Flat
Lay a 2 m straightedge (or long spirit level) across the floor in several directions. Measure the largest gap under it.
- Laminate and engineered wood: usually max 3 mm over 2 m.
- Rigid vinyl and large tiles: often max 2 mm over 2 m, sometimes stricter.
- Thin flexible vinyl: as flat as you can get it; every bump shows.
Grind or sand high spots. Fill hollows with a self-levelling compound after priming. Small dips can be filled with a trowel-applied repair mortar.
Dry
New sand-cement screed dries slowly: a rough rule is a week per centimetre for the first 4 cm, and much longer beyond. Test it with a moisture meter or a CM test before laying wood or laminate. Anhydrite (calcium sulphate) screeds have much stricter limits than cement screeds.
On concrete and screed, a floating floor needs a vapour barrier: 0.2 mm PE foil with 20 cm overlaps, taped and turned up at the walls. For timber subfloors, do not use foil; check the moisture of the boards instead.
Clean and firm
Remove old adhesive ridges, nails and staples. Screw down any creaking timber boards. Vacuum thoroughly; grit under a floating floor causes squeaks.
Old floors underneath
Many floating floors can go over existing tiles if they are firm and flat; fill wide grout lines. Never lay over carpet or soft vinyl.