Before any floor finish goes down, someone needs to answer one question: is the screed dry enough? Guesswork, a hand on the surface or a date on the programme won’t do. The answer comes from a proper moisture test.
There are two main methods used in the UK: the hygrometer test, which measures relative humidity, and the calcium carbide test, usually called the CM test. This guide explains how each one works and what to watch out for.
The hygrometer (relative humidity) test
The hygrometer test measures the relative humidity of a small pocket of air trapped against the screed surface. When that air reaches balance with the screed underneath, the reading shows how much moisture the screed is still giving off.
How it is done
- A yellow hood box is placed on the screed and sealed to the floor so no room air can get in or out.
- A probe measurement instrument is connected to the hood and reads the humidity of the trapped air.
- The hood is left in place to settle before a final reading is taken. Settling time is normally 72 hours, but it is good practice to check the manufacturer’s guidelines for the screed and the instrument.
- The reading is recorded along with the date, time, location, room temperature and conditions.
Things to watch
- Seal it properly. A leak around the hood will give a false reading.
- Don’t rush it. Reading too early, before the air has reached balance, can be misleading.
- Steady conditions. Big swings in temperature, heating left on, or doors and windows opening and closing can all affect the result.
- Several locations. One reading doesn’t represent a whole floor. Test in a number of places, including areas likely to dry slowly, such as corners, thicker sections and rooms with poor airflow.
The calcium carbide (CM) test
The CM test measures the actual moisture content of a sample taken from the screed. It is a preferred testing method of the Screed Scientist and many proprietary screed manufacturers, because it looks at the material itself rather than the air above it.
How it is done
- A sample of screed is taken from the floor, through the depth of the screed, and broken down.
- A weighed amount of the sample is placed in a silver flask along with a calcium carbide reagent and steel balls.
- The flask is sealed and shaken. Moisture in the sample reacts with the calcium carbide and produces gas.
- A pressure gauge on the flask shows the gas pressure, which converts to the moisture content of the sample as a percentage.
Things to watch
- Take a representative sample. The sample should come from the full depth of the screed, not just the drier surface.
- Work promptly. Samples should be tested straight away so they don’t dry out or pick up moisture.
- Follow the instrument’s instructions for sample weight and shaking time.
Which result do you use?
The acceptable readings for laying a floor finish come from the screed manufacturer and from the floor covering and adhesive manufacturers. Check what limits they set and which test method they accept for the readings you’ve got. BS 8204 and BS 8203 also give guidance on testing and on preparing screeds for floor finishes.
If readings are borderline or results from different methods don’t agree, talk to the manufacturers before laying anything.
Record everything
Whichever method you use, keep a written record: test locations marked on a plan, dates, times, readings, instrument details, room temperature and humidity, and who carried out the test. Add photographs. That record helps the flooring contractor plan their work, and it gives everyone a clear picture of the floor at handover. Our article Avoiding Disputes: Records and Sign-off covers this in more detail.
In short
- The hygrometer test uses a sealed yellow hood box and a probe instrument, usually left for 72 hours to settle.
- The CM test uses a screed sample, a silver flask and a pressure gauge.
- The Screed Scientist and many proprietary screed manufacturers prefer the CM test.
- Always check the manufacturers’ acceptable readings, test in several places and keep records.
Next in the series: Screed Drying Times by Type
