You can make wood fire retardant by applying chemical solutions that slow ignition and reduce flame spread. The most accessible DIY method uses a borax and boric acid solution brushed, sprayed, or soaked into the wood surface. For higher-performance protection, professional pressure treatment forces fire-retardant chemicals deep into the wood cells. The right approach depends on whether you need code-compliant protection for a building project or basic flame resistance for a workshop, set piece, or outdoor structure.
How Fire Retardants Actually Work
Fire retardants don’t make wood fireproof. They change the way wood burns by promoting the formation of a protective char layer on the surface instead of allowing open flames to spread. When treated wood is exposed to heat, the chemicals trigger reactions that create this dense, insulating barrier between the flame and the unburned wood underneath. The char blocks heat transfer and limits the release of flammable gases that would otherwise feed the fire.
Some treatments work by releasing water vapor when heated, which absorbs energy and cools the surface. Aluminum-based compounds, for example, undergo a reaction that pulls heat out of the burning process while forming a ceramic-like protective layer. Phosphorus-based chemicals work differently: they react with the wood to increase char production, essentially converting more of the wood’s surface into a stable carbon shield rather than letting it decompose into combustible vapor. Calcium carbonate reacts with phosphorus compounds during burning to form thermally stable complexes that further reinforce this barrier.
DIY Borax and Boric Acid Solution
The simplest fire-retardant treatment you can make at home uses borax powder and boric acid dissolved in water. A formula recommended by the National Fire Protection Association calls for 9 ounces of borax powder and 4 ounces of boric acid per gallon of water. Mix thoroughly until dissolved, then spray it onto the wood or brush it on in heavy coats. You want the solution to saturate the surface, not just dampen it.
For larger batches or deeper saturation, a common ratio is 6 parts borax to 5 parts boric acid in 100 parts water (roughly 6 pounds of borax and 5 pounds of boric acid in 12 gallons). You can soak smaller pieces directly in the solution until the wood is fully impregnated, which gives better penetration than surface application alone. This treatment yields about 8 to 12 percent chemical weight gain in the material, which is the range needed for meaningful flame resistance.
A few practical notes: the solution mixes best in cool or lukewarm water. You may need to repeat the application for thicker or denser wood. The treatment can leave a slight white residue on the surface, so test on a scrap piece first if appearance matters. Borate treatments are water-soluble, which means rain or repeated moisture exposure will eventually wash the chemicals out. This makes them best suited for interior applications or covered outdoor areas.
Professional Pressure Treatment
Commercial fire-retardant wood is made through pressure impregnation, a process that forces chemicals far deeper into the wood than any brush or spray can reach. The wood is loaded into a large steel cylinder called a vacuum pressure vessel. An industrial vacuum pump removes air from the cylinder and from within the wood cells themselves. The cylinder is then flooded with a fire-retardant solution, and high pressure drives the chemicals deep into the wood’s cellular structure.
Cycle times and pressure settings vary based on the wood species and the retention level needed. Once the cycle finishes, the cylinder is drained and a final vacuum extracts excess solution for reuse. The treated wood then sits on a drip pad for 24 to 48 hours before it’s tagged with information about the treatment type, chemical retention level (measured in pounds per cubic foot), and the inspection agency that certified it.
You can’t replicate true pressure treatment at home, but you can buy pressure-treated fire-retardant lumber from building supply companies. This is the only option that meets building code requirements for fire-rated assemblies. If your project needs to pass inspection, DIY surface treatments won’t qualify.
Why Wood Species Matters
Not all wood absorbs fire-retardant chemicals equally, and not all untreated wood burns the same way. The differences can be dramatic. Douglas fir and southern pine have similar densities, but their flame spread behavior is wildly different: Douglas fir scores 70 to 100 on the flame spread index, while southern pine ranges from 130 to 195. Density alone doesn’t predict how a softwood will burn.
Hardwoods are more predictable. Their flame spread correlates more reliably with density, and their limited extractive content (natural resins and oils) reduces variability in fire performance. However, hardwoods contain less lignin than softwoods, which means they produce less residual char when burning. Since fire retardants work partly by boosting char formation, treatments are essentially compensating for a natural disadvantage in hardwoods.
Treatability, meaning how deeply chemicals can penetrate, also varies by species. On a scale researchers use to rate transverse penetration, highly treatable species score around 36 (with deep, even absorption) while resistant species score as low as 3. Pine generally absorbs treatment well. Dense hardwoods like oak and maple are harder to treat effectively with surface application, making pressure treatment more important for those species.
Fire Rating Classes and What They Mean
Fire-retardant effectiveness is measured using the ASTM E84 test, which assigns a flame spread index (FSI) based on how quickly fire travels across a material’s surface. The results sort materials into three classes:
- Class A: Flame spread index of 0 to 25, smoke developed index of 0 to 450
- Class B: Flame spread index of 26 to 75, smoke developed index of 0 to 450
- Class C: Flame spread index of 76 to 200, smoke developed index of 0 to 450
Untreated southern pine, with its FSI of 130 to 195, falls into Class C. Commercially pressure-treated fire-retardant lumber typically achieves Class A, the highest rating for combustible materials. DIY borate treatments can reduce flame spread, but they generally won’t achieve a certified Class A rating because the chemical penetration isn’t deep or uniform enough to pass standardized testing.
If your local building code requires a specific fire class for interior wall finishes, ceilings, or structural elements, you’ll need to use commercially treated and certified lumber rather than a homemade solution.
How Long Fire Retardant Treatments Last
The biggest enemy of fire-retardant treatments is water. Traditional phosphate-based treatments are highly effective at reducing flammability, but phosphate salts dissolve in water. Over time, rainfall and even vapor condensation can wash the active chemicals out of the wood, gradually reducing its fire resistance. This is the main reason most fire-retardant wood is specified for interior or sheltered use.
Researchers have been developing more durable alternatives. One approach chemically bonds phosphorus and nitrogen compounds directly to the wood’s cell structure through a process called phosphorylation and carbamylation. In lab testing at Pinus sylvestris (Scots pine) samples, this method created stable chemical bonds that resisted two full weeks of water leaching under the European EN 84 standard, which simulates years of outdoor moisture exposure. Some free, unreacted chemicals did wash out, causing a slight reduction in fire performance, but the treated wood still achieved the highest fire classification for wooden materials even after the leaching test.
For a DIY borate treatment on interior wood, you can expect reasonable longevity as long as the wood stays dry. If the wood gets wet regularly, plan to reapply the treatment periodically. For outdoor projects where leaching is a concern, commercially treated lumber with newer bonded-chemical formulations offers significantly better durability than anything you can apply at the surface level.
Applying a DIY Treatment Step by Step
Start with clean, dry wood. Sand the surface lightly to open the grain and improve absorption. Remove all dust with a tack cloth or damp rag, then let the wood dry completely.
Mix your solution: 9 ounces of borax and 4 ounces of boric acid per gallon of warm water. Stir until fully dissolved. For small items like shelving, decorative panels, or theatrical set pieces, submerge the wood in the solution for several hours or overnight. For larger pieces like framing lumber or plywood panels, apply the solution with a pump sprayer or wide brush in heavy, saturating coats. You want the surface visibly wet, not just misted.
Apply at least two coats, letting each coat dry before the next. Drying typically takes several hours depending on humidity and temperature. After the final coat dries completely, you can paint or finish the wood normally. Keep in mind that some finishes (particularly oil-based stains) may reduce the surface effectiveness of the treatment, so water-based finishes are generally a better match.
Wear gloves and eye protection when mixing and applying the solution. Work in a ventilated area, and avoid inhaling spray mist. Borax and boric acid are relatively low in toxicity compared to many industrial chemicals, but they can irritate skin and eyes with prolonged contact.