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Wet Screed

John Alcock on anhydrite screeds over underfloor heating

A few weeks after my article on anhydrite screeds (CFJ July 2013), a technical support enquiry landed on my desk highlighting the issue of anhydrite screeds over underfloor heating.
A domestic barn in the Midlands was being converted into a high-end residential property. As seems more common with refurbishment jobs of this standard, underfloor heating had been fitted, claimed to have been commissioned by the book – a subject we’ve dealt with in past articles.
An anhydrite screed of around 50mm thickness had then been installed.
All had seemed fine for about six months, until some of the LVT’s in the kitchen had begun to lift.
Visiting the property and inspecting the floor, I discovered that there was still moisture in the screed which had caused the usual issues with the coverings above.
Clearly, the problem had come from the issue of not letting the screed dry enough. In most circumstances ensuring an RH of 87% or below in the screed is sufficient to start putting down a surface membrane, but the situation is more complicated where underfloor heating is concerned.
My advice, based on problem projects such as the one above, is that if the screed has been installed above underfloor heating then it’s wise to wait until it is fully dry – below 75% – before applying the membrane.
This seems to be a common mistake, although in fairness I imagine in many situations this is down to pressures to get on the floor and complete the job as soon as possible.
It’s also worth bearing in mind that moisture problems like this can affect a wide range of floorcoverings.
I’ve seen expensive porcelain tiles lift from the floor, with further inspection revealing the tell-tale ettringite residue on the underside – a sure sign that the adhesive has reacted with the screed due to excessive moisture in the floor.
Put simply, although it might not make you any friends in the short term, it’s well worth having those tough conversations to explain why the floor needs to dry further before continuing work.

Ultimately, although when a floor fails it is often us who has to put it right, the delays this causes are also a problem for the main contractor and building owner.
John Alcock is technical specifications manager at Bostik
T: 01785 272727

This article has been reproduced from the Contract Flooring Journal website. You can find them at