In short: as a rule of thumb, screed dries at about 1mm a day up to 40mm thick, then about 0.5mm a day after that, in good conditions (20°C and 65% humidity). A 65mm screed takes about 90 days. Sand and cement screed usually needs about 21 days before the underfloor heating goes on. Calcium sulphate (anhydrite) screed can be warmed up from 7 days, which also speeds up drying.
Your dates
Day by day warm-up plan
How the dates are worked out
- Drying time: about 1mm a day for the first 40mm, then about 0.5mm a day (2 days per mm) after that, in good drying conditions of around 20°C and 65% relative humidity. This rule of thumb is widely quoted from BS 8204 guidance.
- When the heating can go on: about 21 days for sand and cement screed. Calcium sulphate screeds can be warmed up from 7 days.
- Warm-up plan: start at about 20°C for 24 hours, raise the flow temperature at the manifold by 5°C a day up to the maximum, hold it for 5 days, then lower it by 5°C a day. Turn off and let the floor cool for 48 hours before moisture testing. This follows the commissioning steps that screed makers publish. Record each day, as many warranties ask for it.
Moisture testing before the floor finish
Every floor finish has a moisture limit. Many need the screed at or below 75% relative humidity, which is about 0.5% moisture content. Some finishes on calcium sulphate screed allow more, and engineered wood often needs less. Check the limit with your flooring and adhesive maker before you lay anything.
Do not rush it
Never use the underfloor heating at full temperature on new screed. Heating too fast can crack the screed and damage the floor finish later.
Frequently asked questions
How long does a 65mm screed take to dry?
About 90 days by the rule of thumb: 40 days for the first 40mm and 50 days for the next 25mm, in good conditions. Calcium sulphate screed dries much faster once the underfloor heating is used to force dry it.
When can I turn underfloor heating on after screeding?
After about 21 days for sand and cement screed, and from 7 days for calcium sulphate (anhydrite) screed. Then warm it up slowly, 5°C a day.
How do I commission underfloor heating in new screed?
Start at about 20°C for 24 hours, raise the flow temperature by 5°C a day to the maximum, hold for 5 days, then lower by 5°C a day. Let the floor cool for 48 hours and moisture test before laying the finish.
Can I use underfloor heating to dry screed?
Calcium sulphate screed can be force dried with the underfloor heating from 7 days. For sand and cement screed, wait about 21 days, then follow the slow warm-up plan.