Short answer: screed usually cracks because it dried too quickly, was laid too thin for the build-up, was not compacted properly, or had no room to move at walls, doorways and corners. Most cracking is preventable with the right depth, edge strip, joints and a steady drying process.
The most common causes
- Drying too fast. Hot weather, draughts or heaters dry the surface before the body of the screed, causing shrinkage cracks and curling at the edges.
- Underfloor heating switched on too early, or turned up to full temperature on a new screed without a gradual heat-up.
- Too thin for the build-up. A floating screed on insulation needs enough depth to spread loads without bending.
- Poor compaction in a sand and cement screed, leaving weak, crumbly areas.
- Missing edge strip or movement joints. A screed needs to move slightly. Without compressible strip at walls and joints at doorways and re-entrant corners, the stress shows as cracks.
- Uneven insulation that rocks underneath the screed.
- Too much water in the mix, which weakens it and increases shrinkage.
Cracks, curling and hollow spots
Fine hairline cracks from early shrinkage are common and often harmless under most floor finishes once the screed is stable. Curling is when edges or joints lift as the top dries faster than the bottom. A hollow sound when tapped usually means a bonded screed has lost its bond to the slab, often because the slab was not properly prepared.
How we prevent it
- Correct depth for the screed type and build-up
- Perimeter edge strip on every wall and around columns
- Joints planned at doorways and awkward shapes
- Fibre reinforcement where it helps
- Thorough compaction and a careful finish
- Clear advice on drying and the heating start-up schedule
Can a cracked screed be repaired?
Often, yes. Stable cracks can be opened up and filled with resin, sometimes with metal stitching across the crack. Hollow or crumbling areas are cut out and re-laid. If cracking is widespread, the cause needs finding first, or it will return. See our floor preparation service.
Quick answers
Will tiles crack if the screed is cracked? They can if the crack is still moving. Stable cracks can be treated, or an uncoupling membrane used, before tiling.
Is some cracking normal? Fine shrinkage cracks can be. Wide, growing or uneven cracks are not and should be checked.
Written by Ben Phillips, director of Chelmsford Floor Screeding Ltd, floor screeding contractors based in Billericay with 20+ years of experience across Essex and London. More about us.