The International Building Code requirements for fire-retardant-treated wood sit in Section 2303.2: the lumber or panel must be impregnated with chemicals under pressure during manufacture, produce a flame spread index of 25 or less under ASTM E84 (or the identical UL 723), and continue to perform through an extended 30-minute version of that test with the flame front advancing no more than 10.5 feet past the burner and no evidence of significant progressive combustion. Meet those thresholds and the product qualifies as FRT wood. Miss any of the surrounding rules on where it can be used, how it’s labeled, how dry it is, how it’s fastened, or how design values are adjusted, and the installation fails inspection.
Surface coatings and intumescent paints do not satisfy the code for the structural uses covered here. The treatment has to penetrate throughout the piece, on every surface, permanently.
Where the Code Allows FRT Wood
The IBC assigns each building a construction type based on the fire resistance its structure must provide. FRT wood is the mechanism the code uses to let wood appear in types that would otherwise demand noncombustible framing.
Type I and Type II
These are the noncombustible construction types, built primarily of steel and concrete. IBC Section 603.1 carves out three FRT wood exceptions:
- Nonbearing interior partitions where the required fire-resistance rating is two hours or less.
- Nonbearing exterior walls where no fire-resistance rating is required.
- Roof construction, including girders, trusses, framing, and decking.
The roof allowance has a height limit. In Type IA buildings over two stories, FRT wood cannot be used in the roof if the vertical distance from the uppermost occupied floor to the roof is less than 20 feet.
Type III
Type III requires noncombustible exterior walls but permits combustible interior elements. Section 602.3 allows FRT wood framing and sheathing inside exterior wall assemblies rated at two hours or less.1International Code Council. International Building Code Chapter 6 – Types of Construction This covers wall studs, plates, and wall sheathing. It does not cover floor sheathing, rim joists, or floor joists, even when those pieces sit in the plane of the exterior wall. Structural wood panels on the exterior face count as part of the wall assembly and must be FRT.
Type IV
Section 602.4 permits FRT wood framing and sheathing within exterior wall assemblies of Type IV-HT (Heavy Timber) buildings with a two-hour rating or less.1International Code Council. International Building Code Chapter 6 – Types of Construction The newer mass timber subtypes (IV-A, IV-B, IV-C) rely on cross-laminated timber and noncombustible coverings rather than FRT wood, and have their own protection rules.
Interior Versus Exterior Products
FRT wood is sold in two exposure categories, and the difference is not cosmetic. The chemicals in interior-grade product will leach or absorb moisture in weather exposure, degrading the fire performance.
Interior FRT wood is intended for locations continuously protected from weather. The code splits interior product into Type A and Type B; Type A carries additional elevated-temperature testing and is the appropriate choice for roof sheathing and similar hot locations. Interior product must show a moisture content of no more than 28 percent when tested at 92 percent relative humidity under ASTM D3201.
Exterior FRT wood is formulated to survive rain, dew, and other weather. It must carry the “Exterior” designation on its label and show no increase in flame spread rating after the Standard Rain Test under ASTM D2898. Decks, balconies, soffits, and any exterior wall sheathing that could see weather require exterior-rated product. AWPA U1 labels these categories UCFA (interior) and UCFB (exterior), and those designations appear on treatment certificates.2American Wood Protection Association. AWPA Standard U1
Flame spread class comes from the E84 result. Class A covers 0 to 25, Class B 26 to 75, Class C 76 to 200. All FRT wood is Class A by definition. The code also caps smoke-developed index at 450 for interior wall and ceiling finishes. Untreated softwood lumber typically tests in the Class C range, 100 to 200.
Labels: What the Inspector Looks For
Every piece of FRT lumber and plywood must carry a permanent label under IBC Section 2303.2.4. Missing or illegible stamps get material rejected on site. A compliant label shows the mark of an approved third-party inspection agency accredited under Section 1703.5, the treating manufacturer, the treatment name, the wood species, the flame spread index and smoke-developed index from testing, the drying method (typically kiln-dried after treatment, or KDAT), and confirmation of conformance to the relevant IBC subsections. Exterior-rated product must also carry the statement that there is no increase in the listed classification when subjected to the Standard Rain Test (ASTM D2898).
Labels are usually stamped near the ends of lumber or on the face of plywood sheets. Verify them before cutting. Once a stamped end is trimmed off, there is no practical way to re-verify the product during inspection.
Moisture Content and Storage
IBC Section 2303.2.8 caps moisture content at 19 percent for structural lumber and 15 percent for wood structural panels. Above those limits the wood can warp, lose capacity, and let the retardant chemicals migrate unevenly.
Manufacturers generally kiln-dry after treatment, but the code limits kiln temperatures to those used on the test specimens that established the product’s fire rating. Overheating during drying degrades the chemicals. On site, check moisture with a pin or pinless meter before installation, particularly if the material has been stored for a while. Stack FRT wood off the ground on level stringers, protect it from rain and standing water, and cover it loosely enough for air to move through. Wood that soaks in storage may exceed the moisture limits even if it arrived compliant, and re-drying in the field without controlled kiln conditions is unreliable.
Structural Design Value Adjustments
Fire-retardant chemistry reduces the strength of wood. FRT lumber looks identical to untreated lumber but cannot carry the same loads. Most treatments cut strength properties by 10 to 25 percent depending on the chemical and the property measured. Averaged across studies, bending strength (modulus of rupture) drops roughly 13 percent and stiffness (modulus of elasticity) about 5 percent.
ASTM D6841 sets the methodology treatment manufacturers must follow to publish adjustment factors for their specific formulations.3ASTM International. Standard Practice for Calculating Design Value Treatment Adjustment Factors for Fire-Retardant-Treated Lumber The factors are not universal. Each manufacturer publishes its own, and the engineer of record must apply them to the base design values for the species and grade. Adjustments cover bending, tension parallel to grain, compression parallel to grain, horizontal shear, and modulus of elasticity.
Roof framing carries an additional concern because FRT lumber in a hot attic sees cumulative heat exposure over decades. D6841 addresses this by testing specimens at both 72°F and 150°F and producing adjustment factors that account for long-term thermal degradation.3ASTM International. Standard Practice for Calculating Design Value Treatment Adjustment Factors for Fire-Retardant-Treated Lumber The standard’s thermal parameters assume roof slopes of 3-in-12 or steeper with ventilation meeting national standards. Specifying FRT trusses or rafters without applying the manufacturer’s factors is an engineering error that can produce long-term deflection or, worse, structural failure.
Cutting, Fasteners, and Finishes
What You Can and Cannot Cut
The code demands that fire-retardant treatment protect every surface of the piece permanently. Ripping or planing removes the treated outer layer and exposes untreated wood, which voids compliance. Simple crosscuts at the ends are generally accepted because the exposed area is small and treatment typically penetrates the end grain more than a saw kerf deep. Resawing a 2×10 into thinner pieces or running FRT lumber through a planer destroys the rating. For non-standard sizes, order pre-cut from the treater or specify the correct dimension before treatment. Anything beyond a simple end cut in the field should be cleared with both the treating manufacturer and the listing agency.
Fasteners
Many fire-retardant chemistries corrode standard carbon steel, especially where moisture is present. IBC Section 2304.9.5 requires fasteners and connectors in contact with FRT wood to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze, or copper. For FRT wood used outdoors or in wet or damp locations, that requirement applies without exception. For interior applications, fasteners may follow the treatment manufacturer’s recommendations instead; where the manufacturer offers no guidance, the exterior-grade rules apply by default.4International Code Council. Significant Changes to the International Building Code 2009 Edition – Section 2304.9.5 Plain steel or standard zinc-plated nails in FRT wood is one of the most common installation errors, producing corrosion, stains, and weakened connections over time.
Coatings
FRT wood accepts paint and stain, but surface prep matters. Sand lightly, clean, and let the wood dry before finishing. Latex and oil-based paints and both water- and solvent-based stains are compatible with most FRT products under normal conditions. In humid environments, a compatible primer under the topcoat helps prevent adhesion problems from chemical salts drawing to the surface, a condition called blooming that appears as white crystalline deposits. Follow both the FRT manufacturer’s finishing directions and the coating manufacturer’s label.
Jobsite Safety and Disposal
Cutting FRT wood generates dust that carries the treatment chemicals. OSHA’s permissible exposure limit for wood dust is 15 mg/m³ total dust and 5 mg/m³ respirable dust over an eight-hour time-weighted average.5Occupational Safety and Health Administration. WOOD DUST Chemical Data Wear a mask rated for fine particulates and work in ventilated areas.
FRT wood is not classified as hazardous waste under federal law, and disposal is not federally regulated by the EPA. Do not burn scraps; the smoke and ash can release the chemicals in concentrated form. Chipping or composting is likewise inadvisable because it can release those chemicals into soil. Landfill disposal is acceptable in most jurisdictions, but state and local rules vary, so check with the local waste authority before disposing of large quantities.6National Pesticide Information Center. Regulation and Disposal of Treated Wood