Why timber stairs creak, gap and move, and how moisture content prevents it
Written by Scott Jones, The Stair Guys, independent staircase measuring and sourcing specialists·Last updated
A staircase is a lot of timber in one place, and timber is never completely still. It takes up and gives off moisture with the air around it, and as it does it swells and shrinks a little. That is normal and it is not a fault, but if it is ignored it is what causes the two complaints people bring: small gaps that open up, and creaks that come and go with the seasons. Getting the moisture right is what keeps a stair quiet and tight.
Moisture content and the heated home
Timber settles at a moisture content in balance with its surroundings, its equilibrium moisture content. That target is not the same indoors and out. For internal joinery in a normally heated UK home it sits around 9 to 13 percent, and in a warm, continuously heated house it can dry further, toward 6 to 10 percent. A sensible working band to aim at is roughly 8 to 12 percent.
This is the single most important number, and it is where cheap or rushed work goes wrong. The 16 to 18 percent moisture content quoted in a lot of general timber articles is for external, exposed timber, not for an interior staircase. If a stair is made or stored at that moisture and then fitted into a warm, dry house, it will shrink as it dries down to its indoor level, and that is when gaps open at the joints. The heating season makes it worst: through winter the house dries the timber, it shrinks and can gap or creak, and in summer it takes moisture back and closes up again. A stair fitted at the right moisture in the first place moves far less either way.
Which way wood moves, and why quarter-sawn is steadier
Wood does not move evenly in all directions. It moves most across the growth rings (tangentially) and about half as much along them (radially), and hardly at all along its length. That matters for how a board is cut. A flat-sawn board moves and can cup more across its width; a quarter-sawn board, cut so the rings run through its thickness, is noticeably more stable across the width, roughly half the movement. It costs more, but on the parts where stability shows most, treads and strings, quarter-sawn is the calmer choice.
Acclimatise before you fix
Because timber moves toward the moisture of its surroundings, the trick is to let it get most of the way there before it is fixed solid. A staircase brought from a cold, damp store straight into a warm house and screwed down that day will do its shrinking after it is fitted, which is exactly when you do not want it. Letting the stair sit in the house, in the conditions it will live in, so it is within a couple of percent of its settled moisture before final fixing, means most of the movement has already happened. It is unglamorous, and it is one of the quiet differences between a stair that stays tight and one that starts talking.
So, gaps and seasonal creaks
Put together, this explains what you are seeing. A small gap that opens in winter and closes in summer is normal timber movement, not a failure, and it is far smaller on a stair fitted at the right moisture. A creak that comes and goes with the heating is usually the same story, the timber shrinking and rubbing slightly at a joint as it dries. That is different from a creak that is there all the time, which is more likely a loose wedge or glue block, covered in how a staircase is put together. Knowing which is which tells you whether it is the wood behaving normally or a joint that needs attention.
SourceMoisture in timber and in-service moisture bands: TRADA Wood Information Sheet, based on BS EN 942
Frequently asked
Why is my wooden staircase creaking in winter?+
Usually because central heating is drying the timber. As wood loses moisture it shrinks slightly, and a joint can rub as it moves, which you hear as a creak that comes and goes with the heating season. A stair fitted at the right moisture content for a heated home, around 8 to 12 percent, moves far less. A creak that is there all year round is more likely a loose wedge or glue block rather than moisture.
Why are gaps appearing in my wooden stairs?+
Timber shrinks as it dries, and a heated house dries it through winter, so small gaps can open at the joints and often close again in summer. The usual cause of larger gaps is a stair that was fitted too wet, at an external moisture content nearer 16 to 18 percent, then shrank as it dried to its indoor level of around 8 to 12 percent. Fitting at the right moisture, and using quarter-sawn timber, keeps movement small.
Do you need to acclimatise timber before fitting a staircase?+
Yes, ideally. Letting the staircase sit in the house in the conditions it will live in, so it reaches close to its settled moisture content before final fixing, means most of its shrinking or swelling happens before it is screwed down rather than after. Fixing a stair solid the moment it arrives from a cold, damp store is what leaves the movement to show up later as gaps and creaks.
Sources
Primary sources we used and reconciled before publishing.
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