Liquid screed is designed to create a smooth, level floor surface, but cracks can sometimes appear as it dries. On London building projects, very fine hairline cracks are quite common during the curing process and are usually cosmetic. However, wider cracks, straight splits, or jagged lines can indicate movement or stress in the screed and should be checked before the final floor covering is installed.
Common causes
Natural shrinkage during curing
As moisture evaporates from the screed, it naturally shrinks. This can lead to small spider-web style cracks that are typically harmless.
Rapid or uneven drying
Strong sunlight, open windows, heaters, or fans can cause the top layer to dry faster than the rest of the screed. This uneven drying can result in cracking. Underfloor heating should normally be left off for about a week before being gradually increased.
Poor preparation of the base
Dust, oil, or debris on the subfloor can prevent proper bonding. If the screed doesn’t adhere well, movement in the base may lead to cracking.
Missing edge strips or movement joints
Screed poured tightly against walls or across large areas without joints may crack as it expands or contracts.
Incorrect mix or excess water
Too much water in the mix weakens the screed and increases shrinkage during curing.
Insufficient thickness
If the screed is poured thinner than recommended, often around 40mm for anhydrite systems, it may curl or crack more easily.
Hairline vs structural cracks
Hairline cracks are usually superficial and rarely affect the floor finish. However, cracks wider than around 1–2mm, cracks that feel loose, or long straight cracks across the floor should be inspected before flooring is installed.
Fixes
Hairline cracks often require no repair.
Moderate cracks can be filled using resin or polymer repair compounds.
Structural cracks may require epoxy injection or stitching.
If the screed has debonded, sections may need to be removed and replaced.
Preventing repeat problems
Correct preparation, proper screed depth, movement joints, controlled drying, and gradual underfloor heating commissioning all help prevent cracking.
Why choose Floor Screeding London?
With more than 15 years of experience across London and the South East, Floor Screeding London provides reliable screed installations and repairs with clear advice and tidy workmanship.