What is screed curling?

Screed curling (Dutch: schotelen, literally “saucering”) is the phenomenon where a screed ends up higher at its edges and joints than in the middle: the floor curls up like a saucer. It occurs because the top of a cement-based screed dries faster than the bottom. The dry top shrinks more, pulling the edges of a floor bay upwards. Where bays are divided by a contraction joint or expansion joint, curling is often seen precisely along those joints.

Curling is usually temporary: as the screed dries further and the moisture content becomes even through its thickness, the edges largely settle back. However, part of the deformation can be permanent. Anhydrite (calcium sulphate-based) screeds are generally less prone to curling than sand-cement screeds.

Application

Consequences: if the floor is finished while it is still curled, tiles and natural stone near the edges may end up hollow or crack when the edges later settle back. Under parquet, PVC or a seamless poured floor, the height differences will show through.

Prevention and limitation:

• Let it dry slowly and evenly: avoid draughts, strong sun and rapid heating (do not switch underfloor heating on too early or too high; follow the supplier’s heating-up protocol).
• Cure it: cover a fresh cement screed with sheeting for the first few days or keep it damp, so that the top does not dry out too quickly.
• Good bay layout: have contraction joints made at the locations specified by the supplier or consultant, especially in large or irregular rooms.
• Measure before finishing: check the residual moisture content (for example with a CM test) and the flatness with a straightedge before tiling or laying.
• Do not level or grind too early: grinding off the raised edges straight away will leave a sunken edge later when the floor settles back.

Related Terms

• Screed floor
• Poured screed
• Anhydrite screed
• Shrinkage
• Seamless floor
• Parquet floor

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