Screed: checking readiness for flooring, CM measurement and drying time correctly

What readiness for flooring means

After installation, screed contains more water than it needs to set. This water must evaporate before a covering that traps it is laid (parquet, PVC, textile coverings, tiles in thin bed depending on the system). Otherwise swelling, deformation, detachment, odour and mould are the risk.

A screed is belegreif when the measured residual moisture is below the limit value that applies to the combination of screed type, covering, adhesive and heating. The age of the screed alone says nothing.

Which types of screed are there?

For the screed standard DIN 18560 and the flooring works (DIN 18365 in VOB/C), the exact requirements are to be looked up there.

The CM measurement

The calcium carbide method is the usual method on site:

1. Take a sample (material from the entire screed thickness, crushed).
2. Weigh it into the CM bottle with the calcium carbide ampoule.
3. Shake the device: the water from the sample reacts and the pressure is measured.
4. Read the value (CM-% relative to the sample mass) and compare it with the limit value.

Measuring points should be chosen to be representative: not just at the edge but also in the open area, in rooms with different loads and at critical places (door thresholds, water points, heating circuits). With underfloor heating, the screed is function-heated and readiness-heated before the measurement, and the measuring points are chosen so that heating pipes are not damaged. The exact number and position depend on data sheets and the client’s requirements.

Guide values (practice, for orientation)

These values are guide values for common coverings. What is authoritative is the information from the screed, covering and adhesive manufacturers and the data sheets of the trade associations. For vapour-tight or particularly moisture-sensitive coverings, lower limit values often apply.

Drying time: rule of thumb and reality

A well-known rule of thumb says: about 1 week per cm of screed thickness up to 4 cm, above that about 2 weeks per cm (for cement screed under favourable conditions, about 20 °C and 65 % relative humidity). That is a planning value, not proof. Construction moisture, cold rooms, closed windows, high humidity and additional sources of water extend the drying time considerably.

Measures for drying:

• Ventilate (in a controlled way, not just tilted windows), heat and, if necessary, use construction dryers.
• With heated screed: heating record with heating-up and cooling-down phases.
• Fast-setting screeds only with the manufacturer’s proof.

The measurement record

A usable record contains:

1. Construction project, room, screed type, thickness, installation date.
2. Measurement date, measuring device, tester.
3. Measuring point (sketch or photo) and measured value for each point.
4. Room climate (temperature, humidity).
5. Limit value and its source (manufacturer’s information, data sheet).
6. Result: ready for flooring yes or no.
7. Signatures of the tester and, if possible, the client (Auftraggeber) or site manager.

Who is responsible?

• Screed layer: Owes a screed free of defects. It cannot release the screed for any covering, only certify its condition.
• Flooring installer: Must check the substrate (including moisture) and, if it has concerns, notify them in writing before starting, for example if residual moisture is too high or the heating record or documentation is missing (§ 4 para. 3 VOB/B). See Bedenkenanmeldung.
• Client/site management: Scheduling. Anyone who forces the date before the screed is ready bears the risk if the flooring installer has properly notified concerns.

If the flooring is delayed because the screed is still too damp, this can be a hindrance if the cause lies within the responsibility of the client or the previous trade. See Behinderungsanzeige.

Typical mistakes

• Judged by laying time only.
• Measurement at one point and the result transferred to the whole area.
• Heated screed covered without a heating record.
• Wrong limit value (cement instead of anhydrite, with or without underfloor heating).
• No documentation: in a dispute the screed is regarded as “probably too damp”.
• Screed wetted afterwards through open windows in rain, water damage or plastering work.

Frequently asked questions

Sources and status

• § 4 para. 3 VOB/B – concerns; § 13 para. 3 VOB/B – defect claims
• DIN 18560 (screeds in building construction), DIN 18365 (VOB/C, flooring works) and data sheets of the trade associations: not checked word for word here, limit values reproduced in substance as orientation.
• Stand: 19.10.2026. The manufacturers’ information and the current data sheets are authoritative.
• Quotations of statutory text are given in translation; the German wording is authoritative.