What is laitance?
Laitance is a weak, friable layer of fine particles that forms on the surface of concrete and screed as it cures. It is made of hydrated cement and very fine aggregate carried to the top by excess water. It is seldom more than 5 or 6mm thick, but it is a common cause of floor failures.
On calcium sulphate (anhydrite) screeds, a laitance layer forms as part of normal curing. The Tile Association's Tiling Guide says this layer must be removed, because it is too weak to tile onto and it also slows the drying of the screed.
What causes laitance
The Designing Buildings Wiki lists four common causes on concrete:
- Excess water in the mix. Water moving up through the concrete carries fines to the surface.
- Rain on the surface while it is being placed.
- Over trowelling the surface.
- Excessive tamping or vibration, which brings fines and water up.
On calcium sulphate liquid screeds the binder itself forms a surface layer as the screed cures, so laitance is expected even when the screed is mixed and laid perfectly. It is a normal stage of the job, not a defect.
Bleeding and laitance: the difference
Bleeding is the water rising to the surface of fresh concrete or screed. Laitance is what that movement leaves behind: the fine, weak material it carries up, which hardens into a skin. Trowelling or power floating while bleed water is still on the surface makes things worse; the Concrete Society warns that it risks delamination of the surface.
How to check for laitance
The Designing Buildings Wiki suggests scratching the surface with a screwdriver or other metal edge until you reach the main aggregate, then measuring the depth of the weak layer. If the surface powders, scratches easily or leaves a dusty residue on your hand, the laitance has not been dealt with.
How laitance is removed
On calcium sulphate screed
Gyvlon, a major calcium sulphate screed brand, advises that its standard screeds should be sanded ideally 3 to 10 days after installation, depending on site conditions, to open up the surface, remove any loose friable material and help the screed dry. It notes that its Advanced range with the E2C additive does not need sanding at that stage. In both cases it points to the Contract Flooring Association recommendation to mechanically abrade and vacuum the surface just before the floor finish goes down, to remove site contamination and give a key for the primer.
On concrete and cement screed
The Designing Buildings Wiki lists the usual methods:
- Shot blasting, firing abrasive at the surface at high speed. Fast on large open areas.
- Mechanical planing, with rows of rotating tungsten tipped cutters, for thicker layers.
- Scabbling, a heavy method using compressed air tools.
- Grinding, with handheld tools for smaller areas and edges.
- Acid etching, only once the other options have been ruled out.
Some cement based liquid screeds are made not to need laitance removal before tiling. Cemfloor, for example, says cement based tile adhesives can be applied directly to its screed surface without sanding, sealing or priming.
Why laitance matters
Tiles, adhesives, levelling compounds, resin and paint all bond to the top few millimetres of the floor. If that layer is laitance, the bond is only as strong as the laitance, and the finish can lift, crack or sound hollow even though the screed below is sound. On calcium sulphate screeds there is a second reason: the layer slows the drying of the screed beneath it, so taking it off is part of getting the floor dry enough to cover.
Common mistakes
- Tiling or levelling straight onto a new anhydrite screed without removing the laitance.
- Missing the sanding window on a calcium sulphate screed. Gyvlon gives 3 to 10 days for its standard screeds; check the window for the screed that was laid.
- Priming over dust. After sanding or grinding, the floor must be vacuumed before the primer goes on.
- Assuming a power floated surface is free of laitance. Over working is one of the causes.
- Using acid first. It is a last resort, not a quick fix.
How this relates to our services
Removing laitance is part of our floor preparation service, using shot blasting or concrete floor grinding to suit the base. On the anhydrite flow screed floors we pour, we hand over written guidance on ventilation, moisture testing and laitance removal for your flooring installer. Our guide to anhydrite screed explains why calcium sulphate behaves differently from cement.
Sources
- Laitance, Designing Buildings Wiki, accessed 25 October 2026
- The Tiling Guide, The Tile Association, accessed 25 October 2026
- Post installation guidance, Gyvlon (Anhydritec), 2024
- Installation of Cemfloor screed, Cemfloor, accessed 25 October 2026
- Floated and trowelled finishes, The Concrete Society, June 2025, accessed 25 October 2026
Part of our screeding glossary, which defines every term we use on site.
