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AS 5604 (timber durability): the natural durability classes

AS 5604:2005 sets the natural durability rating of timber heartwood for in-ground (Class 1-4) and above-ground use. Selects species for posts, bearers, decks.

By Dan Waring. Last updated 16 May 2026. Verified 16 May 2026.

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In plain English

AS 5604:2005, Timber, Natural durability ratings, is the Australian Standard that classifies the natural durability of the heartwood of Australian and imported timber species, for both in-ground and above-ground exposure. The standard is the reference table every architect, engineer, and builder reads to match a timber species to an outdoor application: posts, deck bearers, deck boards, joists exposed to weather, retaining-wall timbers, pergolas, decks. AS 1684 (residential timber-framed construction) and AS 2870 (residential slabs and footings) both cross-reference AS 5604 when nominating timbers for exposed elements (verified 2026-05-16).

The two rating dimensions:

ClassIn-ground service lifeAbove-ground service life
Class 1Over 25 yearsOver 40 years
Class 215 to 25 years15 to 40 years
Class 35 to 15 years7 to 15 years
Class 4Under 5 yearsUnder 7 years

A timber’s class differs between in-ground and above-ground service, and the standard publishes the rating for each species against both. Some species sit at Class 1 above-ground but Class 3 in-ground (e.g. some grey ironbark variations), so the application dictates the species choice.

Critical builder takeaway: AS 5604 rates HEARTWOOD only. The sapwood of even a Class 1 species is rated as low as Class 4. Sapwood-included timber for in-ground use needs preservative treatment (per AS 1604) to bring the sapwood up to the heartwood’s durability.

What it requires

For the timber supplier and the designer:

  1. Species identification. Each piece must be identifiable to a species or species group named in the standard. “Hardwood” alone is not a durability class; the species and origin (Australian native, imported) determine the rating.
  2. Heartwood proportion. Where the timber section includes sapwood, the durability of the assembly drops to the sapwood rating (Class 4 unless treated).
  3. Application-specific rating. Use the in-ground column for posts, bearers in contact with soil, retaining-wall timbers. Use the above-ground column for decking boards, joists clear of ground, exposed beams.
  4. Reference cross-checks. AS 1684 calls up AS 5604 for outdoor framing. AS 2870 calls it up for stumped-floor timbers in contact with ground. AS 1604 (Specification for preservative treatment) is the treatment standard for sapwood-treated and lower-class timbers.

What it doesn’t cover

  • Engineered timber durability. LVL, glulam, plywood, MDF, particleboard have their own durability classifications (commonly via product certification rather than species rating).
  • Treated-timber durability. AS 5604 is natural durability; preservative-treated timber sits under AS 1604.1 (Sawn and round timber) and the H-class system (H1 to H6).
  • Timber strength. Strength classes (F-grades, MGP, JD) are separate. AS 5604 says nothing about how strong the timber is, only how long it will last.
  • Fire resistance. Bushfire timber selection follows AS 3959 Appendix F (bushfire-resisting timbers); AS 5604 is durability-focused.

Practical implications

  • In-ground vs above-ground choice is non-substitutable. Spec’ing Class 2 cypress pine for posts (above-ground Class 2 hardwood) is wrong if the post is buried. Use Class 1 species for in-ground (e.g. ironbark, tallowwood) or treat to H5 minimum.
  • Sapwood selection on visible decks. A Class 1 species deck board with sapwood edges will rot at the sapwood within 5 to 7 years even as the heartwood remains sound. Specify “kiln-dried hardwood, sapwood removed” or “treated to H3 minimum” on visible deck boards.
  • Imported decking species need an AS 5604 reference. Spotted gum from Australia maps to a specific class; “imported hardwood” from Indonesia may map to a different species with different durability. Ask for the species name, not just the colour.
  • In-ground stumps under stumped floors. AS 2870 cross-references AS 5604 here. Where natural durability is short, the engineer specifies an in-ground steel stirrup or galvanised post anchor to lift the timber off ground contact.
  • The standard does not address termite resistance separately. Class 1 durable species (ironbark, etc.) are often termite-resistant by coincidence, but AS 3660.1 termite management is a separate compliance pathway, not a side-effect of AS 5604.

References

See also

Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.

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