Covering readiness and the CM test: limits, procedure, right timing

Covering readiness is the moment from which a screed may be covered with flooring without trapped moisture causing damage. It is not a feeling and not a calendar date but a measured state — and the CM test is the recognised way to prove it. This article explains the limits, the test procedure and how continuous monitoring delivers the right test date.

What covering readiness means

A screed is ready for covering when its residual moisture has fallen far enough that the planned flooring remains permanently free of damage. In Germany the frame of reference is DIN 18560 (screeds in construction) together with the flooring trade rules; in Switzerland, SIA standards (in particular SIA 251 for floating screeds) govern the analogous context. Material classification follows EN 13813 across Europe.

Crucially, covering readiness depends on the flooring. Vapour-tight coverings (PVC, rubber, parquet) demand lower residual moisture than diffusion-open ones. The flooring manufacturer’s specifications and contractual agreements always take precedence.

The CM limits

Recognised guide values for covering readiness:

Screed type	unheated	heated
Cement screed (CT)	≤ 2.0 CM-%	≤ 1.8 CM-%
Calcium sulphate screed (CA/CAF)	≤ 0.5 CM-%	≤ 0.3 CM-%

The much lower values for calcium sulphate screed are no misprint: CAF is moisture-sensitive — residual moisture under a tight covering can damage the structure long-term.

How the CM test works

With the calcium carbide method (CM), screed material is chiselled out of the lower third of the layer at the measuring point, crushed, weighed and sealed in a pressure flask with a calcium carbide ampoule. The contained water reacts with the carbide to form acetylene; the resulting pressure is read off the manometer and converted to CM-%.

The test is destructive (a drill/chisel point), takes around 20–30 minutes per point and delivers a point value at the time of testing — no more, no less. This is exactly where its two practical problems come from:

1. Tested too early: the value is above the limit, the test was wasted, a new date is needed. Costs and drill holes add up.
2. Tested too late: the screed may have been ready for weeks — lost construction time nobody gets back.

Monitoring schedules the test — it does not replace it

Continuous moisture monitoring inside the screed solves the timing problem: an embedded sensor measures moisture and temperature in the material and transmits the values continuously by radio through the screed. The curve — displayed in g/m³, comparable with the CM reference — shows how the core moisture approaches the threshold range.

This defines clearly when the CM test is scheduled: only when the measured curve makes passing highly likely. The normative proof remains the CM test — monitoring prevents flying blind between installation and proof and makes the drying curve documentable for everyone involved: screed layer, site management and flooring installer see the same status.

For planners who want to specify monitoring, specification templates are available for download — details in the article moisture monitoring in the specification.

Responsibility and documentation

The duty to check before installation lies with the flooring installer; responsibility for drying lies with site management during construction. In practice this creates a familiar tension: the installer wants certainty, site management wants deadlines. A documented, continuous moisture curve defuses the discussion — it shows objectively whether and when the screed is drying and makes decisions traceable instead of ad hoc.

To try this in your own project: the AntOne pilot project includes sensors, drying forecast and documentation for site management and planners.