How to prepare a subfloor
Almost every failed floor starts underneath it. This guide shows you how to get the subfloor clean, dry and flat before the first plank or tile goes down – and how to work out exactly how much self-leveling underlayment to buy.
Work out your floor area first →
Whatever you lay on top – laminate, vinyl, tiles or parquet – is only as good as the subfloor beneath it. A damp or uneven base comes back months later as lifted joints, cracks or hollow-sounding spots. So get the ground right first: the free planner gives you your room's exact area, and the self-leveling underlayment calculator turns that area and your layer thickness into the number of bags. Then work through the five steps below.
Preparing the subfloor in 5 steps
Assess the subfloor
The subfloor has to be clean, dry, sound and load-bearing before anything goes on it. Strip out old coverings and scrape off any residual adhesive, sweep and vacuum, then tap for hollow or crumbling spots – a base that is already failing will pull your new floor down with it.
Moisture-test the screed
A screed that looks dry on top can still be damp inside, and trapped moisture ruins everything laid over it. Have the screed checked with a calcium carbide moisture test and wait until the reading is under the limit for your floor type before you go any further.
Check the flatness with a straightedge
Lay a 10 ft straightedge across the floor in several directions and look for gaps under it – no more than about 3/16 in of deviation should show. Mark the high spots and the hollows with a pencil, so you know exactly where to grind down and where to fill.
Prime and level the low spots
Roll on the primer the compound needs and let it dry, then pour self-leveling underlayment into the hollows – or across the whole floor for a badly uneven base – and spread it with a smoother. Let it cure fully before you walk or build on it; rushing this is the most common mistake.
Clean and re-check before you lay
Once the compound has cured, vacuum the floor again and run the straightedge over it one last time to confirm it is flat and sound. Only then roll out the underlayment and start installing – a subfloor checked twice almost never comes back to haunt you.
Frequently asked questions
- How flat does the subfloor have to be?
- The common rule is no more than about 3/16 in of deviation under a 10 ft straightedge. Small dips can be filled with self-leveling underlayment; a badly uneven floor is worth leveling across the whole surface, because every bump telegraphs through a thin floor covering and stresses the joints.
- Do I need a primer before the self-leveling underlayment?
- Almost always, yes. The primer binds the dust, stops the base sucking the water out of the compound too fast and helps it flow out level. On very absorbent screeds you may need two coats. Skipping it is the usual reason self-leveling underlayment cracks or fails to bond.
- How long does self-leveling underlayment take to dry?
- You can usually walk on it after a few hours, but it needs far longer before a floor goes down – often a day or more, and longer for thick layers. Follow the bag's stated times and, for moisture-sensitive coverings, re-test before you lay.
Buy the right amount of self-leveling underlayment
Self-leveling underlayment is sold by the bag, and each bag covers only so many square feet at a given thickness. Draw your room in the free planner to get the exact floor area, then let the self-leveling underlayment calculator turn it into bags for your layer thickness – so you order enough in one go instead of making a second trip to the shop.
Open the free planner →