Heated Screed Commissioning Calculator — Drying & Ramp Schedule

For anhydrite screed, allow a minimum of 7 days (preferably 21 days) before starting commissioning. For sand:cement screed, the minimum is 21 days. The laitance must be removed from anhydrite before commissioning.

Set the flow temperature to a maximum of 25°C and hold it for 3 days first — BS EN 1264-4 requires this initial hold to avoid thermal shock and cracking. Only then increase the flow by no more than 5°C per day up to the design temperature. Hold at the maximum operating temperature for a minimum of one week (about 7 days, per BAL TN 24.23 and Tarmac Topflow Screed A), then cool back to ambient in 5°C/day steps. Never exceed 55°C.

For tiles on anhydrite: ≤0.5% CM (carbide method). For moisture-sensitive finishes such as wood and laminate over underfloor heating the target tightens to ≤0.5% w/w (or ≤75% RH) — and ≤0.3% CM for glued timber over UFH (Chaunceys). The ≤1.0% w/w figure applies to permeable finishes only, not wood. Test at multiple locations, especially in corners and near external walls.

It depends mostly on the screed's drying time. Anhydrite (calcium-sulfate) flow screed dries at about 1 day/mm up to 40mm, then 2 days/mm beyond 40mm (BS 8204-7 / BAL / Tarmac) — so a 50mm screed needs ~60 days and a 60mm screed ~80 days of initial drying. The heating cycle then adds a 3-day initial hold at 25°C + ~4–8 days ramp up + ~7 days hold at max + ~4–8 days ramp down (roughly an extra 18–25 days). For a 50mm anhydrite screed at 45°C flow that totals about 78 days. Sand:cement is slower again (1 day/mm to 50mm then ~2 days/mm, BS 8204-1); fast-drying proprietary screeds are quicker — follow the product PDS.

Yes. Any screed over underfloor heating (wet or electric) needs the same commissioning procedure. The screed must be dried out gradually to prevent cracking and ensure proper moisture levels.