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The use of durability Class timber framing in Subfloor areas

May 19, 2019

NCC 2019 has amended subclauses 3.4.1.2 (e) (iii) (A) & (B) which now clarify what durability class of timber is required where the subfloor space is excessively damp or subject to frequent flooding if the subfloor ventilation cannot be increased by 50% or the ground in the subfloor space is not sealed with an impervious membrane.

The subfloor framing when used above the ground must be Class 1 or 2 durability timbers or Class H3 preservative treated timbers; and where used in the ground must be Class 1 or 2 durability timbers or H5 preservative treated timbers in accordance with the AS 1684 Residential timber framed construction series of Australian Standards.

The question we often get asked is what is a durable Class 1 or 2 timber or a H3 and H5 preservative treated timber?

Durability Class timbers are expressed as one of four classes. The value for each species is based on trials of the resistance to both decay and termites of untreated heartwood in the ground.

  • A Class 1 Timber is of the highest natural durability, expected to have a life greater than 25 years in the ground and greater than 40 years exposed above ground. (e.g  Tallowwood, blackbutt, ironbark, or spotted gum)
  • A Class 2 Timber is of high natural durability, expected to have a life of about 15 to 25 years in the ground and 15 to 40 years exposed above ground.  (e.g blue gum, Jarrah, Karri)

Class H preservative timbers are classified under AS 1604.1 2012 as hazard treated timbers and range from Class H1 – H5. The amount of preservative treatment of the timber will then depend on the situations that it can be used in.

  • H3 preservative treated timbers are only suitable for above ground use like framing, decking boards, bearers and joists. This level of treatment is required to protect the timber from attach by decay fungi and insects including termites.
  • H5 Preservative treated timber can be used above ground and in the ground for house stumps, retaining walls and builders poles. This level of treatment is required to protect the timber from attack by insects, including termites, and very severe decay.

All preservative treated timber is required to be legibly marked with the treatment plant number, preservative code number and Hazard class. (i.e. 099 01 H3) so you know exactly the class of timber to use in the specific circumstance.

For further information, please call 1300650620 to speak to an HIA Technical Adviser.