When Is Screed Ready? Drying and Checking

Being able to walk on screed does not mean it is ready. How to check residual moisture, strength and flatness before laying the floor.

Once screed has turned a lighter colour and you can walk across it comfortably, many people assume it is ready. But for floor coverings, what counts is not how the surface looks but how much moisture remains inside the layer. Parquet laid too early swells and gaps, tile adhesive bonds poorly, and a musty smell develops under laminate. Every one of these problems is the price of saving a few weeks of waiting.

Curing and drying are not the same

Two processes happen in cement-based screed. The first is curing: cement reacts chemically with water and gains strength. The second is drying: the surplus water not used by that reaction evaporates out of the layer. Screed can be strong enough to walk on while still holding a considerable amount of water.

Conditions matter for both. In the first days, the surface should not dry too quickly, or the top layer stays weak and cracks. That is why direct sun, strong draughts and heaters close by are avoided. Later on, moderate ventilation of the room helps the moisture escape.

What affects drying time

It is hard to name an exact period in advance, because several factors work together:

• Layer thickness: moisture has further to travel in a thick layer, and drying time grows disproportionately.
• Mix composition: wet, semi-dry and special fast-drying mixes all behave differently.
• Air temperature and humidity: in cold, humid seasons the process slows down.
• Ventilation: in a closed room where the air is not changed, moisture leaves the screed but stays in the air.
• Type of base: a layer laid over a damp-proof membrane can only release moisture upwards.

For proprietary mixes, the manufacturer's instructions give drying guidance. But that assumes ideal conditions and has to be confirmed by checks on site.

The simple plastic sheet test

There is a method for a first check that anyone can do. Clear polythene sheets are laid in several places on the screed, particularly in a corner, in an area far from windows and where the layer is thickest, and their edges are taped down tightly. If, after a day, droplets appear under the sheet or the screed shows a darker patch, the layer is still giving off moisture.

This test gives only a direction. A pass is not enough for parquet or other sensitive coverings, because it does not accurately show how little moisture remains.

Measuring moisture

An informed decision needs measurement.

• The carbide method is regarded as one of the most reliable site methods. Samples are taken from different depths of the layer, mixed with a reagent in a sealed vessel, and moisture is determined from the pressure.
• Electronic moisture meters give quick readings and are handy for comparative checks across the surface, but their values are approximate and depend on the composition of the mix.

The permissible residual moisture depends on the final floor. Parquet and other timber floors are the most demanding, while tiles and some resilient coverings are relatively tolerant. The limit is stated in the instructions of the parquet, adhesive or covering manufacturer, and the measurement should be compared with exactly that figure. Recording the result in a written note protects you if warranty questions come up.

Strength and surface condition

Dry does not automatically mean good. These checks matter too:

• Scratch a cross into the surface with a metal tool. If the screed crumbles and leaves a deep groove, the top layer is weak.
• Tap it with a hammer. A dull sound can indicate that the layer has come away from its base.
• Check for dusting. If a lot of dust comes off on your finger, priming or additional treatment is needed.
• Note any cracks. Fine hairline surface cracks and cracks running through the full thickness call for different remedies.
• Check flatness with a long straightedge. If it does not meet the requirement of the final covering, a self-levelling compound is planned.

If there is underfloor heating

Once a heated screed has dried, the system is not switched straight to full power. The temperature is raised in stages, held for a period and then lowered in stages again. This heating-up protocol both dries the layer and prepares it for thermal expansion. The specific steps are based on the requirements of the heating system and floor covering manufacturers and should be documented. Rushing the heating can cause cracking.

Before the floor covering goes down, run through this list one last time:

• Moisture has been measured and meets the covering's requirement.
• The surface does not crumble, dust or sound hollow.
• Cracks have been assessed and repaired where needed.
• Flatness has been checked with a straightedge.
• If there is underfloor heating, the heating protocol is complete.
• Movement joints and the perimeter edge strip are in place.

Zaferoğlu İnşaat builds renovation schedules around the drying and testing stage of the screed so that floor coverings are never laid in a hurry. If you have questions about the sequence of floor works, feel free to write or call us.