How much gap should I leave for timber to move?

Everyone knows timber moves; the practical question is what to do about it. The answer is always the same shape: identify which dimension is going to change, decide how much, and build a detail that lets it happen without loading a joint or a fixing. The alternative — fixing everything rigidly and hoping — reliably produces splits.

Step by step

1. Identify which dimension will change.Width and thickness change; length does not. So a run of cladding boards moves in the direction across the boards, a wide table top moves across its width, a solid door panel moves across the panel, and a stud does not meaningfully move at all.
2. Estimate the amount from the width and the exposure.As a rough working allowance, take 1 per cent of the width for timber staying in a stable heated room, and up to 2 per cent for anything moving between conditions or living outdoors. A 600mm wide solid top wants six to twelve millimetres of freedom across its width.
3. Let panels float.A solid panel in a frame sits in a groove, unglued, with a small gap at each edge. Glue it only at the centre of the top and bottom edges if you must locate it, so it expands symmetrically. This is the single oldest solution to the problem and it still works.
4. Use slotted or oversized fixings.Where a wide solid component is fixed down to a frame — a worktop to a cabinet, a top to rails — use slotted holes running across the grain, buttons that slide in a groove, or figure-of-eight fasteners that pivot. The screw holds it down without holding it still.
5. Lap or tongue joints rather than butt them.Cladding, panelling and floorboards use shiplap, tongue-and-groove or an overlap so that a gap opening between boards is hidden behind the lap rather than becoming a hole. A butt joint between two boards shows every millimetre of movement.
6. Leave a deliberate expansion gap at the perimeter.Anything laid across a whole room — boarding, panelling, a run of cladding — needs a gap at each end that is then covered by trim fixed to the wall rather than to the boards. The trim hides the gap and lets the boards slide under it.
7. Fix at one point and let the rest move.Long components should be pinned in one place and free everywhere else, so the movement all goes in a predictable direction. Fixing both ends rigidly guarantees a break somewhere in the middle.
8. Acclimatise before you fit, and fit at a sensible moisture content.The allowance you calculate assumes the timber is somewhere near its eventual equilibrium. Timber fitted very wet will only shrink, and timber fitted very dry will only swell, so let it settle in the room first and your allowance can be split either way.

Tips

• Fit tightly in summer and you will get gaps in winter; fit with gaps in winter and it closes up in summer. If you can choose, fit at a middling time of year and split the difference.
• Man-made boards let you dodge the whole problem for wide flat parts — a veneered MDF or plywood panel of the same appearance moves so little that it can be glued in solidly.
• Breadboard ends, the classic solution for the end of a wide table top, work because the end piece is pinned only at the centre and slotted at the outer pins. Copy the principle rather than just the look.

Common mistakes

• Gluing a solid panel into its frame — The panel cannot get narrower, so it splits, or it pushes the frame apart and opens the frame joints. Panels float — that is what the groove is for.
• Butting cladding boards tightly together on installation — They were probably at their driest when you fitted them. When they swell they have nowhere to go, so they cup, buckle or force their fixings out.
• Allowing for movement along the length — Length change is negligible and a gap left at the end of a run in the length direction just looks like a mistake. Put the allowance across the grain where the movement actually is.

Questions people ask