How flat does my subfloor need to be?
Flatness and level are different things, and floating floors care about the first and not the second. Knowing how to measure it tells you whether you need to do anything at all.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 3 min
Short answer
Lay a long straightedge across the floor in several directions and look for gaps underneath. Flatness is about local dips and humps under that straightedge; level is about whether the whole floor slopes, which usually does not matter for a floating floor. Manufacturers state a tolerance over a given length — that figure, not your eye, is the test.
This is the question people skip, and it is the one that decides whether a new floor creaks, whether the joints open and whether a click system fails early. A floating floor spans small hollows and rocks over humps, and every step flexes the joints. An old house can be far out of level and still be perfectly flat enough to floor.
Step by step
- Understand the difference between flat and level.Level means horizontal. Flat means without local dips and bumps. A floor can slope by a noticeable amount across a room and still be flat, and that is usually fine for a floating floor. A floor that is dead level but has a dip in the middle is a problem.
- Find the tolerance the flooring actually requires.It is in the fitting instructions, expressed as a maximum deviation over a stated length. Different products and different fitting methods have different figures, which is exactly why there is no universal number.
- Check with a long straightedge, not a short spirit level.A two-metre straightedge or a long level laid on edge across the floor. A short level bridges too little to find a broad dip and reports the local slope instead.
- Move it around and rotate it.Check in both directions and diagonally, and walk the straightedge across the whole area. A hollow that only shows on one diagonal is easy to miss and will still be there under the floor.
- Measure the gaps rather than judging them.Slide something of known thickness under the straightedge to find the size of the worst gap. Comparing that number against the manufacturer's tolerance is the whole test.
- Mark the high spots and the hollows on the floor.Write on the subfloor in pencil. High spots and hollows need different treatments, and it is very easy to lose track of where they were once you start work.
- Take high spots down before filling hollows.Grinding or planing a hump is usually less work than raising everything around it. Do that first, then re-check, because removing the humps often brings the floor inside tolerance without any filling.
- Fill the remaining hollows with a suitable compound.Levelling or smoothing compound for the substrate you have. GuideHQ has a guide on why these fail, and the causes are nearly all preparation.
- Check again after the work, before the floor goes down.The straightedge takes two minutes and this is the last moment it costs nothing.
Tips
- A floor that rocks a straightedge when you press one end is worse than one with a gap under the middle — that is a hump, and humps cause more trouble than hollows.
- On a suspended timber floor, a dip that has appeared recently is worth understanding before covering it up.
Common mistakes
- Checking with a short spirit level. — It reports slope over a short distance, which is not what the tolerance describes. Broad dips pass a short level easily.
- Trying to make an old floor level rather than flat. — It is usually unnecessary, sometimes impossible, and a great deal of work for no benefit to the floor covering.
Questions people ask
Does a slope matter at all?
Not to a floating floor's performance. It may matter to you — furniture and doors notice a pronounced slope — but it is a separate decision from whether the floor can be laid.
Can underlay take up unevenness?
Only a very small amount. Underlay is for acoustics, comfort and sometimes moisture. Relying on it to bridge a dip is how joints fail.