Heartwood
Heartwood is the dense, decay-resistant inner wood of a tree. AS 5604 durability ratings apply only to heartwood. Sapwood is always Class 4 regardless of species.
Ask Chalkline about this →Heartwood is the dense, dry, decay-resistant inner wood of a tree, formed when the living sapwood cells die and become saturated with extractives (natural anti-rot, anti-insect compounds) that the tree produces as it ages. In the milled section, heartwood is visibly darker than the surrounding sapwood, typically denser to the touch, and lower in moisture content at felling. The AS 5604:2005 natural-durability ratings apply only to heartwood: a Class 1 species’ heartwood lasts 25+ years in ground; the same species’ sapwood is Class 4 (under 5 years in ground).
The spec phrase to read on every Australian hardwood order:
| Spec phrase | What it means |
|---|---|
| ”heartwood-in” (or “heartwood only”) | Sapwood is excluded; the timber delivered is all heartwood. Premium, more expensive. |
| ”merchantable” or “select” | Sapwood is allowed up to a stated percentage (commonly 25% of the cross-section). |
| “trade quality” | Sapwood is allowed; quality grade refers to the absence of major defects, not species durability. |
| Species name alone (e.g. “Spotted Gum decking”) | Sapwood is allowed by default. The Class 1 durability of Spotted Gum heartwood is irrelevant to the sapwood edges. |
Why this matters:
- In-ground posts: must be heartwood-in or treated to H4 minimum. Sapwood at the soil contact zone fails first.
- Exposed deck bearers and joists: heartwood-in for Class 1 species like Spotted Gum or Ironbark; otherwise the sapwood splits and rots within 5 to 7 years even on a Class 1 species.
- Visible deck boards: heartwood-in is the premium spec. Many builders accept sapwood-included boards and accept the aesthetic defect at year 5.
- Internal structural framing: usually softwood (radiata pine), which is treated rather than heartwood-selected. Heartwood selection is mostly a hardwood discussion.
Inspection on delivery:
- Pull the bundle and check 6 to 10 random pieces.
- Look at the end-grain colour contrast: heartwood centre vs lighter sapwood edge should be obvious.
- Measure the proportion of sapwood across the cross-section; reject pieces above the contract limit.
- Reject the whole bundle if more than 10 to 20% of inspected pieces fail the spec.
Heartwood vs durability vs preservative treatment:
- AS 5604 says heartwood durability is intrinsic to species.
- AS 1604.1 says sapwood can be preservative-treated to H-classes (H1 through H6) to match heartwood durability in defined applications.
- A spec for “heartwood Class 1 or H4-treated softwood” gives the trade two acceptable solutions.
Also known as: heart timber; dense core; (informally) “the dark wood”.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.