Moisture log for floor fitters

Why moisture ruins floors

Moisture can put a floor finish at risk. Before you lay the floor, find out whether the subfloor meets the requirements for the covering you plan to use. The flooring maker’s instructions always win. Use their test method and limit, rather than treating a general figure as permission to lay any floor.

The sheet does not choose a safe limit for you. Enter the figure from the relevant instructions and write down its source. A Pass on the sheet means the entered reading is no higher than the entered limit. It is not a separate judgement that the floor is ready for every product.

How to test a subfloor

For a hygrometer test, a sealed, insulated box sits on the floor. It reads the humidity of the trapped air as % RH. 75% RH or less is the usual figure for a dry base under BS 8203. Some wood floors over underfloor heating need 65%. Check the flooring maker’s instructions for your floor.

Switch underfloor heating off at least 96 hours before a hygrometer test. Record the date it was switched off. Leave the box until the reading settles. That is often 72 hours and can be longer on thick slabs. Do not treat the usual settling time as a reason to stop before the reading has settled.

Test where the floor is likely to be wettest. Look at corners, thick areas and rooms with poor airflow. A hand-held meter helps find those spots. It is for finding wet areas, not for proving a floor is dry. Keep that distinction clear when you choose a test location and write up the result.

For a timber subfloor, use a wood moisture meter and the flooring maker’s figure. Record that method on the log. Do not use an RH result or a CM limit in place of the figure required for the timber test.

RH and CM in plain words

RH means relative humidity. In the hygrometer box test, it describes the humidity of the air trapped over the floor. It is an air humidity reading, not the percentage of water by weight in a piece of screed.

A CM test uses a sample from the full depth of the screed. The sample is shaken in a flask with a reagent. The gauge gives moisture as a % by weight. The limit comes from the screed maker and the flooring maker, so record where your chosen limit came from.

RH and CM describe different things. Label the method beside your readings. Do not carry a limit from one method over to another.

How to record readings

Start with the job address, customer, floor covering and subfloor type. Add the test method, meter make and model, calibration date and the person who tested. Write down the limit used and its source so someone reading the log can see what the result was compared with.

For each reading, record the place, date, time, reading, room temperature and room humidity. Give the location a clear name rather than just calling it the floor. Take photos and keep them with the log. The photos can help you find the same spot when you look back at the test.

Keep each reading in its own row. Check the entries before you make the PDF, especially the method and limit. The row result stays blank if a reading or limit is missing. Save a copy of the completed record with your other job sheets, rather than relying only on the browser’s saved form.

What to do if it fails

If the reading fails the required limit, wait and retest. Improve heat and airflow, or ask the flooring maker about a surface damp proof membrane. Follow their instructions for the next step. Do not change the limit in the log just to turn a failed reading into a Pass.

Keep the failed reading and record the next test as a separate entry. Use the location notes and photos to keep the record understandable.

Check the method and limit again. The flooring maker’s instructions always win.

Hygrometer method and figures: Contract Flooring Association guidance