Screed moisture testing
Before flooring goes on, the screed is tested for moisture. The usual limit is 75% relative humidity. Calcium sulphate: 75% RH or 0.5% moisture. Cement-based: under 75% RH, about 2% by the carbide method.
Three methods: a hair or digital hygrometer, a carbide bomb, or an oven test. With underfloor heating, the heating is switched off for 48 hours before testing.
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The limits
| What goes on top | Calcium sulphate | Cement liquid (cement-based liquid screed) |
|---|---|---|
| Cement-based adhesive or smoothing compound | 75% RH (0.5%) | Under 75% RH (about 2% carbide) |
| Calcium sulphate based adhesive or compound | Often 87% RH (1.5%): ask the maker | Not applicable |
| Wood or vinyl | 75% RH (0.5%) | Under 75% RH |
| Surface liquid DPM | Only after 28 days, under 87% RH, and no underfloor heating | Possible: follow the DPM maker |
The methods
- HygrometerA sealed hood or box on the screed surface; the air inside comes to balance with the screed and the meter reads its relative humidity. A hygrometer with a humidity box is the usual kit.
- Carbide test
A sample is drilled out and sealed in a pressure vessel with calcium carbide; the pressure gives the moisture content as a percentage.- Oven test
A sample is weighed, dried and weighed again. - Carbide test
Wood floors are stricter
BS 8201 treats 75% RH as acceptable for wood flooring, but many wood flooring suppliers say that's for floating wood floors only and ask for drier screeds. Get the wood supplier's figure in writing before the test, so everyone is working to the same number.
Who tests, and when
- It's the floor fitter's job to measure the moisture and choose the adhesive and preparation.
- With heating: after the commissioning cycle and 48 hours off.
- Test where it dries slowest: the thickest part, corners, and away from windows.
- Keep the result with the date, the method and where it was taken. It's the record if a floor fails later.
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Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. These are his jobs.
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Moisture testing questions
What moisture level does liquid screed need before flooring?
75% RH for most finishes. 75% RH or 0.5% for calcium sulphate. Under 75% RH, about 2% carbide, for cement-based.
How is screed moisture tested?
With a hygrometer under a sealed hood, a carbide test on a drilled sample, or an oven test.
Can tiles go on at 87% RH?
On calcium sulphate, with a calcium sulphate based adhesive, sometimes. 87% RH may be possible with some adhesives: ask the adhesive maker first.
Who is responsible for the moisture test?
It's the floor fitter's job to measure the moisture and choose the right adhesive and preparation.
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