Verification
Moisture Testing Methods and Acceptance Criteria for Liquid Screed
Moisture testing confirms a liquid screed has dried sufficiently for the specified floor finish, using recognised RH probe or carbide methods rather than guesswork.
Need moisture testing guidance for your screed?
We can advise on realistic drying and testing timelines for liquid screed installations across Exeter and the South West.
Or call our landline on 01392 237700
Before any floor covering, adhesive or heating system is committed to a liquid screed, the moisture content of the slab needs to be verified rather than assumed. Calcium sulfate and cement-based screeds both release residual construction moisture over time, and laying a moisture-sensitive finish too early is one of the most common causes of costly failures on site.
Two testing methods dominate UK practice: in-situ relative humidity (RH) testing using probes inserted into the screed, and the carbide bottle (CM) method which chemically reacts a crushed sample with calcium carbide to measure moisture release. Each has its place depending on the floor finish specified and the screed type installed.
Specifiers should agree the test method, number of test points and acceptance criteria before installation begins, not after a covering has already failed. This avoids disputes about whether a floor was 'dry enough' and gives a documented, defensible record for the project file.
Moisture Testing — the points that decide it
- RH testing follows the principles set out in BS 8203 and BS 5325 for the specific floor finish being laid
- Carbide (CM) testing gives a percentage moisture content and is common for calcium sulfate screeds
- Test point numbers and locations should relate to floor area, not be arbitrary
- Ambient conditions (temperature, ventilation) affect drying rates and should be recorded alongside results
- Acceptance criteria differ by covering — timber, resin and vinyl are typically more sensitive than ceramic tile
- Testing too early gives a false sense of security and can mask ongoing moisture migration
- Results should be recorded in writing and retained as part of the handover documentation
- Where heating is present, commissioning should generally precede final moisture testing
Reference points for moisture testing
| Common RH threshold (general flooring) | Typically around 75% RH — confirm against finish manufacturer |
|---|---|
| Typical carbide threshold (calcium sulfate) | Often below 0.5% CM — confirm against manufacturer data |
Choosing between RH and carbide testing
RH probe testing measures the humidity of air trapped within a sealed hole drilled into the screed, left to equilibrate before a reading is taken. It is widely used because it is non-destructive beyond the small drilled hole and gives a reading that correlates well with moisture-sensitive finishes such as timber and resin.
The carbide method involves extracting a small sample of screed, crushing it and reacting it with calcium carbide reagent in a sealed pressure vessel; the pressure generated is converted to a moisture percentage. It is quick to perform on site and is commonly specified for calcium sulfate screeds where manufacturers publish percentage thresholds for particular finishes.
Setting sensible acceptance criteria
Acceptance criteria should be agreed with reference to the flooring manufacturer's requirements and the screed manufacturer's data sheet, since these can differ. A resin flooring specification may demand a materially lower moisture reading than a ceramic tile installation on the same screed.
Where results are borderline, it is more sensible to extend drying time and retest than to proceed and risk delamination, blistering or mould growth beneath an impermeable covering. Building this contingency into the programme early avoids pressure to skip or shortcut testing later.
In short
Moisture testing confirms a liquid screed has dried sufficiently for the specified floor finish, using recognised RH probe or carbide methods rather than guesswork.
Need moisture testing guidance for your screed?
We can advise on realistic drying and testing timelines for liquid screed installations across Exeter and the South West.
Or call our landline on 01392 237700
Moisture Testing — your questions
Need moisture testing guidance for your screed?
We can advise on realistic drying and testing timelines for liquid screed installations across Exeter and the South West.