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Technical guide 2 min read

Subfloor Requirements

What the surface under your floor has to do, how to check it properly, and what to do when it fails. This applies to every floor type we supply.

Applies to
Every floor type
What matters
Dry, flat, sound, clean
Tools
Straight edge and moisture meter
Usual fix
Levelling compound
1/ 6
01

Why this is the page that matters

Almost every flooring complaint we see traces back to the subfloor rather than to the boards. A floor that bounces, a joint that has opened, a plank that has lifted at one end, a vinyl surface with a visible ridge: in each case the material did what it was always going to do over the surface it was given.

The other reason to take it seriously is commercial. A subfloor problem is invisible once the floor is down and expensive to reach afterwards, because reaching it means lifting the floor you have just paid for.

Good to know: Preparation is boring, cheap and permanent. Correction is interesting, expensive and temporary. Spend the time at the start.
02

The four requirements

Every subfloor has to meet all four. Three out of four is a failure, not a pass.

Dry
Measured with a meter, not judged by eye or by touch. Concrete and screed hold water long after the surface looks and feels dry.
Flat
Checked with a long straight edge in several directions including diagonally, not by looking along the floor.
Sound
No movement, no loose screed, no flaking paint, no crumbling patches. A floor fixed to something loose is loose.
Clean
Vacuumed, free of dust, old adhesive and grit. Adhesive will not bond to dust and grit telegraphs through thin floors.
Important: The tolerances that apply are the ones in your product fitting instructions. They differ by construction and by fitting method, and where they state a figure that figure governs.
03

How to check properly

Twenty minutes with two tools tells you more than any amount of looking.

1
Sweep and vacuum first
You cannot assess a surface you cannot see, and loose grit under a straight edge gives a false reading.
2
Walk the straight edge across the room
Move it in a grid and then diagonally. Mark every hollow and every high spot on the floor itself with chalk as you find them.
3
Measure moisture in several places
Test near external walls, in the middle of the room and anywhere that looks darker. One reading in one spot proves nothing.
4
Check for movement
On a timber subfloor, walk it and listen. Screw down anything that flexes or creaks before the new floor goes over it.
5
Photograph what you find
If a problem emerges later, the record of what the subfloor looked like is worth having.
04

When it fails the check

A failed check is not a disaster. It is information you got at the only point where acting on it is cheap.

Hollows and general unevenness in concrete are usually best answered with a levelling compound, which is faster and more reliable than trying to grind an existing surface true. High spots and old adhesive ridges need grinding back. A timber subfloor that moves needs fixing down or overboarding. Moisture is the one problem that cannot be worked around: it needs time, or a suitable barrier where the product allows one, and neither is optional.

Hollows and dips
Levelling compound, applied over the whole area rather than patched locally.
High spots and ridges
Grind back. Filling around a high spot does not remove it.
Movement or creak
Fix the boards down properly or overboard. A new floor will not stiffen an old one.
Failed moisture reading
Wait, or use a barrier where the product permits one. Do not fit and hope.
Important: Fitting over a subfloor that failed a moisture test is the single most common cause of a floor that has to be replaced.
05

Questions about subfloors

How dry does a concrete subfloor need to be?
In UK practice following BS 8203 a hygrometer sealed to the slab should read 75% relative humidity or below for resilient and multilayer floors and 65% for solid timber. A new screed dries at roughly a day per millimetre for the first 50 mm under BS 8204, so the reading, not the calendar, says when it is ready.
How flat is flat enough?
Flat enough for the product and the method: the fitting instructions state a tolerance under a straight edge, and that figure governs. Check across the room and diagonally; fill hollows with a levelling compound and grind high spots rather than hoping the floor will bridge them.
Can I lay a new floor over old floorboards?
Yes, once they are screwed down where they move and levelled with a plywood overlay where they are uneven. A glued floor needs the ply; a floating floor can go over sound, level boards on an underlay. The void underneath must be ventilated, so check the airbricks first.
What if the subfloor fails the check?
A failed check is information you got at the only point where acting on it is cheap. Hollows and unevenness in concrete are answered with a levelling compound, damp with a surface damp proof membrane or more drying time, and loose or springing boards by screwing down and overlaying. What does not work is laying the floor and hoping.
06

Related reading

Not sure your subfloor is ready?
Tell us what you have found and what you are planning to lay. We would rather answer the question now than replace a floor later.
Ask our team Guide centre
Continue reading
Technical guide Moisture, Humidity and Room Climate Fitting guide Engineered Wood Installation Guide Fitting guide Herringbone Installation Guide
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