How long screed needs to dry, and how to test it
By Michał Białas · Last updated
A common rule of thumb for sand-cement screed is one day per millimeter for the first 40 mm (about 1 1/2 in) and two days per millimeter after that, so a 2 in (50 mm) screed needs about two months. The rule is only a planning estimate: the floor is ready when a moisture test says so, and the limit depends on the floor covering going on top.
Why drying time matters
Screed is mixed with far more water than the cement needs to set, and the rest has to leave through the surface. Laying a floor too early traps that moisture. Tile can debond, laminate and wood swell and cup, vinyl adhesive can fail and bubble, and mold can grow under the underlayment. Moisture problems are one of the most common reasons for flooring failures, and they usually show up weeks after installation.
Drying time by product (rules of thumb)
| Product | Typical rule | 2 in / 50 mm example |
|---|---|---|
| Sand-cement screed | 1 day per mm up to 40 mm, then 2 days per mm | About 60 days |
| Fast-drying cement screed | Per product; often about 1 week for 50 mm | About 7 days |
| Liquid anhydrite screed | 1 day per mm up to 40 mm, then 2 days per mm, if ventilated | 40 mm → about 40 days |
| Self-leveling compound | Walkable after about 24 hours; finishes per product | Thin layers: days, not weeks |
These are typical values that assume good conditions: around 68 °F (20 °C), moderate humidity and steady ventilation. Cold, damp or unventilated rooms can double them. The screed calculator shows the estimate for your product and thickness under the result.
Example 1 (meters): planning a 65 mm floating screed
- First 40 mm: 40 days.
- Remaining 25 mm at 2 days per mm: 50 days.
- Total: about 90 days before the moisture test — around three months.
- Compare a 50 mm screed: 40 + 20 = about 60 days. The extra 15 mm add a month.
Example 2 (feet): 2 in of sand mix
- 2 in = 50.8 mm.
- First 40 mm: 40 days; the remaining 10.8 mm × 2 = 21.6 days.
- Total: 61.6 → about 62 days. If the schedule can’t wait, a fast-drying screed or a thinner bonded build-up is the usual answer.
What speeds drying up, and what slows it down
| Helps | Hurts |
|---|---|
| Steady ventilation, a few minutes of cross-draft several times a day | Closed, humid rooms |
| Room temperature around 68 °F (20 °C) | Cold rooms; unheated buildings in winter |
| A dehumidifier once the screed can be walked on | Fresh plaster, paint or other wet trades in the same space |
| Thinner layers | Thick layers; the last millimeters dry slowest |
| Following the heat-up schedule on radiant floors | Forcing the heat up early, which can crack the screed |
Don’t cover fresh screed with plastic or boards once the initial curing period on the data sheet has ended, and don’t blow hot air directly at it: the surface dries and shrinks faster than the inside, which leads to cracks and curling at the edges.
How to test moisture
Drying time tells you when to test, not when to lay the floor. The common tests are:
- In-situ relative humidity probes, widely used in North America: holes are drilled into the slab and probes measure the humidity inside after they settle. Many floor coverings state a maximum RH, often somewhere around 75–85%.
- Carbide (CM) test, common in Europe: a sample is taken from the full depth and reacted with calcium carbide in a pressure vessel. Values often quoted are about 2.0% for cement screed (1.8% heated) and about 0.5% for anhydrite screed (0.3% heated), but the flooring maker’s limit is what counts.
- Plastic sheet test: tape a square of clear plastic to the floor and check for condensation or darkening after a day or so. It shows that moisture is coming up, but it can’t prove that a floor is dry enough.
- Electronic surface meters: quick for finding wet spots and comparing areas; not a substitute for the tests above.
Test in several places, including the thickest areas and spots away from windows, and record the results. Flooring suppliers often ask for them if there is a warranty claim.
Before the finish goes down
- Wait at least the rule-of-thumb drying time for your product and thickness.
- Test moisture at several points and compare with the limit for your floor covering and adhesive.
- On anhydrite screed, sand off the surface laitance and vacuum before priming.
- Prime as the adhesive or leveler requires (see primer before leveling and tiling).
- For floating floors, lay a vapor barrier where the installation guide calls for it.
Frequently asked questions
Can I tile on screed after a week?
Only on a fast-drying screed made for it, or with a tile adhesive the manufacturer approves for young screeds. Standard sand-cement screed needs weeks, and a moisture test before tiling.
Does a thicker screed take proportionally longer?
Longer than proportional. The rule of thumb doubles the time per millimeter beyond about 40 mm, because water from deep layers has further to travel.
Can I use radiant heating to dry the screed faster?
Only by following the screed maker’s heat-up schedule, which raises the temperature slowly over days. Turning the heat up early can crack the screed.
Is self-leveler dry after 24 hours?
Usually walkable, but not always ready for every finish. Thin layers under tile are often ready within a day or two; vinyl and wood may need longer. Check the data sheet.