Floor Screed Mix Ratios, Curing Times & Moisture

Q: What if the UFH pipes have <60mm cover above them?

Below the recommended cover, thermal cycling stress on the screed can crack it above the pipes. If cover is 40–50mm, use a reinforced anhydrite mix or add a fibre-reinforced cement mix. Below 40mm, redesign the buildup.

Q: Do I always need to prime before tiling?

Yes, on anhydrite absolutely (compatibility). On cement screeds, primer improves adhesion and reduces adhesive water-loss into the substrate. Skip it and adhesive can dry too fast, causing hollow spots.

Q: Can I use ready-bagged floor levelling compound as a screed?

Only as a topping over an existing sound screed, typically 3–20mm depth. Levelling compounds are not screeds — they are thin bond-coats or top-outs. Not structural, not for depths beyond product spec.

Q: The customer wants to lay tiles tomorrow — what do I say?

If the screed was poured today, no. If yesterday, no. If a week ago, still no. The customer's programme urgency does not change the physical curing timeline. Explain in writing what happens if they force it, and put your name to a proper timeline. That's your protection.

Q: Why does anhydrite fail with cement adhesive?

Anhydrite is calcium sulphate. Cement-based adhesive contains portland cement which reacts with residual calcium sulphate + moisture to form ettringite, an expansive crystal that destroys the bond over weeks or months. Use calcium-sulphate compatible adhesives (F ball F46, Mapei Ultralite Flex, etc.) on anhydrite substrates.

Summary

Floor screeds are one of the least-forgiving construction elements. Get the mix wrong, cure it wrong, or tile onto it too early and the result is a bond failure that's only visible weeks or months later when tiles lift, cracks appear, or wood floors cup. Rectification is almost always a full lift and re-lay — there is no partial fix.

For a plasterer, floor-layer or tiler, screed knowledge sits at the boundary between two trades and is the point where blame lands when a floor fails. The tiler blames the screeder, the screeder blames the drying schedule, the customer blames whoever they can find. Documented pre-tile moisture testing protects everyone.

The rise of anhydrite (calcium sulphate) liquid screeds in the last 15 years has changed the game — they are faster to lay, self-levelling and better for UFH — but they demand rigorous drying discipline before covering. Cement screeds are more forgiving but slow. Choose the right system for the programme.

Key Facts

Cement:sand ratio — 1:3 by volume is the modern standard for domestic sand-and-cement floor screeds; 1:4 acceptable for lightly-loaded areas. Never go weaker than 1:4.
Sand type — sharp (concreting) sand only. Building/plastering sand contains fine particles that weaken the mix and encourage cracking.
Minimum depth (BS 8204-1) — 25mm bonded to concrete; 65mm unbonded (with DPM); 75mm floating (with insulation); 75mm minimum over UFH pipes measured above pipe crown.
Foot traffic — cement screed at 24 hours; anhydrite at 24–48 hours; both keep loading light until 7 days.
Full cure — cement screed reaches ~95% strength at 28 days. Do not tile before then unless using a rapid-set primer/adhesive system.
Curing method — polythene sheet over cement screed for first 7 days minimum; keep from drying too fast in warm/windy conditions.
Anhydrite/calcium sulphate screed — self-levelling, poured to depths of 40–75mm typical, sets to walkable in 24–48 hours. Cannot be laid over reactive substrates (see below).
Anhydrite laitance — a chalky calcium-sulphate hydrate film forms on the surface. Must be removed by mechanical sanding before ANY covering; adhesives will not bond through it.
Moisture — cement screed — must be below 75% relative humidity (BS 8203) or 2% CM (calcium carbide) before covering. As a rule of thumb: 1 day per mm of thickness under good drying conditions.
Moisture — anhydrite screed — must be below 75% RH or 0.5% CM before covering. Anhydrite is more moisture-sensitive: residual moisture reacts with cement-based adhesives, forming ettringite and destroying the bond.
Underfloor heating drying — commissioning heat cycle is a way to accelerate drying but must be programmed properly. Cement: turn on at day 21, ramp 5°C/day to 45°C max, hold 3 days, ramp back down. Anhydrite: commission from day 7 per manufacturer schedule.
Reinforcement — polypropylene fibres at 0.6–1kg/m³ in the mix reduce plastic shrinkage cracking. A197 mesh is unnecessary in domestic floors <100mm.
Movement joints — required at doorways, at changes in bay width, and in bays >40m² for cement (>1000m² for anhydrite).
Contamination — do not mix salt-contaminated sand, seawater or admixtures without checking compatibility. Chloride content limits are set by BS 8204.

Quick Reference Table

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Screed Type	Depth	Foot Traffic	Foot-Load OK	Tile-Ready
Cement (bonded, 25mm)	25 min	24 hr	3 days	21–28 days
Cement (unbonded, 65mm)	65 min	24 hr	7 days	28+ days
Cement (floating, 75mm over insulation)	75 min	24 hr	7 days	28–35 days
Cement (over UFH, 65–75mm)	65 above pipe	24 hr	7 days	28 days min + heating cycle
Anhydrite (40–50mm)	40 min	24–48 hr	7 days	40+ days with sanding
Anhydrite (75mm over UFH)	75 above pipe	48 hr	7 days	60+ days with heating

Mix Reference (cement:sand by volume)	Application
1:3 sharp sand (with polypropylene fibres)	Domestic, standard load
1:4 sharp sand	Domestic light-load only
1:3.5 with rapid cement	Fast-track domestic
1:4:1 (cement:sand:crushed 6mm)	Industrial / heavy-load

Detailed Guidance

Preparation before screeding

Substrate check — concrete slab must be sound, clean and free of laitance, dust, oil or debris. Sweep, vac, prime.
Bond coat (bonded screed) — SBR/water/cement slurry brush-scrubbed into wet substrate immediately before screed placement. For anhydrite, use manufacturer's primer.
DPM (unbonded screed) — 1200g polythene sheet, joints lapped 100mm and taped. Turn up walls 100mm; trim after screed set.
Insulation (floating screed) — 100mm PIR or EPS to Part L. Turn up around perimeter with edge foam expansion strip (10mm min).
UFH pipe fixing — clip to insulation or staple to insulation with 15mm cover minimum around pipe (60mm cover above crown recommended for cement, 65mm above crown for anhydrite).
Perimeter isolation — always fit 10mm compressible edge strip at all walls and vertical fixings. Screeds move as they cure; unrestrained movement means no cracking.

Mixing a sand-and-cement screed

Never batch by weight of sand — sand batches by volume with allowance for bulking (moisture content adds ~25% to loose volume). A good rule:

1 part OPC (Ordnance Portland Cement)
3 parts sharp sand (BS 882 medium grade)
Water: START at 0.4:1 water:cement ratio. Add sparingly. The mix should be "damp earth" — squeeze a handful and it just holds shape without free water.
Polypropylene fibres 0.9kg per m³ of screed volume — mix dry, then add water.

Over-wet mix is the number-one cause of cracked/soft screeds. If your screeder is pouring wet-looking mixes to make finishing easier, expect cracks and dust when it cures.

Laying and finishing

Datum levels — screed to laser or dot-and-dab levels, set to finished floor level minus tile + adhesive (usually 15–20mm).
Bay layout — for cement, screed in bays or continuously. Free movement joints at doorways.
Compaction — screed must be compacted with a straight-edge rocked side-to-side and forward. Air pockets = weak zones.
Finish — wood float initially, then closed with a steel trowel or power float. Do not over-trowel — this brings fines/laitance to the surface.
Curing — cover with 1000g polythene as soon as the surface will not mark. Overlap joints, tape or weigh down edges. Leave for 7 days minimum for cement.

Anhydrite / liquid screed process

Anhydrite is different in three critical ways:

Self-levelling — poured through a pump, spread with dappling bars only. No trowelling. Levels to within 2mm across 2m.
Fast strength gain — 24-hour walkable, 48-hour light load, 7-day full load. Reduces programme by weeks.
Laitance formation — a 2–5mm thick chalky calcium-sulphate skin. This layer is porous, weak, and blocks adhesive bonding.

Laitance removal is non-negotiable:

Sand at 7–10 days — mechanical floor sander with 40–60 grit paper. Vacuum thoroughly.
Prime — manufacturer-approved primer (usually a two-part epoxy or specific latex primer for anhydrite).
Adhesive compatibility — must use a calcium-sulphate compatible adhesive (F ball F46 or similar). Standard C2 cement adhesive on anhydrite = bond failure within weeks.

Skipping laitance removal or using the wrong adhesive is the #1 reason tiled anhydrite floors fail. Document the sanding step with photos before laying.

Moisture testing — critical, always

Both screeds MUST be tested before covering. Never rely on time alone — a "42-day-old" screed in a damp new build can still be 4% moisture, well above spec.

Methods (BS 8203):

Hygrometer (75% RH limit) — insulated box sealed to floor for minimum 4 hours (24 hours preferred). Reads relative humidity at the floor surface.
Calcium carbide (CM) meter — sample extracted, reacted with carbide, reads pressure of acetylene gas. 2% max for cement, 0.5% max for anhydrite.
Handheld capacitance meter — indicative only. Cannot be used for compliance. Useful for spot checks during drying.
Sleeve test (BS 8203) — 500mm × 500mm rubber mat weighted to floor, left 24 hours. Condensation underneath = still wet. Rough qualitative check only.

Document every reading with location, date and reading. If a floor covering later fails, the moisture record proves you did your part.

Drying conditions

Screed dries at approximately 1mm per day under ideal conditions:

20°C ambient temperature
50–65% relative humidity
Gentle ventilation (not draughts)
No standing water, no covering

Under UK winter conditions in a new build with wet trades still working, drying rate can drop to 0.3mm/day. A 65mm screed can take 6 months to dry naturally in poor conditions.

Force-drying methods:

Portable dehumidifiers — remove ~10L/day from a 30m² room. Effective for cement AND anhydrite.
UFH commissioning cycle — ramp heating up 5°C/day from 20°C to 45°C, hold 3 days, ramp down. Reduces drying time by 40–60%.
Space heating + ventilation — extract vents, warm supply air. Faster than heat alone.

Never force-dry cement screed before day 21 — sudden drying causes plastic shrinkage cracks. Anhydrite can be commissioned earlier per manufacturer.

Cracks and defects

Plastic shrinkage cracks (0–24 hrs) — surface cracking during curing. Cause: too dry a mix / drying too fast / no polythene. Cosmetic only if minor; structural if wide.
Drying shrinkage cracks (weeks 1–4) — random cracking, usually at re-entrant corners or over pipe runs. Movement joints prevent most.
Curling — edges lift due to differential drying (top dries first, contracts, curls up). Common on unbonded screeds without adequate curing. Bond breaker helps.
Delamination — bonded screed lifts from substrate. Cause: dirty substrate, laitance on slab, poor slurry bond. Sound-test with a hammer at 500mm centres — hollow patches = delaminated.
Powdering / dusting — surface too weak. Cause: over-wet mix, over-trowelling bringing fines up, walking on green screed. Prime and seal, or lift and re-lay.

When to insist on lift-and-relay

If any of these:

Screed fails BS 8204 in-service crushing strength (cored sample <30 N/mm² Group A)
Delamination >25% of area
Moisture stubbornly above spec after 60 days force-drying (usually indicates buried moisture source)

Do not tile onto a suspect screed. The re-lay cost is £30–£60/m²; a bond failure claim on a large tiled area can be 10× that.

Frequently Asked Questions

Can I tile onto a screed at 21 days?

Only if you've measured the moisture and it's below spec, AND you're using a fast-set adhesive system. Standard practice is 28 days minimum for cement screed at 65mm. Rushing this is the #1 cause of tile bond failure in domestic work.

What if the UFH pipes have <60mm cover above them?