Timber preservatives

Most designers using timber are used to specifiying a range of preservatives. Without question all softwood whether external or internal is routinely coated or injected against a perceived host of rots and insect infestations. Most commonly specified preservative are not benign to people or the environment (see below) yet a huge chemical industry rides on the back of an argualbly small or non-existent risk. Prior to 1940 virtually no preservative treatment was carried out, yet the vast majority of buildings (including the houses many of us live in) are still standing, free of rot and infestation.

The problem with timber preservatives

• During the production of impregnated wood, hazardous and carcinogenic substances are released to air, surface water and soil.

• Workers and consumers are exposed to carcinogenic substances during the handling and processing of impregnated wood.

• Impregnating wood hampers the sustainable reuse of wood.

• Hazardous and toxic components of impregnated wood leach to the environment during the installed life of the wood, mainly via direct contact with surface water, but also via contact with rain and moist soil.

• Impregnated wood forms a threat to the environment in waste incineration and landfills.

• Household or uncontrolled outdoor burning of impregnated wood forms a big threat to human health and the environment.

• Treated timber is classed as 'Hazardous Waste'

Specifying preservatives

First choice: • Don’t use preservatives unless required by BS 5589 AND BS 5268:Part 5.
See 'Wood preservation' for alternatives.

Second choice: • If you have to use preservatives, use Boron based compounds. Borate Oxide (SBX) - Boron compounds are well known, non-toxic preservatives, but are water soluble. Treatment can only be carried out on green timber with a moisture content of over 50% (useful for UK sourced timber)

Not so good: • Cu & Zn Naphthanates, Acypectas Zinc. Recently developed alternatives to CCA (Copper Chrome Arsenate): ammoniacal copper quartenary (ACQ), copper azole and copper citrate

Avoid: • Creosote, Arsenic, Chromium Salts, Dieldrin (banned in the UK), PCP, Lindane, Tributyl Tin Oxide, Dichofluanid, Permethrin, CCA (from June 2004 the EC has restricted its use in buildings other than housing).