Your screed looks dry and you can walk on it, so it's natural to want the new floor down straight away. But a screed that feels dry on top can still hold a lot of water further down, and that trapped moisture is one of the most common reasons new floors fail. Here's how long each type of screed really needs, and how to tell when yours is ready.
The type of screed and how thick it is make the biggest difference. These figures assume decent drying conditions, roughly 20°C with air moving through the room, so treat them as a starting point rather than a promise.
A 65mm sand and cement screed, which is common over insulation in extensions, can easily take three months or more to dry naturally. That's why so many projects that start with a cement screed end up behind schedule.
Those drying times assume warm, well-ventilated rooms, and a new extension in Bolton in November often isn't one. Cold weather, high humidity and rooms without windows or doors fitted can double the time it takes a screed to dry. Moisture only leaves the screed if the air around it can take it away.
You can help it along. Once the screed has cured enough, usually after about 7 days, keep the building weathertight, open windows when the weather's dry, and keep some gentle heat going. A dehumidifier in a closed room works well. Don't put fan heaters straight onto the surface or use strong heat in the first week, because drying it too quickly can make it crack, curl or go dusty on top.
If you have underfloor heating, commissioning it properly is the best way to speed up drying, especially with liquid anhydrite screeds. Starting too hot is a common mistake, and it can crack the screed or make it lift at the edges.
A typical schedule looks something like the list below, but check what your screed and heating system manufacturers recommend, as they have the final say.
The only reliable way to know if a screed is ready is to test it. Surface moisture meters give you a rough idea, but the proper method is a hygrometer test under BS 8203. A sealed hood is placed on the screed for at least 72 hours to measure the relative humidity of the air trapped above it. For most vinyls, LVT and carpets, the target is 75% relative humidity or less. Wood flooring and some adhesives usually need it lower, often 65%. With anhydrite screeds, some manufacturers use a CM (calcium carbide) test instead, with a limit of around 0.5% moisture.
If the programme can't wait, a surface damp proof membrane can be applied to a screed that's still too wet. It works, but it costs extra and doesn't suit every floor covering, so it's best planned for rather than used as a last-minute fix.
Trapped moisture has nowhere to go but up into your new floor. Vinyl and LVT can blister, lift or go wavy. Adhesives can fail. Engineered and solid wood can swell, cup and gap. Tiles laid onto anhydrite screed without the right preparation can come loose, because the cement in tile adhesive reacts with the calcium sulphate. That's why anhydrite screeds need the thin surface layer (laitance) sanded off and a suitable primer applied before tiling.
Putting right a failed floor usually means lifting it, letting the screed dry properly and starting again, which costs far more than waiting a few extra weeks. If your deadline is tight, it's better to choose a fast-drying screed or a liquid screed with underfloor heating from the start.
Yes. You can usually walk carefully on liquid screed after 24 to 48 hours and on sand and cement screed after about 24 hours. Being able to walk on it doesn't mean it's dry enough for flooring, so keep heavy loads and other trades off it for the first week if you can.
It usually does overall, mainly because it can be laid thinner and dried with underfloor heating from about a week after it's poured. Left to dry on its own in a cold, damp building, it can take just as long, so conditions still matter.
Your flooring installer should test the screed before laying anything, and many flooring warranties require a recorded hygrometer reading. If your screeder and flooring fitter are different firms, agree who is responsible for testing before work starts so nobody assumes the other one has done it.