Sand-cement screed

One part cement, three parts sand, and a drying clock nobody can cheat.

Sand-cement screed — Floor Slabs & Screeds

Last updated 2026-07-28 by the BuildPedia Editorial Team.

What is Sand-cement screed?

The traditional sand-cement screed is the oldest way to turn a structural slab into a floor, and still the most common: a 1:3 to 1:4 cement-sand mix with just enough water to compact, laid at 65–75 mm, ruled to level off datum dots and trowelled closed. It can be laid bonded to the slab with a slurry or bonding agent, unbonded on a membrane, or floating over insulation and underfloor heating — and in each case its job is the same: provide the flat, level, correctly-cured surface that tiles, timber, vinyl and carpet silently assume they are standing on.

The screed's virtues are robustness and tolerance; its vice is time. A traditional screed dries at roughly a millimetre per day under good conditions — a 75 mm floating screed is a ten-week wait to safe moisture levels for resilient finishes, and no programme pressure changes the physics. Rushing floor coverings onto a wet screed is among the most reliable generators of defects claims in the industry: blistered vinyl, cupped timber, debonded tiles and mould. The moisture test — a hygrometer hood giving a real RH reading, typically 75% or below for sensitive finishes — is the only voice that gets a vote on readiness, and it is recorded, not assumed.

Workmanship decides everything else. The mix must be consistent — gauged by volume or weight, mixed to a workable earth-moist consistency, used within its working life. It must be genuinely compacted: screed laid loose or just ruled off sounds hollow when tapped, debonds, and eventually cracks and curls at the edges. Day joints are formed vertically and compacted against a stop end, not feathered into nothing. And the first 48–72 hours decide the surface: a screed allowed to dry too fast in sun or draught loses strength and dusts. None of this is sophisticated; all of it is routinely got wrong by people in a hurry.

How does Sand-cement screed work, step by step?

Step 1: Prepare the base and set the level datum

Prepare the base and set the level datum — Sand-cement screed, step 1

The slab is cleaned and prepared for the screed type — slurry or bonding agent for bonded, membrane for unbonded, insulation with perimeter edge strip for floating. UFH pipes are pressure-tested and held under test. Datum levels are taken from the site benchmark and transferred as screed dots or rails across the area, because the screed's flatness is set here, not at the trowel.

Step 2: Batch and mix to the specified proportions

Batch and mix to the specified proportions — Sand-cement screed, step 2

The mix is gauged — 1:3 to 1:4 cement to sharp sand, with admixture where specified — and mixed to an earth-moist consistency that holds together in a squeezed ball without bleeding water. Retempering dead mix is banned: a batch past its working life goes in the skip, not in the floor. Consistency between batches is what stops colour, strength and curling variations across one floor.

Step 3: Lay, compact and rule to level

Lay, compact and rule to level — Sand-cement screed, step 3

Screed is placed in workable bays, spread proud of the dots, and genuinely compacted — tamped or beaten — before being ruled off to level with a straightedge working between the datums. Compaction is the difference between a floor and a hollow-sounding future defect. Bay sizes are kept to what the gang can compact and rule while the mix is alive.

Step 4: Trowel closed and form the day joints

Trowel closed and form the day joints — Sand-cement screed, step 4

The surface is wood-floated then steel-trowelled to a closed, dense finish — closed enough to resist dusting, not burned to a polish that rejects adhesives. Day joints are formed vertically against a stop end with the edge compacted, so the next day's bay butts to a solid face. Coverings and ducts are protected, and edges and columns get their perimeter isolation strip continuous and uncut.

Step 5: Cure and protect the first week

Cure and protect the first week — Sand-cement screed, step 5

The screed is protected from rapid drying — polythene or curing membrane for the first days, draughts and direct sun excluded. Foot traffic is kept off for 48 hours minimum and heavy loads for a week or more. UFH stays off: commissioning with heat comes later, gradually, to a protocol, and never as a way to force drying.

Step 6: Test soundness and moisture before finishes

Test soundness and moisture before finishes — Sand-cement screed, step 6

Before any finish, the screed is tapped or chained for hollow areas — debonded zones come up now, while the repair is local and cheap. Moisture is measured with a hygrometer hood or carbide meter at representative points, recorded against the finish manufacturer's limits, and only a recorded pass releases the floor. The drying arithmetic — roughly a day per millimetre — tells you when to test, not whether to.

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