Plywood for FRT sheathing is typically Douglas fir. The treatment chemical penetrates softwoods more effectively than hardwoods, which is why hardwood FRT products are rare. Always confirm the species and treatment combination has an ICC-ES evaluation report.
Size Guide
FRT lumber is available in standard dimensional sizes: 2×4 through 2×12, and 4×4 posts. FRT plywood: 4×8 sheets in 15/32", 19/32", and 23/32" thicknesses. Lengths: 8' through 20' for framing lumber. FRT glulam beams are available for heavy timber applications requiring both fire rating and long spans.
Grade Guide
FRT lumber is graded the same as untreated lumber (#1, #2, Select Structural) but carries an additional FRT stamp from the treating plant. The stamp must include the treater's name, the evaluation report number (ICC-ES ESR), the flame spread classification, and whether it is rated for interior or exterior use. Always check that the stamp is legible and matches your spec.
Budget Breakdown
Budget: Interior-rated FRT 2×4 studs at $8–$12 each (vs. $4–$6 for untreated). Mid-range: Exterior-rated FRT framing at $12–$18 per piece. Premium: FRT plywood or FRT glulam beams at 2–3× the cost of untreated equivalents. Overall, expect FRT materials to add 40–100% over standard lumber costs.
Common Mistakes to Avoid
- Using interior-rated FRT in exterior or high-humidity locations — it can lose effectiveness
- Cutting FRT lumber and not retreating the cut ends, which voids the fire rating at that point
- Using incompatible fasteners — some FRT chemicals accelerate corrosion of standard galvanized hardware
- Assuming FRT lumber makes a building fireproof — it reduces flame spread but does not prevent combustion
- Not verifying the ICC-ES report number before purchase, leading to inspection failures
Pre-Purchase Checklist
- Confirm code requirements: Is FRT required or would non-combustible be better?
- Specify interior or exterior FRT rating
- Verify the ICC-ES evaluation report number for your product
- Use compatible fasteners (stainless steel or approved coatings)
- Plan for structural derating in your engineering calculations
- Store FRT lumber under cover — some treatments are water-soluble
Frequently Asked Questions
No, but it is commonly required for roof construction in Type III and Type V buildings, exterior wall assemblies in wildfire zones, and concealed spaces in multi-family construction. Check your local code.
Yes, but some manufacturers require cut ends to be field-treated with a brush-on fire retardant. Check the product's installation instructions.
FRT lumber may have a slight greenish or brownish tint depending on the chemical used. It can be painted or stained once dry. It typically cannot achieve a natural clear finish.
FRT is a pressure-impregnated chemical treatment throughout the wood. Intumescent coatings are surface-applied paints that swell when heated to form an insulating char. Both reduce flame spread but are specified differently.