How Much Expansion Gap Should You Leave on a Timber Floor?

Leave a 10mm expansion gap at every wall and fixed obstruction for a domestic room up to six metres, and add 2mm for every additional metre. A 10 metre run needs 18mm, not 10mm, and a solid oak floor needs slightly more than an engineered one.

The gap is not optional and it is not a detail to be hidden with mastic. A floor laid tight to the wall will lift at the joints or tent in the middle as it takes on moisture.

Why does timber need an expansion gap?

Because wood moves with humidity across its width, not its length. In a centrally heated house the seasonal movement can be 3 to 8mm across a four metre room.

Without a gap, that movement has nowhere to go except up. The visible result is a ridge in the middle of the floor or a joint that opens at the end of the room.

How is the gap measured?

Between the floor and the wall, under the skirting, using packers of the specified thickness. Measure at both ends and the middle of each wall.

The gap is hidden by the skirting, not filled. A bead of sealant across it defeats the purpose entirely.

Does engineered flooring need a smaller gap?

Slightly, because the cross-lamination reduces movement, but the gap is still required. A 10mm minimum applies, and the manufacturer’s figure overrides this article.

Engineered boards still move across the width. The plywood core limits the movement, it does not remove it.

How much does solid oak move?

Roughly 0.2 to 0.3 percent across the grain per one percent change in moisture content. A 200mm board can move 1mm for a typical seasonal swing.

That sounds small until it is multiplied across a five metre room. The gap exists because the total movement is the sum of every board.

What about a floating floor?

A floating floor needs the gap on every side, including under door frames and around pipes, and it must not be pinned anywhere. Any fixing defeats the principle.

Floating floors are also more sensitive to the gap than glued floors, because the entire surface moves as one sheet.

How are door frames handled?

Under-cut the frame and casing so the floor slides beneath with the gap maintained. Cutting the floor around the frame leaves a fixed point that the floor will fight.

Use an offcut as a spacer when undercutting. It is the simplest way to get the gap exactly right.

What is the gap at a threshold?

The same 10mm plus the allowance for the run length, covered by a profile that allows movement. A rigid threshold strip screwed through the floor removes the gap entirely.

Where two floors meet, each needs its own gap and the profile has to bridge both. A single profile fixed on one side only is the correct detail.

Does underfloor heating change the requirement?

It increases movement, because the floor is drier in winter and warmer at the surface. Use a larger gap and a flooring product rated for heated substrates.

Solid timber over underfloor heating needs careful specification: a stable species, narrow boards and a controlled humidity range.

How long should the flooring acclimatise?

Forty-eight hours minimum in the room, unopened and stacked with spacers, and longer for solid timber. The moisture content of the boards should be within 2 percent of the room’s equilibrium.

Acclimatising in a garage or a cold conservatory is not acclimatising. It has to be the room it will be laid in.

What happens if the gap is too small?

The floor lifts at a joint, a ridge appears, or the boards cup. All three are irreversible without lifting the floor and relaying it.

The failure usually appears in the first heating season, which is exactly when the floor is new and the installer has been paid.

What if the gap is too big?

The skirting may not cover it, and draughts appear at the wall. 20mm is usually the practical maximum before a deeper skirting or a scotia bead is needed.

A scotia bead hides the gap and allows movement. It is the standard remedy for a renovation where the skirting cannot be removed.

Does the gap apply to laminate?

Yes, and laminates often need more, typically 10 to 12mm, because the material expands more than timber. The manufacturer’s figure is the one to follow.

Laminate also needs gaps at every fixed point, including kitchen units and island plinths. A floor pinned by cabinets tents at the nearest joint.

How is a large room divided?

With a movement joint or a threshold at eight to ten metres, or at a change of direction. The joint is a designed feature, not a failure.

A metal profile in a doorway reads as intentional. A ridge in the middle of a room does not.

What about pipes through the floor?

Each pipe needs a clearance hole of 10 to 15mm around it, with a collar covering the gap. A board cut tight to a pipe is a fixed point.

Radiator pipes are the usual culprit, because they are rigid and the floor moves around them.

How much does a floor-laying job cost?

15 to 40 per square metre for labour, depending on the fixing method and the board format. Underlay and trims add 3 to 8.

The expansion gap costs nothing to get right and everything to get wrong. It is the cheapest part of the specification.

What is the right humidity for a timber floor?

40 to 60 percent relative humidity, maintained year round. Below 40 the floor shrinks; above 60 it swells.

A hygrometer costs less than a metre of oak. Reading it during the first winter tells you whether the heating is drying the floor out.

Should the gap be checked before handover?

Yes, with a feeler gauge at four points per wall. A 10mm packer that will not slide under the floor means the gap is missing.

Check before the skirting goes on. Afterwards the gap is invisible, which is the point, and also the problem.

What is the most common installation error?

Fitting the floor tight to one wall and leaving the gap on the other side only. The floor then moves in one direction and lifts at the tight edge.

The second most common is pinning the floor through a threshold. Both are quick to avoid and expensive to correct. The guide on floor level tolerance covers what the substrate has to be before the boards go down.

Expansion gaps belong to the same family of details as movement joints in tile, and both are decided by the room’s dimensions. The guide on movement joints covers the tile equivalent.