Natural durability ratings

*Notes are numbered as they are included in Construction Timbers in Queensland.

In this guide:

Note 6*: Natural durability of timber

Timbers in schedules A, B and C are allocated separate natural durability ratings for above-ground and in-ground situations, in accordance with AS 5604-2005 Timber—natural durability ratings. A rating is given for each timber species in Schedules A, B & C (under ‘Natural durability ratings’).

Because many variables are involved, it is very difficult to classify timbers accurately in terms of their resistance to decay, so the ratings are based on a range of years of expected service life.

The rating system used in AS 5604-2005 Timber—natural durability ratings is based on the average life (range in years) of test specimens of sound, untreated heartwood (35 mm x 35 mm for the above-ground tests and 50 mm x 50 mm for the in-ground trials). Where no data exist to confirm an above-ground rating, a provisional above-ground rating denoted by brackets—e.g. (2)—is given, based on the timber’s in-ground rating.

The performance and life expectancy of timber used in specific applications and environments will be influenced greatly by many other factors in addition to the natural durability ratings. These other factors include:

  • presence or absence of preservative treatment
  • the manufacturing process
  • detailing, supplementary protection and maintenance
  • climate and environmental conditions
  • member size and orientation of the wide surface.

Member size is important because an increase in sectional area generally results in a corresponding increase in expected service life. This is why the target design lives for some building applications extend to 50 years, despite the life expectancy indicated by durability rating definitions described in AS 5604-2005 Timber—natural durability ratings and reproduced in the table below.

In construction, timber member dimensions would normally exceed the section sizes of specimens used in trials and the construction timber should provide satisfactory performance, providing normal good building practice and maintenance is followed and the timber meets the grade quality requirements for the application.

Natural durability rating system for heartwood

Durability class Aboove-ground life expectancy In-ground life expectancy
1 > 40 years > 25 years
2 15 to 40 years 15 to 25 years
3 7 to 15 years 5 to 15 years
4 0 to 7 years 0 to 5 years

Source: AS 5604-2005 Timber—natural durability ratings (Standards Australia 2005)

Note 7*: Untreated sapwood

The untreated sapwood of all timber species is regarded as class 4 (non-durable), irrespective of the heartwood rating.

Note 8*:   Regulatory durability performance requirements

The building regulation framework in Australia is performance-based, and specifically addresses health, safety and amenity as primary objectives. While the Building Code of Australia (BCA, Australian Building Codes Board, 2006) currently does not have specific durability performance requirements, it does have implicit requirements, and it contains prescriptive deemed-to-comply solutions, acceptable construction practices and verification procedures.

The Australian Building Codes Board has published a guideline on durability in buildings (Australian Building Codes Board 2002). This explains the implicit requirements of the BCA that should be followed by manufacturers and specifiers wishing to satisfy the BCA’s requirements.

The administration and application of the BCA is devolved, by legislation, to state and territory authorities and/or private certifiers, who then have to interpret and apply relevant standards or acceptable solutions.

The hierarchy of building regulations in Australia for timber and durability is as follows:

  • The BCA is adopted by all states and territories under an inter-governmental agreement (IGA) between the Australian Government and the states and territories.
  • The BCA, in turn, calls on primary references such as Australian Standards and documents, including the AS 1684 series (Standards Australia 2006), also known as the Timber framing code and AS 1720.1-1997 (Standards Australia 1997), also known as the Timber structures code and in some cases, individual states may adopt variations that call on state-specific references such as Construction Timbers in Queensland (CTIQ).
  • BCA primary-referenced documents call up secondary references such as the AS 1604 series for the specification for preservative treatment of timber (Standards Australia 2004, 2005) and AS 5604 (Standards Australia 2005) on natural durability ratings.

CTIQ reflects the performance expectations implicit in the BCA as at 2013.

In-ground natural durability ratings

The predicted performance of a timber in contact with the ground (and therefore exposed to decay and termites), is determined by an allocated, in-ground durability rating. The in-ground durability rating refers to the performance of the heartwood. The ratings apply to sound, mature heartwood with a minimum cross-section thickness of 40 mm. The table gives the four rating definitions of the in-ground durability scale.

Above-ground natural durability ratings

Where data are available, timbers have been allocated to a class for above-ground durability. Where reliable data are not available, provisional above-ground durability classifications (based on published in-ground ratings) have been allocated. These provisional ratings are shown in brackets—e.g. (4) in timber Schedules A, B & C (under ‘Natural durability ratings’). This example Ag (4) means the timber has a provisional above-ground durability rating of 4. The table shows the four ratings (life-expectancy definitions) of the above-ground durability scale.

Note 9*: Round timbers

Round timbers with a complete annulus of preservative-treated sapwood (H4 or H5) will have life expectancies significantly greater than those given in the table Natural durability rating system for heartwood.

Last modified
05/17/2017 - 23:48