Boron Borax vs Other Timber Preservatives: What You Need to Know

Why the Choice of Preservative Matters

Timber preservatives are not interchangeable. They differ in the organisms they protect against, the environments where they remain effective, their toxicity to humans and animals, their effect on the timber and downstream processing, and their regulatory status in different markets. Choosing the wrong preservative for an application wastes money at best and creates health or structural risks at worst.

The main preservative systems used in Sri Lanka are Boron Borax (applied by VPI), Copper Chrome Arsenate (CCA), and — for export packaging — heat treatment under ISPM 15. Each has a distinct profile of strengths and limitations.

Boron Borax (Borate Treatment)

Boron Borax — disodium octaborate tetrahydrate — is the preservative used in VPI treatment for interior and above-ground applications. It works by disrupting the digestive enzymes of wood-destroying insects, making the wood indigestible and lethal to termites, powder post beetles, and wood-boring larvae. It also inhibits fungal growth by interfering with fungal enzyme systems.

The key advantage of borates over most alternative preservatives is safety. Boron Borax is non-toxic to mammals at the concentrations used in timber treatment — it is classified in the same general toxicity category as common salt. It leaves no harmful residue in sawdust or wood waste, does not off-gas volatile compounds in enclosed spaces, and does not require special handling or respiratory protection during installation.

The key limitation of borates is water leaching. Boron compounds are water-soluble and will leach out of timber in repeated water contact. This makes borate treatment unsuitable for ground contact, exterior applications subject to rain wash, or any application where the timber is regularly wetted. For those applications, alternative preservatives with better water resistance are required.

• Effective against: termites, powder post beetles, wood-boring larvae, fungal decay
• Not suitable for: ground contact, repeated water contact, marine applications
• Safety: low toxicity to humans, animals, and the environment
• Applied by: Vacuum Pressure Impregnation (VPI) for full penetration
• Service life: indefinite in dry, interior, sheltered conditions

CCA (Copper Chrome Arsenate)

CCA is a water-borne preservative that contains copper, chromium, and arsenic. It was the dominant structural timber preservative globally for several decades and remains widely used in some markets for ground contact and exterior applications where borate treatment is insufficient.

CCA provides excellent protection against both insects and fungal decay in high-moisture and ground contact applications. Treated timber is visually identifiable by its characteristic green tint from the copper compounds. Once fixed in the wood, the preservative is relatively stable and does not leach significantly under normal outdoor conditions.

CCA is restricted or banned in many markets for residential applications. The arsenic component raises concerns about skin contact during handling and about disposal — CCA-treated timber cannot be burned (the ash contains concentrated arsenic) and in some jurisdictions requires special disposal as hazardous waste. For residential construction, furniture, playground equipment, or any application involving regular human skin contact, CCA is not appropriate and borate treatment is the correct choice.

Creosote

Creosote — a heavy coal tar distillate — is one of the oldest timber preservatives and remains effective for heavy industrial applications: railway sleepers, utility poles, marine pilings, and heavy outdoor structures. It is highly effective against both insects and fungal decay in the most demanding exposure conditions.

Creosote is not appropriate for any application involving human contact or proximity — it is a known carcinogen and skin sensitiser, and its use is heavily restricted in residential and public-access applications. Creosote-treated timber has a strong, persistent odour and cannot be painted. It is a specialist material for specific heavy-industrial uses, not a general-purpose timber treatment.

Choosing the Right Preservative for Your Application

For most construction and furniture applications in Sri Lanka — roof structures, floor framing, door and window frames, interior joinery, and furniture — Boron Borax by VPI is the correct choice. It is effective against the relevant threat organisms, safe for use in occupied buildings, does not affect downstream processing or finishing, and is applied by a method (VPI) that achieves full cross-section penetration.

For ground contact applications — fence posts, decking support posts, pergola uprights set in the ground — CCA-treated timber is more appropriate, because the ground contact and wetting cycles will leach borate treatment faster than it can protect. For heavy industrial or marine applications, consult with a specialist — creosote or alternative heavy-duty systems may be required.