Kiln-dried timber (KD)
Kiln-dried timber is dried in a controlled kiln to 10-15% moisture content, delivering dimensional stability and higher F-grades than green or air-dried stock.
Ask Chalkline about this →Kiln-dried (KD) timber is sawn structural timber dried in a controlled kiln to a target moisture content (commonly 10 to 15%) before sale. The controlled drying produces three benefits over green or air-dried timber:
- Dimensional stability. A KD member moves much less in service than a green member. The shrinkage that would happen on a building site over months happens at the kiln before delivery.
- Higher achievable F-grade and MGP grade. Grading rules under AS 2858 and the MGP standard for softwood treat moisture content as an input; lower moisture content allows the timber to grade higher (e.g. MGP12 cannot be achieved with green stock).
- Reduced check and split risk. Controlled drying avoids the surface check splits that uncontrolled drying produces in heavy-section hardwood and softwood members.
How KD timber is identified:
- “KD” stamp on the end of each piece.
- Moisture content stamp or sticker (e.g. “MC 12%”).
- Grade stamp confirming the F-grade or MGP grade.
- Mill marking identifying the mill and the kiln batch.
The moisture content target depends on the application:
| Application | Target MC at delivery |
|---|---|
| Internal structural framing (kiln-dried) | 10 to 15% |
| External structural framing (less critical) | 12 to 18% |
| Joinery-grade hardwood | 8 to 12% |
| Air-dried “seasoned” timber | 12 to 22% (varies by climate) |
| Green (unseasoned) hardwood | Over 25%, often 30 to 60% |
What to do if green timber is delivered when KD was specified:
- Reject the delivery if the specification called KD. The grade may not satisfy the engineer’s design.
- Test moisture content with a calibrated moisture meter (multiple-pin penetration type). If above 18%, the timber is not stable enough for KD-spec applications.
- Document with photographs and supplier statement before sending back. Some suppliers attempt to substitute air-dried for KD on shortages.
- Check the engineer’s mark-up. The structural design assumes the grade; substituting green for KD changes the design and a re-engineering check may be required.
Common defects to watch for after install:
- KD framing wetted on site (rain, leaks) before being lined re-wets to 20%+, then re-dries inside the wall, causing nail-pop and joint cracks.
- KD hardwood used in long-exposure external positions without further treatment: returns to the equilibrium of its environment, which on an outdoor deck is around 18% (much higher than the KD spec).
- Mixed KD and green members in the same structural assembly: the green members shrink relative to the KD members, causing tilt and binding.
Also known as: KD; kiln-seasoned timber; kiln-dried framing; KD hardwood.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.