How do I cut an expansion gap into a floor that is already down?

A floating floor expands as one continuous sheet. If it is held anywhere at all, the expansion has nowhere to go and the floor lifts in the middle, so the job is finding every restraint, not just the obvious one.

What you'll need

• An oscillating multi-tool with a flush blade
• A plunge saw or a laminate jamb saw
• A pry bar
• A 10mm spacer
• Scotia beading or a pipe collar set
• A vacuum

Step by step

1. Confirm the diagnosis.A floor that peaks, lifts at the edges or opens at the joints when the weather changes is expanding into a restraint. A floor with a single lifted board is a different problem.
2. Map the perimeter.Take the beading off and check the gap all round with a 10mm spacer. It only takes one wall with no gap to lock the whole floor.
3. Check the door frames and architraves.Flooring butted hard against an architrave is one of the two most common causes. It should run underneath the architrave with clearance.
4. Check the pipes.Radiator pipes through the floor need a hole about 20mm larger than the pipe, closed with a two-part collar. A tight hole pins the board at that point.
5. Check under fitted units.Flooring run under kitchen units, wardrobes or a heavy island is held down by their weight. Nothing can be done except cutting the floor free at the unit line if it is causing lifting.
6. Check the thresholds.A threshold strip screwed through the floor rather than into the subfloor pins the boards. Lift it and refix into the subfloor through the gap.
7. Trim the perimeter.A multi-tool with a flush blade run along the skirting takes the board back neatly. A laminate jamb saw or a plunge saw with a guide is faster on a long run.
8. Undercut the architrave rather than cutting the floor short.Where the floor meets a door frame, cutting the architrave up by the floor thickness gives a neater result and a proper gap.
9. Vacuum as you go.The offcuts fall into the gap you have just created and reduce it back to nothing. Clear the gap before covering it.
10. Let the floor relax.A floor that has been under compression takes days or weeks to settle back. Do not judge the result the same afternoon.
11. Cover the gap without pinning the floor.Scotia beading pinned into the skirting, never down through the floor. Pin it into the floor and you have recreated the problem exactly.
12. Deal with any damaged boards.Boards that have been peaked for a long time may have damaged joints. Replace those individually once the floor has relaxed.

Tips

• Ten millimetres is a normal minimum, but larger rooms need more — a common rule is 1.5mm of gap for each metre of floor run, with 10mm as the floor.
• If the floor runs continuously through a doorway into another room, it needs an expansion break at the threshold. Long continuous runs are the hardest to accommodate.
• A floor laid in winter and locked tight will lift the following summer. Timing explains a lot of 'it was fine when it was laid'.

Common mistakes

• Trimming only the wall where the floor is lifting — The floor moves as one sheet, so the restraint is often on the opposite side of the room from the visible damage. Trimming one wall leaves the other pinch points intact and the floor lifts again.
• Pinning the beading down through the floor — Beading is there to hide the gap, not to hold the floor. A pin through the board removes the expansion gap at that point regardless of how much air is behind it.

If it doesn't work

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