Sand-and-Cement Screed for Tile Flooring: The Technical Specification Guide
A screed is the layer of sand-and-cement mortar laid between the structural floor and the tiles. It is the part of a tiled floor you never see — and the part that decides whether the floor stays flat, bonded and crack-free for decades. At TN Stone we lay and tile over sand-and-cement screeds across Melbourne every week, so this guide sets out the specification we work to: mix ratios, water content, minimum thickness, compaction, curing and drying, and the joints and moisture tests that stop shrinkage cracks and hollow spots under tiles.
It follows AS 3958.1, the Australian standard guide to installing ceramic tiles, together with international screed practice from BS 8204 and the UK Contract Flooring Association.
Why the mix consistency matters more than anything else
Traditional screed is a semi-dry, “earth-moist” mortar — typically 1 part cement to 4 parts clean sharp sand by volume (1:3 for heavy-duty or very thin bonded layers). It is not a wet mix. The water content sits at a water-to-cement ratio of roughly 0.35 — just enough for the cement to hydrate fully, low enough to keep drying shrinkage to a minimum.
The site check is the squeeze test: squeeze a handful of mixed screed in a gloved hand. It should hold its shape without crumbling, and no water should weep out. If water runs between your fingers, the mix is too wet — the finished screed will be weaker and far more prone to shrinkage cracking. Never fix a stiff mix by dumping in extra water; adjust in small increments.
Minimum thickness by screed type
How the screed connects to the base dictates how thick it must be:
| Build-up | Minimum thickness | Reinforcement |
|---|---|---|
| Bonded — laid wet-on-wet over a cement slurry coat on prepared concrete | 15 mm | Not required |
| Unbonded (self-supporting) — over a plastic slip sheet or waterproofing membrane | 40 mm | Galvanised steel mesh, compulsory |
| Floating — over insulation or underfloor heating | 40 mm domestic | Mesh or fibre recommended |
These are AS 3958-based figures: a bonded topping should never go below 15 mm, and a self-supporting screed below 40 mm risks cracking outright. Where mortar is used as the tile bed itself rather than a separate screed, a nominal 25 mm bed (15–30 mm acceptable) is the AS 3958 guidance.
Preparing the base
- New concrete needs 6 weeks’ drying before screeding or direct bedding (4 weeks where the screed sits on a separating layer) per AS 3958.1 Table 4.3.
- Steel-trowelled or burnished concrete must be mechanically prepared — scarified, shot-blasted or ground — to remove laitance, curing compounds and contamination.
- For bonded screeds, dampen the base to saturated-surface-dry, broom a cement/SBR slurry bonding coat (no more than about 2 mm thick) over it, and place the screed immediately while the slurry is still wet. If the slurry dries first, the screed will debond.
- For unbonded screeds, lay a double layer of 200–250 micron polyethylene sheet (top layer at right angles to the bottom) or place over the waterproofing membrane.
- Any movement joint in the concrete slab must be carried up through the screed and the tiling. Never bridge it.
Laying and compaction — how hollow spots are prevented
Hollow, drummy tiles almost always trace back to poor compaction or poor adhesive coverage — not the tiles.
- Set rails at finished level, and plan bays so each is completed in one continuous pour.
- Compact everything. Beat the fresh mortar with a timber tamp or use a tamping beam / vibrating screed bar on larger areas. Levelling with a screed bar is not compaction.
- For unbonded screeds, lay the first half of the mortar, embed the galvanised mesh (150 mm laps, wired), then place the rest to full thickness.
- When tiling, comb the adhesive and press tiles firmly into it; check coverage by lifting the occasional tile — minimum 90% coverage, and never spot-fix.
Curing and drying — the schedule that governs your tiling date
Curing and drying are two different things, and confusing them is the classic error:
- Cure (0–7 days): cover the finished screed with polyethylene sheeting, protect it from sun, wind and frost, keep traffic off for 48 hours. Curing holds moisture in while strength builds — it is one of the most important crack-prevention measures there is.
- Dry (day 7 onward): remove covers and let the screed air-dry continuously before tiling.
The accepted drying rule for sand-and-cement screed is 1 day per millimetre of thickness for the first 40–50 mm, then 2 days per millimetre beyond that — and only in good conditions (about 20 °C, low humidity, real ventilation). A 40 mm bathroom screed is roughly 6 weeks before it is reliably dry enough to tile.
The definitive check is a moisture test, not a calendar. AS 3958.1’s acceptance criterion is no more than 5.5% moisture by its electrical-resistance test method; the international equivalents are a hair-hygrometer reading at or below 75% relative humidity, or 2% by the calcium-carbide (CM) method. Tiling onto a wet screed traps moisture, which attacks the adhesive bond and debonds the tiles.
Joints: controlling shrinkage before it cracks your tiles
Every screed shrinks as it dries — the design task is controlling where:
- Perimeter joints: a compressible strip (6–10 mm) wherever the screed and tiling meet walls, columns and fixed elements, so the floor can shrink without restraint.
- Bay joints: within the screed at roughly 5–6 m intervals in large areas, formed during laying or sawn after hardening.
- Tile movement joints (AS 3958): perimeter joints at all restraints, minimum 6 mm, filled with flexible sealant — not grout. Intermediate joints at no more than 4.5 m spacing wherever an internal floor dimension exceeds 9 m (or 6 m in direct sun) and in all external tiling, carried through the full depth of tile and bed.
Missing movement joints are the single most common cause of tile cracking and grout failure in residential work — the damage typically shows up 6 to 18 months after handover.
Common defects and their causes
| Symptom | Usual cause |
|---|---|
| Random shrinkage cracks | Too much water in the mix; no bay joints; screed too thin for its build-up |
| Hollow/drummy tiles | Under-compacted screed; adhesive coverage below 90%; spot-fixing |
| Debonding screed | Slurry coat allowed to dry before placing mortar; unprepared or contaminated base |
| Cracks mirroring slab joints | Screed or tiling bridging a substrate joint |
| Dusty, weak surface | Semi-dry mix never properly compacted; over-watered mix |
TN Stone’s checklist before tiles go down
- 1:4 mix, sharp washed sand, batched by gauge box.
- Squeeze test passed — no water weeping.
- Thickness verified: 15 mm bonded minimum, 40 mm unbonded with mesh.
- Compaction over 100% of the area.
- 7-day cure under polythene, protected.
- Drying time allowed (1 day/mm to 40–50 mm, then 2 days/mm).
- Moisture tested and documented: ≤5.5% (AS 3958.1) or ≤75% RH.
- Movement joints in place, sealant-filled, full depth.
A screed is only as good as the specification behind it. If you are planning a tiled floor in Melbourne — bathroom, kitchen, balcony or a full home — talk to us about getting the base right before a single tile is laid. Find out more about our tiling services.
Planning a tiled floor and want the screed specified properly before tiling starts?
Request a Free QuoteCommon questions
How long does a screed take to dry before tiling?
About 1 day per millimetre of thickness up to 40–50 mm, then 2 days per millimetre beyond — and always confirmed with a moisture test (≤5.5% per AS 3958.1) before tiles go down.
What is the correct sand-and-cement mix for a floor screed?
1 part cement to 4 parts clean sharp sand by volume for standard work, mixed semi-dry at roughly 0.35 water-to-cement; 1:3 for heavy-duty or very thin bonded layers.
Why do tiles sound hollow after installation?
Usually under-compacted screed or adhesive coverage below 90% — voids under the tile. Lift-and-check during laying prevents it.
How thick does a screed need to be?
15 mm minimum for a bonded screed; 40 mm minimum for an unbonded or floating screed (with steel mesh).