How to Test Screed Moisture Before Floor Finishes

A screed can look dry, feel hard and be ready for light site traffic while still holding enough moisture to cause a floor finish to fail. To test screed moisture properly, the reading must be taken at the right depth, with the right method, after the screed has had suitable drying conditions. This is the check that protects timber, vinyl, tiles, resin and adhesive systems from costly problems after handover.

For builders, developers and homeowners, moisture testing should be treated as a planned stage of the programme, not a last-minute task just before the flooring contractor arrives. It confirms whether the floor is ready for its next layer and provides a clear record when several trades are involved.

Why screed moisture matters

Excess moisture can migrate from the screed into the adhesive or floor covering above it. The result depends on the finish, but common failures include adhesive breakdown, bubbles beneath vinyl, warped timber, staining, mould growth and debonded finishes. These defects may not appear on the day the floor is installed. They often develop weeks or months later, when rectification is disruptive and expensive.

The risk is particularly relevant on tightly programmed projects. Liquid screed provides a smooth, level base and is highly effective with underfloor heating, but it still needs appropriate drying time. Faster access for follow-on trades is not the same as acceptance for moisture-sensitive finishes.

Screed type also matters. Cementitious liquid screeds and traditional sand-cement screeds behave differently from calcium sulphate, often called anhydrite, screeds. Their drying characteristics, surface preparation requirements and acceptable moisture levels can vary. The floor covering manufacturer and adhesive manufacturer should always have the final word on the maximum permitted moisture content.

How to test screed moisture accurately

The two principal methods used on site are the carbide bomb, commonly referred to as a CM test, and in-situ relative humidity testing. Surface moisture meters can be useful as a quick indication of variation across a floor, but they should not be relied on alone to approve a screed for final flooring.

Carbide bomb testing

A CM test measures the moisture within a carefully taken sample of screed. The sample is weighed, placed in a sealed vessel with calcium carbide reagent and agitated. Moisture in the sample reacts with the reagent, creating gas pressure that is read on a gauge. The result is expressed as a percentage moisture content.

This is widely regarded as a reliable, practical method for checking screed readiness, provided the sample is taken correctly. It is destructive in a small, controlled area, so the test locations need to be agreed and repaired afterwards. More importantly, the sample must come from the relevant depth rather than simply being scraped from the surface.

For calcium sulphate screeds, a commonly specified requirement before many moisture-sensitive floor finishes is 0.5% CM or lower. A figure around 2.0% CM is often quoted for cement-based screeds. These figures are useful reference points, not universal rules. Some finishes, adhesives and specified systems require lower values, while certain tile installations may allow a different threshold. Check the product data and project specification before releasing the floor.

In-situ relative humidity testing

Relative humidity testing uses a probe placed within a sealed sleeve or hole in the screed. Once it has equilibrated, the probe measures the humidity within the floor build-up rather than only at the surface.

This method can be particularly useful where a specification calls for RH testing or where the floor finish manufacturer sets an RH limit. It requires careful installation, sufficient equilibration time and a calibrated instrument. A reading taken too soon, from an incorrectly drilled hole or in an unsealed test location can be misleading.

Handheld moisture meters

Non-destructive handheld meters are convenient for surveying larger areas. They can identify wetter zones around doorways, external walls, service penetrations or areas that have received rain. However, their readings are influenced by screed density, aggregates, reinforcement and the instrument setting. They are best used to target further investigation, not as the sole acceptance test before fitting a floor finish.

A practical testing process for site teams

A dependable test starts well before the meter or CM vessel comes out. The screed needs to have dried under conditions that support evaporation, and the testing plan should reflect the size and layout of the project.

1. Confirm the screed system and required limit. Establish whether the screed is cementitious or calcium sulphate, then obtain the moisture requirement for the proposed floor covering, adhesive and any smoothing compound.

2. Review the [drying record](https://www.precisionscreed.co.uk/post/liquid-screed-drying-time-on-real-projects). Check the installation date, screed depth, weather exposure, ventilation, internal temperature and whether the building has been made weathertight. A 50 mm floor and a 75 mm floor should not be expected to dry at the same rate.

3. Prepare the surface correctly. Calcium sulphate screeds commonly require laitance removal by sanding before flooring preparation. This is separate from moisture testing, but both activities should be planned together. A weak or contaminated surface can compromise the finish even when moisture levels are acceptable.

4. Select representative test points. Test more than one location on larger floors. Include areas that are likely to dry slowly, such as thicker sections, shaded rooms, external perimeter zones and places where materials have been stored.

5. Record the method and results. Note the date, room, screed type, floor depth, test location, instrument or method, result and required limit. This gives the flooring contractor and client a clear basis for the next decision.

Do not assume that one dry test near a sunny doorway proves the entire floor is ready. Moisture is rarely uniform across a building, particularly after periods of poor weather or when internal conditions have not been controlled.

Drying screed without creating new problems

Good drying conditions are controlled rather than aggressive. Ventilation, moderate heat and dehumidification can all help, provided they are introduced at the right time and in line with the screed manufacturer’s guidance. Opening the building to cold, wet outside air may achieve the opposite of what is intended.

Underfloor heating can assist the drying process, but it must be commissioned properly. The system should not simply be switched to maximum temperature. A staged heat-up programme allows the screed to acclimatise and reduces the risk of thermal stress. Once the prescribed programme is complete, the heating is normally brought back down before moisture testing and floor installation, following the requirements of the floor finish manufacturer.

There is a trade-off here. Turning up the heat or bringing in large dehumidifiers may appear to recover lost time, but rapid and poorly managed drying can increase the chance of cracking, curling or uneven moisture distribution. A controlled programme is more reliable than an emergency response at the end of the build.

Common mistakes when testing screed moisture

The most frequent error is confusing drying time with test evidence. Manufacturers may give indicative drying rates, often under ideal site conditions, but these are not a guarantee for an individual project. Thickness, ambient humidity, ventilation and construction moisture all affect the result.

Another issue is testing only the surface. A surface can be dry because the room has been heated or ventilated, while moisture remains deeper in the screed. Similarly, testing before the building is weathertight or after a water ingress event gives little assurance that the floor will remain suitable.

It is also unwise to choose an acceptance level based only on the screed. The finish above governs the risk. Porcelain tiles fixed with an appropriate flexible adhesive may tolerate a different condition from engineered timber bonded across the full floor area. Each build-up needs a compatible specification from base to finish.

When should a screed be tested?

Arrange an initial assessment early enough to identify whether the programme is realistic, then carry out formal acceptance testing close to the flooring installation date. If the result is above the required limit, there is time to improve drying conditions and retest rather than postponing the flooring contractor at short notice.

On projects with underfloor heating, allow for the commissioning and cooling sequence in the programme. On extensions and refurbishments, factor in the higher likelihood of changing site conditions, open doorways and moisture from plastering or decorating.

Precision Screed coordinates screed and underfloor heating installations with the wider floor build-up in mind, helping clients across Oxfordshire, Wiltshire and Berkshire plan for a level, efficient and properly prepared floor.

The right time to test is before a floor finish is ordered for fitting, not after the installer is on site. A documented moisture result gives every trade a clear decision point and keeps the final floor protected for the long term.