Liquid (flow) screed creates smooth, level floors, but it can crack if conditions aren’t ideal. In London projects it’s also common to see fine hairline cracks as screed dries these are often superficial. Wider, jagged or long straight cracks usually signal stress or movement and should be assessed before flooring goes down.
Common causes
- Natural shrinkage while curing: as moisture leaves the screed it shrinks, causing fine “map” or spider-web hairlines (often cosmetic).
- Rapid or uneven drying: sunlight, draughts, heaters/fans, open windows, or turning UFH on too early can dry the surface faster than the base, leading to crazing or larger splits. A common rule is wait ~7 days before starting UFH, then raise temperature gradually (a few °C per day).
- Poor subfloor prep / debonding: dust, oil, debris, or a weak base can stop proper bonding; later movement transfers into the screed.
- No edge strip / movement joints: pouring tight to walls or across large uninterrupted areas can trap expansion and cause straight tension cracks.
- Incorrect mix / excess water: too much water increases shrinkage and weakens strength; wrong additives can create weak spots.
- Too thin / lack of reinforcement: pouring below the recommended thickness (often around 40mm for anhydrite systems) can increase curling and cracking.
Hairlines vs structural cracks
Hairlines are typically surface-only and often fine to tile over. If cracks are wider than a hairline (1–2mm+), feel loose/soft, run long and straight, or track over UFH pipes, treat them as a repair item.
Fixes
- Hairlines: often no repair needed; adhesives/underlays usually bridge them.
- Moderate cracks: chase/grind, vacuum, then fill with resin/polymer repair compound.
- Structural cracks: epoxy injection and “stitching” can lock the screed together.
- Severe debonding: cut out and re-screed the affected section.
Preventing repeats
Clean, stable base + correct depth + perimeter edging/movement joints + controlled curing + gradual UFH commissioning.
Why LMA: 15+ years across London/SE, clear advice, tidy installs, and repairs done properly so your floor finish goes down with confidence.
Don’t let floor cracks undermine your build. Contact LMA today for a free survey and expert advice. Our London-based screeding team will diagnose the issue (often quickly and on-site) and outline the best fix. You’ll see why homeowners, builders, and contractors trust us for every screed job. Call us or fill out the form on our site, and get your project back on track with a strong, level floor you can rely on.