A new floor finish looks perfect on the day it goes down. A few weeks later the seams are lifting, the vinyl is bubbling, or the timber has started to cup. In many cases the finish was fine and so was the fitter. The problem was underneath: the screed wasn’t dry enough when the floor covering went on.
In this Learning Zone article, the Screed Scientist explains why moisture in screed causes so many flooring failures, and what everyone on site can do to avoid them.
Where the water comes from
Every screed starts life wet. Water is needed for the binder to react, and with flowing screed, extra water also helps the material flow and self-level. Some of that mixing water is used up as the screed hardens. The rest has to leave the screed by evaporating from the surface.
Where a damp-proof membrane or insulation sits below the screed, the water can’t drain away downwards. It can only travel up through the screed and out into the air of the room. That process takes time, and the thicker the screed, the longer it takes.
Why the surface can fool you
A screed often looks and feels dry on top long before it is dry all the way through. The surface dries first because it is in contact with the air, while moisture deeper down is still working its way up.
Once a floor finish is laid, it acts like a lid. Moisture still rising through the screed collects underneath the covering, with nowhere to go. That is when the trouble starts.
What goes wrong
When a floor finish is laid over a screed that still holds too much moisture, typical problems include:
- Adhesive failure. Moisture can attack many flooring adhesives, so the bond weakens and the covering lifts or slides.
- Bubbling and blistering. Sheet vinyl and other impervious finishes can trap moisture, which pushes up into blisters.
- Lifting seams and joints. Tiles and planks can move as the moisture changes underneath.
- Swelling and cupping. Timber and wood-based floors take on moisture from below and change shape.
- Odours and discolouration. Trapped damp can cause staining and unpleasant smells in some finishes.
Putting any of these right usually means lifting the finish, waiting for the screed to dry properly, preparing the surface again and laying a new floor. That is slower and dearer than waiting the first time.
Different finishes, different tolerance
Not all floor coverings react in the same way. Some finishes are far more sensitive to moisture than others, which is why flooring manufacturers set their own acceptable moisture readings for their products. The right question isn’t simply “is the screed dry?” but “is the screed dry enough for this particular finish and this particular adhesive?”
That is why the flooring contractor, the floor covering manufacturer and the screed manufacturer’s guidance should all be checked before laying begins.
Measure, don’t guess
The only reliable way to know whether a screed is ready is to test it. Touching the surface, looking at its colour or counting days on the calendar aren’t enough on their own. Drying rates vary with thickness, screed type, temperature, humidity and ventilation, so two floors poured on the same day can dry very differently.
The two recognised methods are the hygrometer (relative humidity) test and the calcium carbide (CM) test. Our article How to Test Screed Moisture Properly explains both. The Screed Scientist, like many proprietary screed manufacturers, prefers the CM test because it measures the moisture content of a sample taken from the screed itself.
The relevant standards
In the UK, the main references for screeds and their preparation for floor finishes are BS 8204 (screeds, bases and in-situ floorings) and BS 8203 (installation of resilient floor coverings). Manufacturers’ own guidance should always be followed alongside them.
The simple rule
A good floor depends on a dry, sound base. Plan enough drying time into the programme, keep the building conditions steady, test properly and keep a record of the results. That approach protects the finish, the programme and everyone’s working relationships.
CSC Screeding, Established in 1987, has laid floor screeds on projects of every size across the UK. The Screed Scientist shares that experience so floors get done right the first time.
Next in the series: How to Test Screed Moisture Properly: Hygrometer and CM Tests
