Screed Types Compared: Sand Cement vs Flowing Anhydrite vs Rapid-Drying — Selection, Drying Times and UFH Use

Quick Answer: Sand-cement screed (traditional 1:3.5 mix, typically 65–75 mm thick) suits most applications but is slow to dry — allow 1 day per mm of depth as a minimum. Flowing (liquid) anhydrite (calcium sulphate) screed is faster to place, self-levelling, and preferred over underfloor heating due to its excellent thermal conductivity, but it requires laitance removal before tiling or direct adhesive application. Rapid-drying screeds can achieve foot traffic in 24 hours and overlay-readiness in 3–7 days, at a significant material cost premium. The key UK standard is BS 8204 (screeds, bases and in-situ floorings), which covers mix design, thickness, drying, and testing.

Summary

Screed is the most underestimated part of any flooring project. Lay the wrong type, the wrong thickness, or too soon, and no amount of expensive floor covering will compensate. Failures — tenting tiles, cupping wood, bubbling LVT — almost always trace back to the screed, not the floor covering itself. Every tradesperson who deals with flooring needs a working knowledge of the three screed families in common UK use: traditional sand-cement (also called sand-and-cement or cementitious), flowing liquid anhydrite (calcium sulphate), and rapid-drying proprietary cementitious blends.

Each has a distinct chemical basis, drying mechanism, and application niche. Sand-cement is the most familiar and most forgiving of imprecise mixing; anhydrite offers faster placement and better UFH performance but fails if it gets waterlogged; rapid-drying products command a premium but justify it on fast-track projects where delaying floor-covering application by months is not an option. Building Regulations Part L, combined with the ubiquity of wet underfloor heating, is pushing anhydrite into more and more residential projects.

UK screed contractors increasingly specialise: a sand-cement gang with a forced-action mixer is a different operation from a liquid screed pump and tanker. Specify clearly which screed type is required before tendering, and ensure the screed contractor is aware of what floor covering will follow — not all screeds are compatible with all adhesives or direct-stick floor covers without priming.

Key Facts

• BS 8204-1 — Concrete bases and screeds; covers design, mixing, laying and curing of cementitious screeds
• BS 8204-7 — Self-levelling screeds; covers flowing (liquid) anhydrite and cementitious self-levelling screeds
• Traditional sand-cement screed mix — typically 1:3.5 (cement:sharp sand) by volume; water:cement ratio kept as low as practicable (0.5 or below) to limit shrinkage and drying time
• Bonded screed — bonded directly to prepared concrete base using SBR latex or cement slurry; minimum thickness 25–40 mm; suitable for direct bonded tiles
• Unbonded screed — laid over DPM or slip membrane; minimum thickness 50 mm (recommended 65 mm) to prevent cracking from curling
• Floating screed — laid over insulation; minimum thickness 65–75 mm to achieve adequate strength and limit curling
• Anhydrite screed (calcium sulphate) — pumpable liquid; typically laid 40–50 mm over insulation with UFH; 35 mm minimum without heating
• Anhydrite drying rate — approximately 1 mm/day for the first 40 mm, then approximately 0.5 mm/day beyond that; 40 mm screed ≈ 40–50 days at 20°C with ventilation
• Sand-cement drying rate — approximately 1 day per mm of depth; 65 mm screed ≈ 65 days; 75 mm screed ≈ 75 days — these are minimums and can double in poorly heated or ventilated conditions
• Rapid-drying screed — proprietary blends using ettringite-forming binders or Portland-calcium sulfoaluminate cement; foot traffic typically at 24–48 hours; 80% RH typically reached in 3–7 days depending on thickness
• Moisture test threshold (before floor covering) — ≤75% RH for most timber and LVT; ≤80% RH for ceramic/porcelain tiles with appropriate flexible adhesive; test to BS 8201 (hygrometer sleeve method)
• Anhydrite laitance — anhydrite screeds form a weak surface layer (laitance) during drying; must be removed (sanded, ground, or scabbled) before adhesive, primer, or direct tiling; failure to do so causes adhesive bond failure
• Anhydrite and water — calcium sulphate dissolves in standing water; never use anhydrite screed in areas subject to flooding, external drainage, or constant wetting; not recommended in wet rooms or plant rooms
• UFH surface temperature — maximum 29°C surface temperature recommended by most screed and floor-covering manufacturers; UFH flow water not exceeding 55°C
• UFH commissioning (anhydrite) — anhydrite screed must be allowed to dry fully before heating begins; then heated slowly (increase by 5°C/day from minimum to maximum), run at maximum for 7 days, then reduce; total conditioning period typically 3–4 weeks
• UFH commissioning (sand-cement) — similar protocol; wait until screed reaches ≤75% RH before starting commissioning; heating accelerates drying only modestly once the screed is dry enough to accept it
• Curing (sand-cement) — cover with polythene sheeting for 7 days minimum to prevent rapid surface drying and cracking; do not allow draughts or direct sunlight onto fresh screed
• Compressive strength classes — BS 8204 uses class designations (e.g. F5, F7 for sand-cement; comparable classes for anhydrite); consult structural engineer or manufacturer for heavily loaded floors

Quick Reference Table

Detailed Guidance

Sand-Cement Screed (Traditional Cementitious)

Sand-cement screed is mixed from Portland cement and sharp (or soft) sand in a ratio of approximately 1:3.5 to 1:4 by dry volume, with the minimum water needed to achieve a semi-dry (biscuit) consistency that just passes the squeeze test — squeeze a handful; it should hold together but not run water. An overly wet mix cracks more as it dries and takes much longer to reach acceptable moisture levels.

Thickness: The correct thickness depends on how the screed is installed:

• Bonded to concrete: 25–40 mm minimum; the concrete base must be prepared by shot-blasting or scabbling and primed with SBR latex or cement slurry before the screed is applied. Bonded screeds are stronger and thinner, but bond failure (due to surface contamination or inadequate preparation) produces hollow, cracked screed.
• Unbonded (over DPM): 50–65 mm minimum. The DPM allows the screed to move slightly, reducing the risk of stress transfer from the subfloor but requiring greater thickness to resist curling.
• Floating (over insulation): 65–75 mm minimum. Thinner floating screeds have a high risk of curling (edges lifting), cracking, and becoming hollow under concentrated loads. For commercial or heavy-use floors, increase to 75–80 mm.

Reinforcement: A single layer of A142 steel fabric reinforcement (welded wire mesh, 6 mm wire at 200 mm centres) placed at mid-depth helps limit crack widths in floating screeds, particularly on long or irregular pours. Polypropylene fibre additives are an alternative that distributes micro-cracks and is easier to handle on site.

Curing: Cover with polythene sheeting immediately after trowelling. Leave covered for a minimum of 7 days. Do not open the building to draughts, do not heat aggressively, and do not walk on the screed for at least 24 hours (48 hours is better). Accelerated drying by forced heating before 14 days causes surface cracking and does not speed up the moisture reduction inside the body of the screed.

Drying time: The 1 day/mm rule is a minimum in ideal conditions (20°C, 60% ambient RH, good ventilation). In an unheated building in a UK autumn or winter, double or triple this estimate. Moisture meters and hygrometer sleeve tests are mandatory before any floor covering is applied.

Anhydrite (Calcium Sulphate) Flowing Screed

Anhydrite screed is delivered by tanker, pumped directly into the space, and self-levels to a smooth, flat surface requiring only light compaction with a dapple roller to break surface bubbles. It contains no cement; the binder is calcium sulphate hemihydrate (the same material as plaster of Paris, formulated for floor use). This gives it different properties from cement-based screed in almost every respect.

Advantages over sand-cement:

• Much faster to lay — a screed pump team can place several hundred square metres per day
• Self-levelling: achieves 3 mm/1.8 m flatness easily; sand-cement requires skilled hand-finishing
• Reduced shrinkage cracking: fewer joints needed; can be poured in large bays
• Better thermal conductivity (approximately 1.0–1.2 W/mK vs approximately 0.7–0.9 W/mK for sand-cement) — important for UFH efficiency
• Thinner screed over UFH pipes: 35–50 mm over the top of the pipe is sufficient vs 65 mm for sand-cement

Disadvantages and cautions:

• Laitance: anhydrite screeds always form a weak surface laitance that must be mechanically removed (floor-sanded with 16–24-grit belt or disc, or floor-grinder) before any adhesive, primer, or tile backer board is applied. Skipping this step is the primary cause of tile and adhesive bond failure on anhydrite screeds.
• Water sensitivity: calcium sulphate dissolves in water. Anhydrite must not be exposed to prolonged wetting, standing water, or areas with drainage channels where water can infiltrate. Not suitable for wet rooms (shower areas, plant rooms with water risk). A correctly sealed surface (after laitance removal) provides adequate protection for domestic bathrooms IF the waterproofing tanking is applied to the screed surface.
• Adhesive compatibility: not all adhesives are compatible with anhydrite without a specific primer. Always prime with a manufacturer-recommended primer before adhesive application. Check adhesive TDS (Technical Data Sheet) explicitly.
• Foot traffic: typically 24–48 hours (faster than sand-cement), but the screed is not dry — it is just hard. Moisture testing still applies.

UFH with anhydrite: Anhydrite is the preferred screed for wet UFH. Its lower thermal resistance means faster response and better efficiency. Pipes are typically clipped to insulation at 100–200 mm centres and the screed is poured to at least 25 mm above the top of the pipe (total 40–50 mm over insulation including pipe depth of approximately 15–20 mm). After the screed has dried (tested to ≤75% RH or per the manufacturer's threshold), a commissioning protocol is followed: raise UFH by 5°C per day from the minimum to the system design temperature, hold for 7 days, then reduce. This removes residual moisture and conditions the screed before floor covering is laid.

Rapid-Drying Proprietary Screeds

Rapid-drying screeds are proprietary formulations using modified binders (typically ettringite-forming or calcium sulfoaluminate-based) that chemically bind water more completely and quickly than Portland cement, dramatically reducing free moisture. Brands include Ardex K-80, Uzin NC 165, Mapei Ultraplan, and others — each has specific mix and application requirements.

Performance claims and reality: Many rapid-drying screeds claim walk-on strength in 2–4 hours and readiness for floor covering in 24–72 hours. This is achievable at optimal temperature and humidity. In cold, damp UK autumn or winter conditions (common on construction sites), these times extend. Always test — do not lay expensive flooring based on elapsed time alone.

Thickness: Most rapid-drying products are available as poured monolithic screeds (40–70 mm) or as thin overlay compounds (3–10 mm). The ultra-thin overlayment products are used to resurface existing screeds, concrete, or to level minor imperfections — they are not structural and do not replace a bonded or floating screed where depth is required.

Cost: Rapid-drying screeds typically cost 3–5× more per m² in material terms than traditional sand-cement mixes. On a fast-track refurbishment where every week of delay costs significant money, the premium easily pays for itself. On a standard new-build without programme pressure, sand-cement or anhydrite is more cost-effective.

Screed Over Underfloor Heating — Key Considerations

UFH pipes or electric heating cables laid in screed are the norm in new and retrofitted housing. The choice of screed type significantly affects UFH performance, installation complexity, and time on programme.

Pipe cover: Manufacturers specify minimum cover above the top of the pipe. For sand-cement: 65 mm total screed above insulation (pipe depth typically 15–20 mm, so approximately 45–50 mm above the pipe top). For anhydrite: as little as 25 mm cover above the pipe top is achievable with some systems, allowing total screed depth of 40–45 mm. Thinner screed means faster thermal response and a lower build-up height — important in refurbishments.

Commissioning before floor covering: For all screed types over UFH, the heating system must not be commissioned (run) until the screed has adequately dried. Running UFH in a wet screed causes the water to migrate unevenly, leading to surface cracks and curl. Once the screed is at or below the required moisture threshold, run the UFH commissioning cycle before the floor covering is laid — this completes the drying and ensures the screed is dimensionally stable at operating temperature.

Expansion joints: Sand-cement floating screeds over UFH require bay joints every 40–50 m², and at doorways. Anhydrite screeds are less prone to cracking but still require perimeter edge strip (compressible foam strip around the perimeter of every room) and joints at doorways.

Frequently Asked Questions

How long must I wait before tiling on a sand-cement screed?

The drying rule of 1 day per mm is a minimum in good drying conditions. A 65 mm screed takes at least 65 days. In UK winter, unheated buildings, or poorly ventilated spaces, allow double this time. Test with a hygrometer sleeve test (BS 8201/BS 8203 method) and confirm readings are ≤75% RH before porcelain/ceramic tiling. Some manufacturers permit up to 80% RH with specific flexible adhesives designed for damp substrates — check the adhesive TDS. Do not rely on a surface scrape or tap test — these are not reliable indicators of internal moisture.

Can I tile directly onto anhydrite screed?

Yes, but only after properly removing the laitance. Use a single-disc floor grinder or belt sander with coarse abrasive (16–24 grit), remove all the surface layer until you reach a consistent, solid surface. Vacuum thoroughly and apply a penetrating primer compatible with anhydrite (such as Mapei ECO Prim T Plus, Ardex P 51, or equivalent). Allow primer to dry fully. Apply flexible tile adhesive to the primed surface using the back-butter method for large-format tiles. Do not use cement-only rigid adhesives — the differential movement between the screed and tile requires a flexible (S1 or S2 class) adhesive.

What is the difference between anhydrite and calcium sulphate screed?

They are the same thing — "anhydrite" refers to the mineral form of calcium sulphate (CaSO₄) without water of crystallisation. Flowing calcium sulphate screeds are sometimes also called "gypsum screeds" in trade references. The terms are used interchangeably. They are distinct from alpha-hemihydrate plasters; screeds are formulated for heavier duty and slower setting to allow workable flow times.

My screed is cracking — what caused it and what do I do?

In sand-cement screed, cracks arise from: too much water in the mix (the most common cause in bonded screeds); insufficient thickness in floating screeds (below 65 mm); lack of bay joints in large pours; lack of compressible perimeter strip (especially over UFH); or the subfloor moving. Hair cracks (less than 0.5 mm) in the screed surface are usually cosmetic and do not affect structural performance. Wider cracks (over 0.5 mm) or hollow sections must be repaired before laying floor coverings. Fill with a compatible low-viscosity epoxy crack injection compound, allow to cure, then rescreed or level as required. Seek specialist advice for extensive cracking — the cause must be established before repair.

Is rapid-drying screed the right choice for every project?

Only if programme is the constraint. Rapid-drying screeds offer real benefits when floor covering must be applied within days of screeding — on fast-track refurbishments, commercial fit-outs with tight handover dates, or where phased working means the screed area must be released quickly. If there is no programme pressure, traditional sand-cement offers excellent performance at a fraction of the material cost. Always weigh programme gain against added cost and check that the rapid-drying product is compatible with the specified floor covering and adhesive.

Regulations & Standards

• BS 8204-1:2003 (and AMD) — Concrete bases and cement sand levelling screeds to receive floorings; mix proportions, thickness by construction type, workmanship

• BS 8204-7:2003 — In-situ floorings: self-levelling screeds; covers anhydrite and cementitious self-levelling screeds including design, application and drying

• BS 8203:2017 — Code of practice for installation of resilient floor coverings; adhesive selection and substrate moisture requirements

• BS 8201:2011 — Code of practice for installation of flooring of wood and wood-based panels; moisture testing methods (hygrometer sleeve)

• BS 5250:2021 — Management of moisture in buildings; relevant for DPM selection and vapour control under screeds

• Building Regulations Part L — Thermal performance; insulation under screed requirements; UFH system efficiency targets

• Building Regulations Part C — Subfloor DPM requirements; moisture resistance of floors

• BSRIA Guides (various) — UFH commissioning protocols in new construction and retrofit

• FeRFA (Resin Flooring Association) guidance — Industry guidance on adhesives, priming, and compatibility with different screed types

• BS 8204 at BSI Knowledge — British Standards Institution; full text of screeds standard

• Anhydritec — Liquid Screed Technical Guidance — UK anhydrite screed contractor and technical guide; laitance removal and UFH commissioning

• Mapei UK — Screed and Adhesive Technical Data Sheets — Manufacturer TDS for primer and adhesive compatibility with anhydrite

• NHBC Technical Standards Chapter 5.3 — Screeds and floor levelling in new dwellings; compliance guidance

• engineered floor installation — acclimatisation, moisture testing, and installation methods for engineered wood over screed

• underfloor heating screed — UFH commissioning requirements in screed

• subfloor preparation guide — preparing screeds and concrete for floor coverings

• levelling compounds — thin-overlay levelling on top of set screeds