Sapwood
Sapwood is the outer layers of a log: lighter, lower density, Class 4 durability regardless of species. Must be excluded or treated for exposed and in-ground use.
Ask Chalkline about this →Sapwood is the outer growth zone of a log, the living wood that conducted water and nutrients while the tree was growing. After felling and milling, sapwood remains visible as the lighter-coloured, lower-density outer band of a milled section, with the older heartwood (the structural wood the tree no longer used) at the centre. Under AS 5604:2005, the natural durability of sapwood of every species is rated as Class 4 (under 5 years in ground, under 7 years above ground), regardless of how durable the heartwood of that species is.
This single rule drives most of the timber-supply decisions on a residential build:
Where sapwood is acceptable:
- Internal framing under cover, where exposure is limited.
- Treated softwood framing where the sapwood has been preservative-treated to AS 1604.1 (commonly H2 minimum for indoor termite protection, H3 for above-ground external).
- Internal joinery in dry, climate-controlled areas.
Where sapwood is unacceptable without treatment:
- In-ground posts and bearers. The sapwood band, sitting in the soil contact zone, rots in 2 to 4 years. Specify “sapwood removed” or H4/H5 preservative-treated.
- Exposed external structural members (deck bearers and joists, pergola posts) where the sapwood band is exposed to weather. Even a Class 1 heartwood species fails at its sapwood edge.
- Decking boards on visible high-traffic decks. The sapwood edge of an otherwise-durable Class 1 board will check, soften, and split within 5 to 7 years; the rest of the board remains sound. The defect is aesthetic and replaceable but unmistakeable.
Identifying sapwood on site:
- Colour contrast: sapwood is usually 30 to 50% lighter than the heartwood of the same species. Some species (white cypress, white mahogany) carry less colour contrast and need closer inspection.
- Density: sapwood is noticeably softer in heartwood-rated species.
- Annual growth rings: sapwood holds the most recent growth rings, often with visible spring/summer wood banding.
- Bark-side vs core-side: sapwood is closer to where the bark used to be.
How AS 1604.1 brings sapwood up to spec:
The H-class preservative-treatment system (H1 through H6) provides controlled chemical treatment that raises the in-service durability of sapwood-containing timber to defined exposure levels:
| H-class | Use | Service exposure |
|---|---|---|
| H1 | Inside, dry conditions | Lyctus borers only |
| H2 | Inside, dry, above ground | Termites and borers |
| H3 | Outside above ground, periodic exposure | Decking, fencing |
| H4 | Outside in-ground, severe biological exposure | Posts, retaining walls |
| H5 | Outside in-ground, severe exposure including water table | Foundation timbers |
| H6 | Marine | Marine pile, jetty |
Common defects:
- Sapwood deck board fails at the edges within 5-7 years; the warranty claim points to the manufacturer or supplier even though the species heartwood is sound.
- Sapwood-included house framing in a Termite-Management Zone breached because the sapwood was untreated; AS 3660.1 termite barrier needed but missing.
- “Sapwood acceptable” specified by mistake on an exposed structural member; the engineer’s design durability is invalidated.
Also known as: sap; soft outer wood; light-coloured edge.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.