Flatness decides, and it is measured
A floating floor rests on nothing: the boards click into one another and lie loose on an underlay. The whole thing forms a single raft that cannot follow an uneven floor — it bridges it. Wherever the subfloor dips, the raft stays up in the air, and that void shows itself underfoot as movement and to the ear as a hollow sound.
That is measured beforehand, not afterwards: a 2 m straightedge laid in several directions, and the largest hollow recorded. The manufacturers state the tolerance their product allows, and that is what decides whether a levelling compound is needed. We make this check during the visit, because it changes the quotation: a levelling compound is not a line item to be discovered on the day of laying.
The second measurement is an invisible one: the residual moisture in the subfloor. A fresh screed or levelling compound holds water that has to leave before it is shut in under a vapour barrier. Wood laid on a subfloor that is still damp swells; the joints close up and the boards lift, a few weeks later.
Wood moves: gaps, movement joints and thresholds
A wood floor lives with the air of the room. It swells when the air is damp, it shrinks when it is dry, and across the width of a room that movement is measured in millimetres. The whole principle of a floating floor is to leave it room to move.
That room is 8 to 10 mm of gap against every wall, around every heating pipe, against every door frame. The gap is then covered by the new skirting boards and pipe collars: it does not show, but it is there. A floor laid tight against the wall has nowhere to go; it lifts in the middle of the room, and there is no repair for that other than taking it up.
Beyond a certain length, which depends on the product, a movement joint is needed, and at every doorway a threshold that separates two independent rafts. It is also what makes it possible to work on one room later without touching the others.
Then the direction of laying. When in doubt, lay along the longest dimension of the room and with the main light: the end joints blend in instead of being counted, and the room looks longer.
Laying over existing tiles, without taking them up
When the tiling in place is sound, well bonded and flat, the floating floor goes straight over it. It is one of the rare cases where you win on every count: no demolition, no dust, no skip, no disposal — and a job measured in days.
Three checks settle it. First adhesion: tapping a few tiles with a mallet is enough to find what sounds hollow, and what sounds hollow is made good. Then flatness, with the straightedge as above, allowing for hollow grout lines if the tiling is old. And finally height: the new floor rises by the thickness of the underlay and the boards, and the doors still have to clear it and the thresholds still have to be walked over.
That last one is the one most often forgotten. We measure the clearance under the doors during the visit, and if a door has to be planed down, it is written into the quotation beforehand, not noted afterwards.
In a co-owned building: impact sound
Replacing a carpet with a hard floor changes the acoustic life of the building — for the neighbour below more than for you. A footstep, a chair pulled back, an object dropped: these are impact sounds, and they travel through the structure.
Many co-ownership rules cover this point and require the new covering not to worsen the original acoustic performance. The answer lies in the underlay: it is not there for comfort underfoot, it is there to deaden. It is chosen according to the subfloor and the performance sought, and the manufacturers publish the corresponding figures.
So we look at two things before pricing: what your co-ownership rules say, and what the subfloor allows. It is a modest item in a quotation, and it is the one that avoids a dispute with the neighbours lasting years.
On an old timber floor the subject comes with a technical point attached: a floor that creaks before laying will creak after it. The boards underneath are screwed down again while the floor is bare, which is the only moment when that is simple.
