Most timbers (especially softwoods like pine) need to be treated to preserve them for outdoor use otherwise they can rot and can be infested by mould and insects.
- Interior-use timbers are usually treated to H1.2 to protect them from wood-eating pests
- Exterior timbers rated for above-ground use only are treated to H3.2
- Exterior in-ground timbers requiring a longish life are treated to H4
- Timber used for house piles is treated to H5
- In-ground timbers suitable for contact with seawater or estuarine ground are treated to H6
Timber preservation treatments are very poisonous. The ‘H’ stands for Hazard and the larger the number following it, the greater the poison rating, and the greater degree of protection.
The most common preservatives are:
- CCA – Chrome Copper and Arsenic
- LOSP (light organic solvent preservative) – usually tributyl tin oxide (TBTO)
- Boron compounds
- Some are naturally occurring in the timber such as in Totara and Macrocarpa
Preservation treatments are implemented by the high pressure application of the preserving formula onto timber in a large vat. This forces the treatment into the first few exterior millimetres of the timber. Because of this, make sure when you saw treated timbers for outside use, to treat the sawn sections with a preservative such as Metalex, normally available from a hardware store.
Never burn or use treated timber in a hangi or bonfire, because the highly poisonous treatment chemicals will be quickly released into the air and do serious harm to lungs.
Use and handling:
- Store treated timber in a covered but well-ventilated area
- Remove any wrapping to allow vapours to dissipate
- Wear long sleeves and gloves when handling treated timbers
- Always wear safety glasses and a filter mask when sawing and sanding.
- Work in a well-ventilated environment but do sawing and sanding outside
- Wait for timbers still damp with treatment to dry before working with them.
- Always wash your hands after handling LOSP timber.
- If your clothing has sawdust on it, wash it separately to avoid contaminating other laundry.
Some semi-hardwood timbers, such as Macrocarpa and Kwila, are so dense that they are naturally resistant to pests and rot and don’t require treatment.