Is Your Floor Dry Enough? The Accurate Way to Check Screed Moisture
Liquid screed is an amazing flooring material, but it does contain significant water volume so don’t let your eyes deceive you. It may look like the surface is completely dry after a few days, but deep down, there’s often residual moisture. This is why the screed moisture test is vital. If you seal that moisture in with tiles or wood, the vapour pressure will eventually destroy the adhesive bond.
At Capital Screeding, we have poured and prepped floors across London, Kent and Essex for years. We know from experience how important moisture testing is, so we want to share with you our process, so you know how and when to test.
Evaporation & How to Test For Moisture Without Damage
As the chemical reaction turns the liquid into a solid floor, the water molecules need a clear path to evaporate into the air – like sweating. If the air in the room is too humid, though, or if the “pores” of the floor are clogged with surface laitance, the evaporation stops. The water remains locked inside the flooring, creating a hidden reservoir of moisture.
The Right Tool for the Job
To measure the moisture in your screed, you can use a hygrometer. This measures the amount of water vapour (humidity) in the air. The instrument you need is a Digital Hygrometer Box (or Hood). This device is designed to be sealed against the floor, creating a trapped pocket of air that reads the moisture coming specifically from the screed, rather than the open room.
Step-By-Step Guide – The Hygrometer Box Method
The surface hygrometer test is an accurate, non-destructive method of testing for moisture. It’s often used before turning on underfloor heating.
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Clean the Spot – Select an area away from direct sunlight and drafts. Make sure that bit of the floor is clean and dust-free.
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Seal the Box – Place the hygrometer box (or hood) onto the screed surface and seal the edges airtight with butyl tape.
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Wait for Equilibrium – Leave the hygrometer alone for at least 72 hours. This allows the moisture in the screed to balance with the air inside the box.
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Check the Relative Humidity – Take the reading. If the RH is above 75%, the screed moisture is still too high for most floor finishes.
3 Ways to Fail the Screed Moisture Test – Without Even Knowing It
Even the most experienced builder can get caught out by liquid screed if they fall into the trap of treating it like concrete. Here are some of the most common mistakes we see on sites:
- The “Touch Test” Trap – This is a BIG red flag. You simply cannot ‘know’ the moisture content by touching the surface or looking at the colour. The surface interacts with air and dries quickly, but deep down, the screed could still be sodden.
- Ignoring the Laitance – This waxy skin that forms on top of liquid screed seals moisture in. So, if you test on top of the laitance without sanding it off first, you’ll get a false reading by measuring the dry skin, not the wet slab underneath.
- Rushing the UFH – Turning your underfloor heating on full blast to “cook” the floor dry will shock the screed, leading to cracks and curling. Force drying needs to be done gently and incrementally.
Remember – Don’t Skip the Final Step!
We have seen too many beautiful floors ruined by soggy screed. It is a costly mistake, and it is 100% preventable. Whether you use the non-destructive hygrometer box we’ve outlined, or you hire a professional to perform the ‘destructive’ Calcium Carbide (CM) test (which involves crushing a sample for absolute precision), the important thing is that you test.
Ready to Pour?
If you are looking for a liquid screed installation that prioritises quality from start to finish, Capital Screeding is your local expert in the South East. Contact our team today to talk through your floor prep – 020 3369 3396.
