Where this starts
A cement-based screed shrinks as it dries. That is not a defect; it is what the material does, and the whole design of a screed — its thickness, whether it is bonded, its bay sizes and its joints — is about giving that shrinkage somewhere to go.
So cracking is usually the shrinkage arriving somewhere it was not given room. The PATTERN is the most informative thing available, and it is visible before anything is measured: cracks running straight across a doorway or diagonally from a re-entrant corner are restraint cracks, where the screed was held while it wanted to move. A fine random network over an area is drying-related. A single crack with a step in level across it is usually the base moving rather than the screed shrinking.
The second question is whether the crack matters, and that depends on what goes over it. A fine shrinkage crack under a floating floor is often of no consequence; the same crack under a rigid tiled finish will transmit straight through into the tiles unless something isolates it.
Several of the causes below are quantities this site computes — thickness, mix proportions, bay area — which is why they are worth checking against what was actually laid rather than treated as unknowable.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Restraint at doorways, columns and re-entrant corners
Most commonA straight crack running across a doorway opening, or a diagonal one leaving an internal corner. The crack is where the screed's plan shape is narrow or notched.
How to check: Look at where the cracks start and end. A crack that joins two corners of an opening is restraint, not a material failure.
Dried too fast
Most commonA fine, random, map-like network over an open area rather than lines between features, appearing early. Often worse near heat, a draught, or a sunlit area.
How to check: Ask what the conditions were in the first days: heating on, windows open, direct sun, or a dehumidifier running all count.
Too much water in the mix
CommonWidespread fine cracking together with a weak, dusty surface that scratches easily — the strength and the cracking fail together.
How to check: Scratch the surface firmly with a coin in a sound area. A surface that powders readily points at the mix rather than the conditions.
Bays too large, or joints omitted
CommonCracking roughly dividing a large area into smaller ones — the screed making its own joints where none were provided.
How to check: Measure the uninterrupted area and look for any movement or day joints. A crack that has subdivided a bay tells you the bay was too big.
Too thin for the type laid
CommonCracking and breaking up concentrated in a specific area, commonly where the base was high, along edges, or where it is loaded.
How to check: Measure the actual thickness at a crack. Bonded, unbonded and floating screeds have different minimums, and the difference between them is substantial.
Debonding from the base
Less commonHollow sound when tapped, sometimes with the screed lifting at the crack edges or curling upward at a corner.
How to check: Tap across the floor and map what sounds hollow against where it has cracked. The two usually coincide.
Movement in the base rather than the screed
Less commonA crack that continues a crack in the structure beneath, or one with a difference in level across it rather than just a gap.
How to check: Run a straightedge across the crack. A level difference means something below moved, and the screed simply followed it.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
The pattern separates restraint, drying and base movement before any measurement, and a dated photograph is what tells you later whether it is still moving.
- 2
A step in level is the one observation that moves this from a screed problem to a base problem, and it changes who needs to look at it.
- 3
It identifies debonding, which often coincides with the cracking and changes the repair from filling to relaying.
- 4
Thickness is the commonest specification to be missed on site and the easiest to check once a crack has opened. It also determines whether the screed can simply be repaired.
- 5
A fine crack under a floating floor may need nothing; the same crack under tiles needs isolating or it reappears in the finish. The remedy depends on the finish, not the crack.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a straightedge shows a difference in LEVEL across a crack, which indicates movement in the structure beneath rather than shrinkage in the screed.
- Where a crack in the screed continues a crack visible in the slab or the walls, which places it in the building rather than the finish.
- Where the screed is over a heated floor and the cracking may have exposed or damaged pipework or cable.
- Where large areas sound hollow and lift underfoot, which is a relay rather than a repair and is worth specifying properly.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.