Fire treated wood is lumber or plywood that has been infused with chemical compounds to make it highly resistant to ignition and flame spread. The treatment doesn’t make wood fireproof, but it dramatically slows combustion, giving people more time to evacuate and firefighters more time to respond. It’s widely used in commercial construction, multifamily housing, and anywhere building codes require reduced fire risk.
How the Treatment Works
The most common method is pressure impregnation. Lumber or plywood is placed inside a large sealed tank, and the air inside the wood is pulled out with a vacuum. A liquid fire-retardant solution is then released into the tank and forced deep into the wood’s cellular structure under high pressure. This isn’t a surface coating. The chemicals penetrate throughout the wood, making the protection far more durable than paint-on products.
After impregnation, the wood is kiln-dried to bring its moisture content back down to usable levels. This step matters because excess moisture can cause the fire-retardant chemicals to migrate toward the surface and form salt crystals, which weakens the treatment’s effectiveness and creates a chalky residue.
What Happens During a Fire
When untreated wood burns, it releases flammable gases that fuel the flames and allow fire to spread rapidly. Fire-retardant chemicals change that process at the molecular level. One of the most studied retardants, boric acid, works by catalyzing dehydration reactions in wood at relatively low temperatures, around 100 to 300°C. Instead of releasing combustible gases, the treated wood sheds water and forms a dense layer of char on its surface.
That char layer acts as a physical shield. It insulates the wood underneath from heat and limits the oxygen available to sustain combustion. At around 420°C, the treated wood undergoes an exothermic polymerization reaction that further strengthens this protective charring. The result is wood that self-extinguishes once the external flame source is removed, rather than continuing to burn on its own. The chemical compounds generated during heating are fundamentally different from those produced by untreated wood, which is why treated wood produces less flame and less toxic smoke.
Where It’s Required and Used
Building codes frequently require fire-retardant treated wood (often abbreviated FRTW) in specific applications. Roof assemblies in commercial buildings are one of the most common uses, particularly in Type III and Type V construction where wood framing is otherwise permitted. It also shows up in exterior wall assemblies, interior partitions near property lines, and balcony structures in multifamily residential buildings.
FRTW gives architects and builders a way to use wood framing in structures that would otherwise require steel or concrete to meet fire codes. This can significantly reduce construction costs while still achieving the required fire ratings. There are two broad categories: interior-type FRTW, designed for covered, dry applications, and exterior-type FRTW, formulated to withstand moisture and weather exposure without the chemicals leaching out over time.
Effect on Structural Strength
The pressure treatment process does reduce wood’s structural capacity, and engineers have to account for this in their designs. The reduction varies depending on the wood species, the specific retardant used, the type of structural load, and the temperature the wood will be exposed to in service. Adjustment factors typically range from 0.80 to 1.00, meaning the wood retains between 80% and 100% of its original design strength depending on conditions.
The biggest reductions show up in roof framing applications where sustained heat exposure is highest. Douglas fir used in roof framing at service temperatures up to 150°F, for example, can see its bending strength reduced by as much as 20%. For wall and floor applications at moderate temperatures, the reduction is much smaller, often only 3% to 12%. Plywood used in shear walls and diaphragms is typically reduced to 90% of its standard allowable values. These adjustments are well documented and built into standard engineering practice, so the finished structure performs safely at its rated capacity.
Choosing the Right Fasteners
One detail that catches people off guard is that fire-retardant chemicals can be corrosive to certain metals. Using the wrong fasteners, connectors, or joist hangers can lead to premature failure, which is a serious structural concern. Recommended fastener materials for FRTW include hot-dip galvanized steel, stainless steel, copper, red brass, and certain grades of aluminum. Plain steel nails and screws should be avoided.
The treating industry recommends hot-dip galvanized or stainless steel fasteners for all types of treated wood, and this recommendation becomes especially important for exterior applications where moisture accelerates corrosion. Galvanized truss plates are standard practice even on untreated wood trusses, and the same principle applies here with greater urgency. If you’re working with FRTW, checking the manufacturer’s fastener compatibility sheet before purchasing hardware will save you from problems down the line.
Fire Treated vs. Pressure Treated
People sometimes confuse fire-retardant treated wood with the green- or brown-tinted pressure-treated lumber sold at home improvement stores. They’re different products solving different problems. Standard pressure-treated wood is infused with preservatives that protect against rot, fungal decay, and insect damage. Fire-retardant treated wood is infused with chemicals that resist ignition and flame spread. The pressure-impregnation process is similar, but the chemical solutions and the resulting properties are entirely different.
Some manufacturers offer dual-treated products that provide both fire resistance and decay protection, but these are specialty items. If a project requires fire-rated wood, standard pressure-treated lumber does not qualify. FRTW carries specific certification stamps from testing agencies that verify it meets fire performance standards, and building inspectors will look for these markings.