Less-Toxic Treated Lumber: A Comprehensive Guide to Borate-Treated Wood

Borates are naturally occurring mineral salts composed of boron, oxygen, and other elements. When applied to wood under pressure, borate compounds penetrate deep into the cell structure. The borate remains inactive in dry wood, but when moisture levels exceed about 20% — the threshold at which decay fungi become active — the borate dissolves and becomes mobile. Fungi and insects that ingest or contact borate-fungal decay and termite attack in buildings.

p>

This moisture-activated mechanism makes borates ideal for applications where wood may occasionally get wet but can dry out again — exactly the conditions that promote fungal decay and termite attack in buildings.

Advantages of Borate-Treated Wood

Safety

According to the USDA Forest Service and the World Health Organization, borates have very low mammalian toxicity. They are not carcinogenic, do not bioaccumulate, and break down into naturally occurring soil minerals. In fact, boron is an essential micronutrient for plants, and borate-based preservatives are registered for use in organic agriculture as a pest control agent. This safety profile makes borate-treated wood ideal for interior structural framing, where off-gassing of chemicals is a concern.

Workability

Unlike CCA-treated wood, which is harder on cutting tools and requires special handling for sawdust disposal, borate-treated wood machines, cuts, and fastens exactly like untreated wood. The treatment does not change the wood’s color, odor, or dimensional properties. Nails and screws go in easily without pre-drilling, and the wood accepts paint, stain, and adhesive normally.

Corrosion Resistance

Borates do not corrode metal fasteners. CCA and other copper-based treatments accelerate corrosion of galvanized and steel connectors, requiring the use of stainless steel or specially coated fasteners. Borate-treated lumber can be used with standard galvanized fasteners, reducing hardware costs.

Limitations of Borate Treatment

Leaching

The most significant limitation of borate treatment is that the preservative can leach out of wood that is exposed to liquid water. This means borate-treated lumber cannot be used where it is in direct contact with the ground, embedded in concrete or masonry (such as mudsills), or in exterior applications that are directly exposed to rain. For these locations, copper-based treatments (ACQ, CA) remain necessary.

Exterior Use Restrictions

Borate-treated wood can be used outdoors only if it is protected from direct weather by a roof, overhang, or durable finish (paint or stain). Decks, porch framing, and roof sheathing in covered areas are all suitable applications. However, exposed decking, railings, and fence boards are not appropriate for borate treatment alone.

Applications in Tropical and Termite-Prone Regions

In tropical climates like Puerto Rico, Hawaii, and the Caribbean islands, termite pressure is extreme, and virtually all structural lumber must be treated. Borate-treated wood has been used successfully in these regions for decades, sold under brand names such as Tim-Bor and Cari-Bor. The key considerations for tropical construction are:

• All interior framing should be borate-treated, including studs, plates, joists, rafters, and blocking.
• Mudsills and ground-contact members should use ACQ or CA treatment — not borates.
• Exterior trim and fascia should be borate-treated only if protected by a roof and painted on all sides.
• Sheathing — borate-treated plywood or OSB is available and effective for walls and roofs.

Borate vs. CCA: Environmental Comparison

Availability and Sourcing

Borate-treated lumber is widely available in termite-prone regions of the United States, including Hawaii, Florida, the Gulf Coast, and California. In other regions, it may need to be special-ordered through lumber yards. For information on borate wood treatment, contact U.S. Borax, Inc. (the primary manufacturer of borate wood preservatives). The International Code Council (ICC) lists borate-treated wood in the IRC as an approved method for termite-resistant construction.

For more on building materials and their applications, see our guides on structural insulated panels, autoclaved aerated concrete, and glass as building material.

Conclusion

Borate-treated lumber represents a significant advance in wood preservation, offering effective protection against termites and decay fungi with dramatically lower toxicity than traditional CCA treatment. While it cannot replace copper-based treatments for ground-contact or exterior-exposed applications, it is the ideal choice for interior framing, covered exterior structures, and any application where human contact with treated wood is frequent. For builders in termite-prone regions, borate-treated wood provides peace of mind without the environmental and health concerns of older preservative chemistries.

The History of Wood Preservation

Wood preservation has a long history dating back to ancient civilizations. The Egyptians used cedar oil to preserve wood, while the Romans applied copper and other metal salts. Modern pressure treatment began in the 1830s with the Bethell process, which used creosote under pressure to force preservatives deep into wood cells. By the early 20th century, waterborne preservatives had been developed, including chromated zinc chloride and, later, chromated copper arsenate (CCA), which became the dominant treatment worldwide from the 1940s through the early 2000s.

CCA’s popularity stemmed from its effectiveness: the copper provided fungicidal protection, the arsenic was toxic to insects, and the chromium locked the chemicals into the wood structure. However, concerns about arsenic leaching from treated wood in playground equipment, decks, and other residential applications led to a voluntary phase-out of CCA for residential use in the United States in 2003. This created a market opening for alternative treatments like ACQ, CA, and borates.

Borate Treatment Penetration and Retention Standards

The effectiveness of borate treatment depends on proper penetration and retention. The American Wood Protection Association (AWPA) standards specify minimum retention levels for different applications:

The “pcf” stands for pounds per cubic foot of wood, measured as boric acid equivalent (BAE). A retention of 0.17 pcf means that each cubic foot of wood contains 0.17 pounds of borate preservative. This is a very small amount — about 2.7 ounces per cubic foot — yet it is sufficient to protect the wood from termites and decay fungi for decades in dry interior conditions.

Penetration is equally important. The treatment must penetrate the full sapwood depth — the outer portion of the tree that is biologically active. Heartwood, the inner core of the tree, is naturally more resistant to decay and does not readily accept chemical treatment. For most softwood species used in construction, the sapwood is 1 to 3 inches thick, and pressure treatment forces the borate solution throughout this zone.

Comparison of Borate-Treated Wood with Other Non-Toxic Alternatives

Several other approaches to termite and decay protection exist for builders seeking alternatives to conventional chemical treatments:

Naturally durable species: Heartwood of certain species — including western red cedar, redwood, black locust, and ipe — has natural resistance to decay and insects. These species contain extractives (organic compounds) that are toxic or repellent to fungi and insects. Using these species avoids chemical treatment entirely, but they are significantly more expensive than treated lumber and are not available in all regions.

Thermally modified wood: Heat-treated wood (subjected to temperatures of 350°F to 450°F in a low-oxygen environment) has improved dimensional stability and decay resistance. The thermal modification changes the chemical structure of the wood, making it less palatable to fungi. Thermally modified wood is available in several species (ash, pine, poplar) but is expensive and has reduced strength compared to untreated wood.

Acetylated wood: A newer technology that reacts the wood with acetic anhydride, modifying the cell structure to make it indigestible to fungi and insects. Acetylated wood (sold as Accoya) offers exceptional dimensional stability and decay resistance with no toxic chemicals. It is expensive but has a 50-year warranty against decay.

Physical barriers: In some termite-prone areas, builders install physical termite barriers — stainless steel mesh, sand layers, or crushed stone — beneath and around foundations to prevent termite entry. These are used in conjunction with treated wood rather than as a replacement.

For more on building materials and their applications, see our guides on structural insulated panels, autoclaved aerated concrete, and glass as building material and cement grades and standards.