Choosing Timber

When choosing timber for a job, then it is important to ensure that a suitable species is selected. There are many things that should be considered to ensure that the life of the timber. This is just as important for home owners as it is for Architects and Builders

The following list of 3 basic questions can be used as a basic checklist to help make a decision on the correct timber selection.

1. Is the timber structural or non-structural?

Timber that is structural should conform to the Australian Standard AS 1684

2. Is the timber for internal or external use?

Timber that is to be used for external purposes should be chosen in terms of its decay durability.

All timber species are assigned to a durability class or “Hazard Class” of 1 to 4. Class 1 durability timbers are the most durable and hence the most suitable for external use, while class 4 timbers are the least suitable.

Durability Class Probable in-ground life expectancy (Years) Probable above ground life expectancy (Years)
1 > 25 > 40
2 15-25 15-40
3 5-15 7-15
4 0-5 0-7

Low durability timbers such as radiata pine (natural durability class 4), can be treated to increase their resistance to decay and termite attack with CCA or LOSP preservatives. The treatment applied to softwoods falls into Hazard Class ratings as per the Australian Standard 1604.

AS 1604 also states that treated timber be stamped to identify

  • the level of treatment
  • the chemical treatment applied
  • the company that treated the timber

The primary treatment carries a 25 year guarantee against failure PROVIDED that all end cuts are treated with an approved “End Seal” application which is then sealed with a suitable paint primer. Currently the only available product to achieve this are Ecoseal (for H3/H2) and EnSeal  (H2 only) by Tanalized.

The following table lists the recommended treatment ratings for specific uses.

Hazard Class Exposure Typical Uses
1 Interior above ground Framing, flooring, furniture
2 Interior above ground Framing, flooring, furniture
3 Exterior above ground Decking, weatherboards, windows
4 Exterior in ground Fence posts, stumps, landscaping
5 Exterior in ground, contact with or in fresh water Retaining walls, stumps and poles, cooling tower fill
6 Marine water Boat hulls, marine piles, pier structural timbers

3. Is the timber to be used in a high fire risk region?

Timbers that are to be used in construction of buildings in high fire risk regions need to conform to the local building codes. The Australian Standards that deal with the requirements of timbers in high fire risk zones are:

  • AS 1720.4 Timber Structures – Fire resistance of Structural Timber Members
  • AS 3959 Construction of buildings in Bushfire-prone areas

The following timbers are considered to be suitable for use in BAL25  high fire risk regions in accordance to AS 3959, provided the thickness is greater than 18mm:

  • Blackbutt
  • Kwila (Merbau)
  • Red Ironbark
  • River Red Gum
  • Silvertop Ash
  • Spotted Gum
  • Turpentine

AS 3959 states that under certain high fire risk conditions, the following timbers can also be used provided they are a minimum of 18mm thick:

  • Balau
  • Forest Red Gum
  • Jarrah
  • Tallowwood
  • Yellow Stringybark