Cracks at Internal Corners of a Liquid Screed Floor

A liquid screed shrinks very slightly as excess water leaves the mix during drying. Along a straight run of wall this shrinkage is uniform and the isolation strip simply allows the slight movement without transferring load into the wall. At an internal corner, however, the screed is restrained on two adjacent sides at once, so the stress has nowhere to relieve itself except by cracking through the body of the screed, usually along the bisecting diagonal of the angle.

This is the same mechanism seen in concrete ground-bearing slabs, and it is why re-entrant corners are flagged in guidance on slab design as locations needing additional detailing or control joints.

The isolation strip should run continuously around every column, pipe upstand, hearth and internal corner, not just the outer perimeter walls. Where a heavily re-entrant corner exists, some specifiers introduce a diagonal cut or additional joint at the corner deliberately, so that any shrinkage crack occurs neatly along a formed joint rather than randomly through the screed.

Where cracking has already occurred and the strip was fitted correctly, the priority is confirming the crack is static (not still moving) before deciding on a cosmetic repair, particularly under a rigid floor finish such as tile.

A useful first step is measuring the crack width at several points along its length and marking the ends with a pencil line and date, then rechecking after a week or two to see whether it has extended or widened. A crack that has stopped moving is a much simpler proposition than one still opening, since ongoing movement points toward ground or subfloor issues rather than settled drying shrinkage.

Tapping either side of the crack with a hard object can also reveal whether the screed has begun to lift or delaminate near the corner, which would change the repair approach from a simple fill to something addressing the loss of bond beneath the surface.

For a static, cosmetic corner crack, a rigid or semi-rigid resin injected along the crack line, followed by surface finishing appropriate to the intended floor covering, is the usual route, restoring a sound surface without needing to lift the screed. Where the crack sits beneath a planned tile or stone finish, a crack-isolation membrane or a formed movement joint carried up through the tile bed is often added as extra insurance against future telegraphing.

Where the corner crack coincides with wider symptoms such as several cracks radiating from multiple corners, or hollow sounding areas nearby, it is worth stepping back and considering whether the isolation strip was omitted more broadly, which points toward a bigger discussion than a single localised repair.