Treated Timber Specifiers Guide

In Australia, there are a number of different timber preservatives produced by Koppers. Each type of preservative can treat timber destined for different applications, however, the way the timber is treated will vary accordingly. To ensure that the correct treatment type and level of treatment is used in an application, a system of Hazard Classes is used according to the definitions in Australian Standards as shown in the table below.

The higher the Hazard Class, the more severe the exposureis, therefore, it is important to purchase preservative treated timber to the correct Hazard Class irrespective of the preservative type. By ensuring you buy preservative treated timber of the correct Hazard Class, irrespective of the treatment type, the same performance can be expected. This means you need to specify the correct Hazard Class for a particular application and ensure that this is marked on the timber that you purchase or your builder uses. There are some restrictions on use of CCA (Copper Chrome Arsenate) in decking and associated posts and handrails, picnic tables, playground equipment and the like, see for more details.  

The interactive treatment guide below, will assist you to determine the correct Hazard Class for the preservative treated timber in your application. Simply point to the circles on each of the products on and around the house.




Click here to download a print friendly version of the guide


Hazard Class Selection Guide

Hazard class exposure specific service conditions biological hazard typical uses
H1 Inside, above ground Completely protected from the weather and well ventilated, and protected from termites Lyctid borers Susceptible framing, flooring, furniture, interior joinery
H2 Inside, above ground Protected from wetting. Nil leaching Borers and termites Framing, flooring and similar, used in dry situations
H3 Outside, above ground Subject to periodic moderate wetting and leaching Moderate decay, borers and termites Weatherboard, fascia, pergolas (above ground), window joinery, framing and decking
H4 Outside, in-ground Subject to severe wetting and leaching Severe decay, borers and termites Fence posts, garden wall less than 1m high, greenhouses, pergolas (inground) and landscaping timbers
H5 Outside, in-ground contact with or in fresh water Subject to extreme wetting and leaching and/or where the critical use requires a higher degree of protection Very severe decay, borers and termites Retaining walls, piling house stumps, building poles, cooling tower fill
H6 Marine waters Subject to prolonged immersion in sea water Marine wood borers and decay Boat hulls, marine piles, jetty, cross-bracing, landing steps, and similar
1. Examples shown in this table are not exhaustive.
2. It is recommended that specifiers nominate the minimum hazard class level appropriate to the specific exposure and service conditions.
3. Extract from AS1604.1, Specification for preservative treatment © SAI Global.