Anhydrite Screed Calculator — Volume, Pump Time & Drying

Multiply your floor area by the screed depth. For 50m² at 40mm depth: 50 × 0.04 = 2.0m³ of screed. Fresh (wet) anhydrite flow screed — the material as delivered and pumped — weighs about 2.2 tonnes per m³ (2200 kg/m³), so 2.0m³ ≈ 4.4 tonnes. Once cured the dry slab is lighter, about 2.0 tonnes per m³ (2000 kg/m³), matching the industry rule of roughly 20 kg/m² per 10mm of depth. Figures per the Tarmac Topflow Screed A technical data sheet (calcium-sulphate / Gyvlon screed to BS EN 13813).

Allow approximately 1 day per mm of depth for the first 40mm, then 2 days per mm (0.5mm/day) above 40mm. A 40mm screed takes about 40 days; a 60mm screed about 80 days. Important: these published rates assume ideal conditions — 20°C, relative humidity not above 65%, with air movement (per BS 8204-7). A typical unheated site dries more slowly, so always confirm with a moisture test (≤0.5% w/w for calcium sulphate) before laying floor finishes. Mechanical dehumidification or commissioning the underfloor heating can shorten the time significantly.

BS 8204-7 (the code of practice for pumpable self-smoothing screeds) requires a minimum 25mm cover over the top of the UFH pipes for a calcium-sulphate screed (cement-based screeds need only 15mm). With 16mm pipes that 25mm cover gives roughly 41mm total depth as an absolute minimum, but most manufacturers (e.g. Cemfloor, Gyvlon) recommend 40–50mm total over the pipes for an optimal UFH installation. BS 8204-7 also sets a minimum overall screed thickness of 35mm domestic / 40mm commercial. Deeper screed stores more heat but takes longer to warm up and to dry.

Anhydrite screed is delivered by mixer/volumetric truck and pumped through a hose. Liquid-screed pumps typically deliver around 12–20 m³ per hour (the range across machines is roughly 5–23 m³/hr — e.g. a Euromair X-PRO D140 peaks at ~14 m³/hr). A 50m² room at 40mm (2.0m³) is poured in well under 10 minutes of actual pumping; a full day's output can reach 1,500–2,000 m².

Yes. Anhydrite screed must be primed before applying tile adhesive, usually in two coats (a diluted first coat then a neat second coat). The laitance layer (surface film) must also be removed by light sanding, typically 4–10 days after pouring. Coverage of about 0.15 L/m² is a useful planning figure, but anhydrite primers range roughly 0.1–0.25 L/m² across two coats — always confirm against your primer's technical data sheet.