The tolerance almost every hybrid, laminate and engineered timber manufacturer specifies is the same: the subfloor must be flat to within 3mm over any 2 metre length, and some also add 1mm over 300mm for local bumps. That is the number your warranty depends on. Here is what it means, how to check it and what to do when the floor fails.
Flat is not the same as level
Level means the floor is horizontal. Flat means it has no humps or dips. A floating floor needs to be flat; it does not need to be level. A slab that falls 20mm from one end of the room to the other but has no local dips is fine. A slab that is perfectly level but has a 6mm dip in the middle is not.
How to check
- Sweep the floor clean and remove any old adhesive lumps, nails or paint blobs.
- Lay a 2 metre straight edge, aluminium level or straight length of steel on the floor. Try to slide a 3mm spacer (a 3mm tile spacer or a coin stack) under it anywhere along its length. If it slides under, that spot is out of tolerance.
- Repeat across the room in a grid, and along both diagonals. Mark low spots with chalk and high spots with a different colour.
- For humps, rock the straight edge: if it pivots, you have found a high spot.
Why it matters
A floating floor is a single sheet of planks locked to each other. Over a dip it bridges and flexes every time it is stepped on, and the click joints work loose and eventually crack. Over a hump the planks rock and the joints open. Bounce, hollow sounds, peaking and gaps all trace back to this one measurement, which is why installers treat it as non-negotiable.
Fixing low spots
Prime the area and pour a self-levelling compound, feathering the edges out to nothing. Small dips can be filled with a patch compound and a trowel. Bags of leveller, primer and patch are on our Cemimax floor levellers page and our floor levelling cost guide gives the prices if you hire it out.
Fixing high spots
On concrete, grind them down with a hired concrete grinder and dust extractor. On timber boards, plane or sand the ridge, or screw down loose boards that have lifted. Where a whole timber floor is uneven, sheeting it with 12mm plywood or 6mm fibre-cement is often quicker than chasing individual boards.
Other subfloor checks
- Dry. Concrete should read below 75 per cent relative humidity or the manufacturer’s moisture limit, and always gets a moisture barrier.
- Clean. No dust, oil, curing compound or old adhesive that stops underlay sitting flat.
- Sound. No crumbling concrete, loose boards or rotten timber.
Once the subfloor passes, the rest of the job is straightforward; our hybrid installation guide takes it from there.
Frequently asked questions
Can I lay a floating floor over tiles?
Yes, if the tiled surface passes the same 3mm over 2 metre check. Fill deep grout lines with leveller.
Does thicker underlay fix an uneven floor?
No. Soft underlay makes bounce worse. Fix the subfloor, then use the underlay the manufacturer specifies.
What tolerance do glue-down vinyl planks need?
The same 3mm over 2 metres, and the surface must also be smooth, because a thin vinyl plank shows every grain of grit and trowel ridge through the surface.