How do I fill gaps between floorboards?

Gaps between floorboards are a draught problem and a dust problem, and almost every filler fails because timber floors are never still.

What you'll need

• Matching timber slivers, or flexible filler
• A sharp chisel and a mallet
• Wood glue and a vacuum

Step by step

1. Accept that the boards will move.Timber shrinks in winter with the heating on and swells in summer. Any fix must accommodate a few millimetres of seasonal movement.
2. Clean the gaps thoroughly first.Vacuum with a crevice tool and scrape out old debris. Filler over dust does not bond.
3. Use timber slivers for the best result.Cut thin wedges of matching timber, glue them, tap them into the gap and plane flush when dry. It is the traditional method and it lasts.
4. Match the timber species and grain.Slivers cut from an offcut of the same floor, or from a board lifted from a cupboard, disappear completely. Different timber shows as a stripe.
5. Use flexible filler for narrow gaps.A flexible acrylic floor filler tinted to match. It moves with the boards where rigid filler cannot, though it is visible on close inspection.
6. Consider rope for wide gaps in period floors.Natural fibre rope pressed into the gap and stained. Traditional, flexible, effective as a draught stop, and easily reversible.
7. Avoid rigid wood filler.It has no give at all, so the first heating season pops it out of every gap. It is the most commonly used and least successful option.
8. Deal with the draught, not just the gap.If the aim is warmth, the underfloor void is the real source. Filling gaps helps; insulating below or fitting a membrane helps far more.
9. Work with the room's humidity.Fill in winter when the boards are at their most shrunk, so the filler is not squeezed out when they expand again.
10. Sand and finish afterwards.Plane or sand slivers flush once the glue is fully cured, then finish the area to match the rest of the floor.

Tips

• Fill in winter when the gaps are widest. Filling in summer means it is squeezed out later.
• Rigid wood filler always cracks out. Use timber slivers or a flexible product.
• If the aim is warmth, the underfloor void matters far more than the gaps.

Common mistakes

• Using rigid wood filler in the gaps — Floorboards move seasonally by several millimetres. Rigid filler has no give, so it is forced out within the first heating cycle and leaves a worse gap than before.
• Filling in summer when boards are swollen — The gaps are at their narrowest, so anything you put in is compressed and ejected when the boards shrink back in winter.

If it doesn't work

Questions people ask