How do you know a screed is really dry?
Only a carbide bomb test counts, on a sample taken through the full depth. A screed that feels dry can still be very wet at its core.
Only a carbide bomb test counts, on a sample taken through the full depth. A screed that feels dry can still be very wet at its core.
This is the most expensive question to answer by eye. A screed that looks light, hard, and dry to the touch can still contain a lot of water three centimeters deep. Installing flooring over this screed means trapping that water under an airtight barrier.
The reference method is the calcium carbide meter, also known as the CM test. A sample is taken with a chisel through the entire thickness, crushed, and then reacted with calcium carbide in a pressure vessel. The resulting pressure indicates the actual moisture content of the mass.
The thresholds commonly required before installing flooring are as follows:
The difference between the two types is significant and often surprising: an anhydrite screed must be significantly drier than a cement screed before receiving flooring. The specifications of the flooring and mortar manufacturer always take precedence over these values.
A pin-type or capacitive moisture meter provides an immediate reading, but it only measures the first few millimeters. It is useful for comparing different areas, not for deciding whether to proceed with installation.
The plastic film test remains useful as a free preliminary check: a square of polyethylene taped down hermetically for 24 to 48 hours. If condensation appears underneath, there is no need to go any further; the screed is not ready.
The rule of one week per centimeter up to four or five centimeters, and much slower thereafter, provides a rough idea for project planning. It never replaces a measurement, as drying also depends on ventilation, the season, and the type of mortar.
Our article drying the screed before tiling or parquet flooring details the timeframes for each type of flooring. And if the schedule is tight, first look at what is reasonable to do to speed up drying.
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