Screeds
Screed drying time estimate (rule of thumb)
- where
- estimated drying time (days)
- drying time per mm up to tb (days/mm)
- screed thickness (mm)
- thickness dried at the first rate (mm): 50 for cement, 40 for calcium sulphate
- drying time per mm beyond tb (days/mm)
Explain this equation
Gives a first estimate of how long a new screed needs before it is dry enough to tile. Use it for programming jobs, then confirm with a moisture test before you start.
The first part counts the days for the screed up to tb at r1, typically 1 day per mm. The second part counts any extra thickness beyond tb at the slower rate r2, typically 2 days per mm. For cement screeds tb is usually 50 mm; for calcium sulphate (anhydrite) screeds it is 40 mm.
Moisture has to travel to the surface to escape, and the deeper it starts the further it has to go, so drying slows as screeds get thicker. The min and max simply split the thickness into the fast part and the slow part.
Sense check: a 50 mm cement screed needs about 50 days; 65 mm about 80 days; 75 mm about 100 days. A 50 mm anhydrite screed needs about 60 days.
Watch out: these figures assume a warm, dry, ventilated room. Before tiling, test: no more than 75% RH for cement screeds, no more than 0.5% CM for anhydrite under cement-based adhesive.
Gives a first estimate of how long a new screed needs to dry before tiling, using the published days-per-millimetre rules.
Enter your values to see the result.
Good to know
Cement:sand screed (BS 8204 guidance as commonly quoted): 1 day per mm up to 50 mm, then 2 further days per mm, so 65 mm ≈ 80 days and 75 mm ≈ 100 days. Calcium sulphate (anhydrite) screed: about 1 mm per day up to 40 mm, then about ½ mm per day (2 days per mm), so 50 mm ≈ 60 days. Set tb to 40 for anhydrite. Before tiling, cement screeds should read no more than 75% RH; calcium sulphate screeds no more than 0.5% moisture (CM method) under cementitious adhesive, or up to 1% with primer and a calcium sulphate-compatible adhesive (check the maker). Anhydrite surfaces also need laitance removed and priming.
Common mistake
Counting calendar days from laying and starting to tile without a moisture test. Cold, damp or unventilated rooms can double the drying time.
Limits
Assumes good drying conditions. Rapid-drying screeds and forced drying with underfloor heating follow the manufacturer’s schedule instead. Only a hygrometer or CM test proves the screed is ready.
Sources
- BS 8204-1:2002+A1:2009 Screeds, bases and in situ floorings – Concrete bases and cement sand levelling screeds – Code of practice
- Stone Specialist: Ask the expert – screeds (BS 8204: 1 day per mm to 50 mm, 2 further days per mm beyond)
- The Tile Association: Tiling to heated floors (2013/2014) – bay sizes, movement joints, thermal expansion coefficients, screed drying, floor loads
- Gyvlon: Post-installation guidance 2024 (calcium sulphate screed drying 1 mm/day to 40 mm then ½ mm/day; 75% RH ≈ 0.5% CM)
- NHBC Standards 9.3.5 Ceramic, concrete, terrazzo and similar tile finishes (movement joints, screed drying, 75% RH)
- Topps Tiles knowledge base: Tiling onto anhydrite (0.5% w/w or 75% RH before tiling)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.