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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 levelling compound 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, and check your room's area in the free planner so you buy the right number of bags of levelling compound before you work through the five steps below.
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.
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 CM measurement (the carbide method) and wait until the reading is under the limit for your floor type before you go any further.
Lay a 2 m straightedge across the floor in several directions and look for gaps under it – no more than about 3 mm 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.
Roll on the primer the compound needs and let it dry, then pour self-levelling compound 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.
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 underlay and start laying – a subfloor checked twice almost never comes back to haunt you.
Self-levelling compound is sold by the bag, and each bag covers only so many square metres at a given thickness. Draw your room in the free planner to get the exact floor area, so you order enough bags in one go instead of a second trip to the shop.
Open the free planner →