When laying floor screed, it’s really important to know that the material is completely dry and hardened before working on it. Not only is it vital for a high quality finish, but it also ensures a safe and secure floor installation. If you’re wondering how long this might take, we’ve gone into detail about the different times and what affects it.
How Does Floor Screed Dry?
Every screed, regardless of mix, passes through two main stages, curing and drying. Curing refers to the initial period where the cement or binder hydrates, gaining strength and stability. This typically lasts around seven days for most screeds. Drying, on the other hand, is the gradual evaporation of residual moisture within the material. This phase determines when the screed is ready to receive its floor finish.
If flooring is laid while the screed is still damp, trapped moisture can lead to lifting, blistering or mould growth beneath the surface. Ensuring the correct drying time and moisture level protects both the screed and the final floor finish.
What Influences Drying Time?
Several variables affect how long it takes a floor screed to dry fully.
- Thickness influences evaporation rate, as deeper screeds take longer to release moisture from the core.
- Temperature and humidity affect evaporation, with cool or humid environments slowing the process and warm, ventilated areas accelerating it.
- Ventilation is essential for consistent drying since closed or sealed areas can trap moisture for longer.
- Mix composition can alter drying rate depending on additives, fibres and polymer binders.
- Underfloor heating, when used correctly, can help stabilise moisture levels and shorten the drying phase.
The Different Drying Times Explained
Below is a general comparison of how different screed types behave under normal site conditions of twenty degrees Celsius and sixty-five percent relative humidity. Always consult manufacturer data for precise performance.
| Screed Type | Approximate Drying Rate | Minimum Curing Period | Ready For Floor Covering* | Typical Applications |
| Traditional sand and cement | approximately 1 mm per day up to 40 mm, 0.5 mm per day beyond | 7 days | around 4 to 6 weeks for 50 mm depth | Domestic floors, extensions, small commercial projects |
| Liquid anhydrite (flowing screed) | approximately 1 week per 40 mm | 7 days | 40 to 60 days depending on conditions | Large areas, underfloor heating, smooth finishes |
| Fast-drying or polymer-modified | 3 to 7 mm per day | 24 to 48 hours | 7 to 14 days typical | Time-sensitive commercial projects, refurbishments |
| Fibre-reinforced screed | 1 mm per day | 7 days | 4 to 6 weeks | Heavily trafficked floors, structural slabs |
*Times assume good drying conditions and adequate airflow. Cooler or more humid environments will extend these figures.
Moisture Testing Before Flooring Installation
Before laying flooring, the screed’s internal moisture must be measured accurately. The two most common testing methods are:
- Hygrometer test measures relative humidity within the screed, with most floor coverings requiring readings below seventy-five percent relative humidity before installation.
- Calcium carbide test provides a direct measurement of residual moisture content, typically below 0.5 percent for cement-based screeds and 0.3 percent for anhydrite.
Skipping these checks risks moisture becoming trapped beneath impervious finishes such as vinyl or epoxy, leading to premature failure.
How It Works With Underfloor Heating
Different screeds perform differently when combined with underfloor heating. Liquid anhydrite and fibre-reinforced screeds are particularly suitable because they surround the heating pipes fully, providing excellent thermal conductivity.
Before commissioning heating, allow the screed to cure completely. The heating system should then be raised gradually, usually after twenty-one days, starting at twenty degrees Celsius and increasing by five degrees per day until full temperature is reached. This helps drive out remaining moisture without causing cracks or surface shrinkage.
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