Screed moisture test (CM method)

What is the screed moisture test and the CM method?

A screed moisture test is an on-site check that shows whether a new screed (poter) is dry enough to take its floor covering. The usual test is the calcium carbide method, called the CM method (CM metóda). A sample of crushed screed reacts with calcium carbide in a sealed device, and the gas pressure gives the moisture content.

The standard for the method is STN EN 13892-10, titled "Method of test for screed materials, Part 10: Determination of moisture content, Calcium Carbide Method". The ÚNMS catalogue says it covers cementitious screed, calcium sulphate screed and magnesite screed made according to EN 13813. Calcium sulphate screed is the family that includes anhydrite screed.

Look up the edition before you quote it. The catalogue shows STN EN 13892-10 released on 1 January 2026 and withdrawn from 1 August 2026. It lists STN EN 13892-10:2026-08 as the replacement. Find the current edition in the catalogue before you cite it in a specification.

The CM method destroys a small sample, so the result describes the spot of the sample. The BAM study says CM and Darr drying need small samples and deliver only punctual information. It adds that they proved not very reliable.

How does the CM test compare with other screed moisture methods?

The table lists the methods named in the BAM study of cement and calcium sulphate screed samples, and one site check from Baumit's guide.

Method	Type of test	What the source says
CM (calcium carbide)	Destructive, small sample	BAM: performed well on calcium sulphate screed samples and failed on cement samples
Darr drying	Destructive, sample dried and weighed	BAM: performed well on calcium sulphate screed samples and failed completely on cement samples
Capacitive and electrical handheld devices	Non-destructive	BAM: studied, with accuracy problems at very low moisture in cement samples. Baumit and Ceresit: indicative only
Microwave and ground-penetrating radar	Non-destructive	BAM: studied as options, promising signal parameters
Vapour-tight foil test, 50 x 50 cm taped to the screed for 24 hours	Non-destructive, indicative	Baumit: no traces of moisture under the foil suggest the screed is dry enough, but the test never replaces a CM measurement

BAM found much greater scatter in cement samples than in calcium sulphate samples and linked it to variations in porosity, pore size and aggregate content. The BAM work used laboratory samples, so it shows how the methods behave and not a site pass mark.

What moisture limits do manufacturers set before floor covering?

The limits are not one national number. Each manufacturer publishes its own, and they differ. The table gives figures from three Baumit and Ceresit documents. Baumit attributes the maximum moisture table in its 2022 guide to STN 74 4505:2013, and that standard's text was not read for this page. The heated values for the Baumit guide follow its rule: 0.5 percentage points lower for heated cement screed and 0.2 lower for heated anhydrite screed.

Source and floor covering	Cement, unheated	Cement, heated	Anhydrite, unheated	Anhydrite, heated
Baumit guide 2022: parquet, laminate	2.5 %	2.0 %	0.5 %	0.3 %
Baumit guide 2022: PVC, linoleum, rubber, cork	3.5 %	3.0 %	0.5 %	0.3 %
Baumit guide 2022: ceramic or stone tile	5.0 %	4.5 %	0.5 %	0.3 %
Ceresit guide: parquet, cork, laminate, PVC, textile, tile in thin bed (CM %)	2.0	1.8	0.5	0.3
Baumit screed sheet 2018, indicative, cement only: wood 2.0 %, ceramic, laminate, PVC, textile 2.5 %	2.0 to 2.5 %	Not stated	Not stated	Not stated

Two points stand out. First, anhydrite screed has a much lower limit than cement screed in both documents. Baumit's table also marks cement-based poured floors as not allowed on anhydrite screed. Second, the same covering gets different limits from different makers. PVC gets 3.5 % in the Baumit guide and 2.0 in the Ceresit guide. The BAM study gives another pair. It puts the ready-for-flooring range at 3.5 M% for cement-based and 0.5 M% for calcium sulphate screed. BAM names no covering. Use the limit that the screed supplier and the covering maker both accept, and write it in the specification. Baumit's table gives weight percent and Ceresit's gives CM %, so compare only like with like.

Why do flooring installers ask for the test before laying wood or vinyl?

Baumit and Ceresit both say the waiting time cannot be set exactly. Drying depends on binder, temperature, humidity, ventilation and thickness. Both say to measure the residual moisture with a CM device before laying. Baumit's guide adds that doubling the thickness may lengthen drying two or three times. It also says the firm that lays the covering does the measurement and judges the screed.

Baumit's guide asks the site manager to record the measuring point, method, time, air temperature, air humidity and result in the site diary. Because CM is a single small sample, measure in more than one place in the room, and record each place.

How is the CM test different from an electrical moisture probe?

The CM test needs a sample and a sealed device. An electrical probe needs no sample and is quicker to repeat, but Baumit and Ceresit call such devices indicative only. The BAM authors found that several commercial devices, including the CM method, failed to find the correct moisture content of cement samples at low moisture. They call for more accurate methods, ideally non-destructive ones. For calcium sulphate samples, CM and Darr drying performed well. The screed type matters as much as the method.

What should you do if the screed is not yet dry enough?

Do not set the floor covering date from the calendar alone. Baumit's guide gives these drying measures:

• Ventilate with windows and doors fully open. A tilted window is not enough.
• If natural ventilation is not possible, use construction dehumidifiers and drain the condensed water out of the room.
• Do not use open-flame heaters, because they add water vapour.
• Keep objects off the drying screed.

For a heated floor, Baumit's screed sheet says to bring the heating up before the covering goes on. At +20 °C and 70 % relative humidity it names 14 to 16 days after laying as the start for its cement screed. It adds that even the prescribed start-up may not dry the screed enough. Follow the instructions of the heating system maker as well. See underfloor heating for the build-up. Test again after the drying measures.

What does a CM result not tell you?

A CM result is a number for one sample at one time. It does not say whether the covering will perform. That depends on the covering maker's limit and on the screed type. Baumit's guide says to consult the covering maker and its installer on glued coverings such as wood and vinyl. Ask which test and which limit the maker accepts before the installer starts. For the screed types, see concrete screed and anhydrite screed, and for the build-up around the screed, see floating floor.