What moisture content should timber be before I use it indoors?

Equilibrium moisture content, the figures for heated and unheated rooms, and why acclimatising matters more than the number on the invoice.

Difficulty: intermediate
Time: 15 min
Read: 4 min

Short answer

Timber in a heated house settles at roughly 8 to 12 per cent moisture content, and in an unheated or outdoor situation at 15 to 20 per cent. Kiln-dried joinery timber typically arrives around 10 to 12 per cent. What matters is letting it reach equilibrium in the room where it will live before you work it.

Wood is hygroscopic: it constantly exchanges moisture with the air until it reaches equilibrium with it. Every shrinkage crack, opened joint and stuck door is that process happening after the work was finished rather than before.

What you'll need

moisture meter (optional)
storage space in the destination room

Step by step

Understand equilibrium moisture content.Timber has no fixed moisture content. It settles at whatever the surrounding air's temperature and humidity dictate, and it moves whenever those change.
Know the target figures.A continuously heated house sits around 8 to 12 per cent. An unheated room or a garage is more like 14 to 16. External joinery equalises around 15 to 18 per cent depending on the season.
Check what you bought.Kiln-dried joinery stock is typically supplied at 10 to 12 per cent. Construction timber is often sold at up to 20 per cent, and treated timber can be wetter than that.
Use a moisture meter if the job justifies it.A cheap pin meter is adequate for relative comparison. Take readings on several boards and on end grain and faces, not one spot.
Acclimatise in the destination room.A week or two, stickered, in the room it will be fitted in, with the heating running as it normally would. This is more important than the number the timber arrived at.
Allow for the direction of movement.Timber moves far more across the grain than along it, and more tangentially than radially. A wide board can move several millimetres across its width between summer and winter.
Design for movement rather than trying to prevent it.Floating panels in frames, slotted fixing holes for wide tops, and expansion gaps at floor perimeters all let timber move without damaging anything.
Do not fit wet timber in a heated room.Skirting fitted at 18 per cent into a house at 10 will shrink noticeably, opening every mitre and leaving gaps at the wall.
Watch out for new-build and post-plastering conditions.A freshly plastered or screeded building holds enormous amounts of moisture. Timber fitted then equalises high and shrinks dramatically once the building dries out.

Tips

The number matters less than the acclimatisation. Timber that has been in the room for two weeks is at the right moisture content by definition.
Never fit timber flooring or joinery in a building that is still drying out from plastering or screeding.
Timber movement across the grain can be many times its movement along it, which is why wide boards need slotted fixings.

Common mistakes

Fitting joinery straight off the delivery lorry — It has been stored in a merchant's yard or an unheated shed at a much higher moisture content. Once inside a heated house it shrinks, and joints, mitres and panels open up.
Fixing a wide solid top rigidly at both edges — The board will move across its width with the seasons. Held rigidly, it splits. Slotted holes or buttons let it move without damage.

If it doesn't work

Mitres opened up a few weeks after fitting

Cause: Timber shrank as it dried to room conditions — Fix: Fill or caulk if it is painted work. Next time acclimatise before fitting.

Solid wood top split down its length

Cause: Fixed rigidly across the grain — Fix: Repair the split and refit with slotted holes or buttons that allow cross-grain movement.

New floor cupping or gapping

Cause: Laid before acclimatising, or into a building still drying out — Fix: Gaps may close seasonally. Boards laid too wet and now shrunk will not recover and may need lifting.

Doors sticking in winter and fine in summer

Cause: Normal seasonal movement plus high humidity — Fix: Ease the sticking edge in the wet season, and address the humidity — this is usually a ventilation problem as much as a timber one.

Questions people ask

Do I need a moisture meter?

Not for most DIY. For laying solid wood flooring, or for a large joinery project in a new build, a cheap meter is worth having as a check before committing.

Is air-dried or kiln-dried better?

Kiln-dried is drier, more consistent and ready for interior use. Air-dried timber is fine for exterior work and for stock that will acclimatise, but rarely gets low enough for a heated house on its own.

What about MDF and plywood?

They move far less than solid timber, which is one of their main advantages. They still swell when wet, especially at the edges, so they are not immune.