Kiln dried or air dried timber — which should I buy?
What each process does to the timber, why merchant carcassing is often soaking wet, and how to tell which you have been sold before you build with it.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 4 min
Short answer
Kiln dried timber has been dried in a controlled chamber to a low moisture content, is straighter, lighter and ready for interior use. Air dried timber has been stacked and left, is at whatever the outdoor air imposes — around 15 to 20 per cent — and needs conditioning indoors before use. Much merchant carcassing is neither: it is wet-graded, sold at whatever moisture content it arrived at, and it will move a lot.
This decides how much your timber is going to move after you have built with it, which makes it more important than most of the things people do worry about. It is also the explanation for the common complaint that a length of timber from the merchant is heavy, cold and damp to the touch — that is not a fault, it is unseasoned stock.
Step by step
- Understand kiln drying.The timber is stacked in a chamber and dried under controlled temperature and humidity over days to weeks, down to a target moisture content — typically around 10 to 12 per cent for joinery stock. It is quick, consistent and it also kills insects and their larvae.
- Understand air drying.Stacked outdoors under cover with sticks between the layers, and left. It takes roughly a year per 25mm of thickness as a very old rule of thumb, and it can only ever reach equilibrium with outdoor air, which in Britain is around 15 to 20 per cent.
- Recognise wet-graded structural timber.Much carcassing is graded and sold green or partly dried, often marked "wet graded" or "WET" on the stamp. It feels cold and heavy, it is often visibly damp on the ends, and it is entirely normal for structural use in a roof or floor where it will dry in place.
- Pick kiln dried for anything indoors and visible.Joinery, furniture, skirting, shelving, internal studwork you will be plasterboarding immediately. It arrives close to where it needs to be, so it stays where you put it.
- Accept air dried or wet stock for outdoor and structural work.A fence, a shed frame, a deck substructure and roof timbers all live at outdoor humidity anyway, so drying them to 10 per cent first would be pointless and expensive.
- Acclimatise whatever you buy in the room it will live in.This matters more than which process it went through. Stack it flat with sticks between layers, in the actual room, for at least a week and preferably longer, before you cut joints in it.
- Check it if the job justifies it.A pin-type moisture meter is inexpensive and settles the argument. Under about 12 per cent is ready for a heated house; 15 to 20 per cent means it is at outdoor equilibrium and will shrink further indoors.
Tips
- Weight is a decent free test. Two identical lengths from different racks, one noticeably heavier, means the heavy one is wetter and will shrink more.
- Kiln drying does not stop timber moving afterwards — it just starts it near the right place. A kiln-dried board left in a damp garage for a month is no longer kiln dried in any useful sense.
- Air-dried hardwood is preferred by some furniture makers because slow drying is gentler on the timber and leaves the colour and workability better. It still has to be conditioned indoors before use.
Common mistakes
- Building interior joinery from wet-graded carcassing — It can be 25 per cent or more when you buy it and will settle at 10 per cent in a heated house. That is a large shrinkage, and it opens every joint, splits panels and twists frames after the job is finished.
- Storing kiln-dried timber outside or in an unheated garage until you need it — It takes moisture back up quickly. Bring it into the room where it will be used as soon as you buy it, stacked flat with air round it.
Questions people ask
Why is my new timber from the merchant soaking wet?
Either it is wet-graded structural stock that was never dried, or it is pressure-treated timber straight from the treatment vessel, or it has simply been standing in an open yard in the rain. All three are normal. Stack it flat with air round every face and let it dry before you build anything precise from it.
Can I dry timber myself?
Yes, slowly and with care: stack it flat on level bearers with sticks between the layers so air reaches every face, keep it out of sun and away from direct heat, weight the top of the stack, and be patient. Drying it fast — in front of a heater, for instance — is how you split and twist it.
Does kiln drying weaken timber?
Badly done kiln drying can cause internal checking and case hardening, which are real defects, but properly kiln-dried commercial stock is not weakened. Structural grading rules account for the drying state, which is why you see wet-graded and dry-graded stamps.