We laid a 65mm sand:cement screed in a kitchen extension in early October. The tiling contractor was quoted a start date of six weeks. He turned up after four weeks, tested the screed with a moisture meter, and the readings were still too high. We delayed tiling by two weeks and he came back again. Still marginally too high, but within acceptable limits if we used the right adhesive. The customer, who had been staying with a relative while the kitchen was out of action, was not pleased.
Screed drying time is the part of floor construction that most project timelines underestimate. The rules are simple but the timelines are longer than people expect.
The standard drying rate
For sand:cement screed, the established rule of thumb is:
1mm of screed depth = 1 day of drying time
This assumes:
- Normal UK ambient conditions (15-20°C, moderate humidity)
- Good ventilation
- Screed has been cured for the first 7 days (kept slightly damp or covered to prevent shrinkage cracking)
- No underfloor heating running
| Screed depth | Standard drying time |
|---|---|
| 25mm bonded | ~4 weeks |
| 40mm | ~6 weeks |
| 50mm | ~7 weeks |
| 65mm | ~9 weeks |
| 75mm | ~11 weeks |
These are minimum guide times in reasonable conditions. Cold weather, high humidity and poor ventilation all extend the timeline.
Anhydrite screed drying
Anhydrite (calcium sulphate) liquid screed dries differently to sand:cement. It is pumped in as a liquid and levels itself, meaning it starts at a consistent depth across the whole area.
The drying profile for anhydrite is approximately:
- Up to 40mm depth: 1mm per day
- Beyond 40mm: 0.5mm per day
A 65mm anhydrite screed would therefore take approximately 90 days. However, anhydrite screeds are almost always used with underfloor heating, and the UFH system can be used to run a heating programme that significantly accelerates drying.
Anhydrite screed also requires a different surface treatment. Before tiling, the laitance (weak surface layer) must be removed by sanding or light grinding. This is essential - do not tile directly onto an unprepped anhydrite surface.
| Screed type | Depth | Approx drying time | Notes |
|---|---|---|---|
| Sand:cement | 50mm | 7 weeks | Standard conditions |
| Sand:cement | 65mm | 9 weeks | Allow extra in winter |
| Anhydrite | 50mm | 7-8 weeks | Can accelerate with UFH |
| Anhydrite | 65mm | 10-12 weeks | UFH programme recommended |
| Rapid-set screed | 50mm | 1-2 weeks | Product-specific |
What affects drying time
Temperature. The biggest variable. Below 10°C, drying essentially stops. In a cold shell building in autumn or winter, factor in a significant extension to any stated times.
Ventilation. Moisture has to go somewhere. A well-ventilated space dries significantly faster than a sealed room. Open windows and doors where possible without creating draughts that cause surface drying (which seals moisture in the depth).
Relative humidity. High ambient humidity slows the moisture gradient. On wet autumn days in an unheated building, drying can be very slow.
Screed type. Rapid-set screeds use additives to accelerate the chemical set. They reach structural strength faster but still require moisture to reduce to acceptable levels for floor coverings. Do not confuse structural strength with moisture readiness.
UFH pipe layout. Screed over underfloor heating has a reduced effective drying depth because the pipes create a barrier. Once the UFH system is commissioned, it can be used to run a drying programme (start at 25°C, increase to 35°C over 7 days, maintain for 3 days, then reduce). Follow the screed manufacturer's specific protocol.
Testing for dryness
Do not rely on the 1mm/day rule alone. Always test before applying floor coverings.
Relative humidity probe (RH hood test). This is the industry standard. A sealed hood is placed on the screed surface and left for 72 hours. The RH reading should be below 75% for most adhesives and floor coverings (65% for wood flooring).
Calcium carbide test. A more aggressive moisture test that gives a direct reading of moisture content by weight. Used on-site for a faster result than the hood test.
Plastic sheet test. Tape a 1m square polythene sheet to the screed for 24-48 hours. Condensation on the underside indicates excessive moisture. This is a simple pass/fail rather than a quantified reading.
| Floor covering | Typically requires RH below |
|---|---|
| Ceramic and porcelain tiles | 75% |
| Natural stone tiles | 75% |
| Engineered wood | 65% |
| Solid hardwood | 60% |
| LVT / vinyl plank | 75% |
| Carpet | 75% |
My tips on screed drying
Build screed drying time into the programme from day one. It is the most commonly underestimated timeline item in residential construction. If the screed goes down in October in an unheated building, do not plan to tile in November.
Start ventilation as soon as the screed has cured. After the initial 7-day curing period, open the building up as much as the weather allows. Every week of good ventilation saves multiple days of moisture testing failure later.
Commission the UFH drying programme. For anhydrite screed with underfloor heating, running the drying programme is not optional - it is how the product is intended to work. Skipping it leaves moisture in the screed and delays floor covering installation.
Identify the cold spots. Ground floor screeds near external walls dry much more slowly than the centre of the floor. When testing, take readings from multiple positions including corners and perimeter zones - not just the middle of the room.
Use the Floor Screed Calculator to work out material quantities for your project.