In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention.

Q: Is there a safe way to speed up screed drying when I’m laying floors?

One of the more common factors involved in natural stone flooring failures is laying stone tiles or slabs on to screeds before they have dried sufficiently.

Moisture release from a screed can lead to staining or discoloration of the stone surface, especially if the stone is of light colour – limestones can be particularly susceptible. However, even darker, much denser stones can suffer problems such as picture framing if the moisture brings salts with it such as lime from the cement.

Ensuring a screed is properly dry before laying a surface on to it is therefore an important aspect of flooring.

Many projects overlook screed drying times and establish a programme that cannot hope to accommodate the length of time it takes.

Screed drying times are given in BS 8204. It states that for every millimetre of screed up to 50mm you should allow one day of drying. For every additional millimetre over 50mm a further two days drying time should be allowed.

In other words, a typical screed of 65mm needs at least 80 days [50+(2x15)] to ensure sufficient drying. Few projects will allow three months for a screed to dry.

Many screeds now incorporate heating systems and there is a temptation to turn these on at an early stage to speed up drying. One problem of doing that is that if drying is too rapid the screed may not cure properly and will have reduced strength.

Screeds are also more susceptible to shrinkage if dried too quickly and cracking and curling may occur. Bonded screeds may lose adhesion and suffer debonding, considerably weakening the floor structure.

However, after 28 days much of the curing and strength gains will have taken place and it is safer to turn heating on.

If there is no heating in the floor the use of dehumidifiers is probably the next best method for removing moisture, coupled with ventilation. Simply heating a room will not necessarily lead to rapid drying. If the floor remains cool, condensation can result and, rather perversely, prolong drying.

There are screed systems available that claim to lock in moisture so they are ready for the installation of natural stone almost immediately. These can be expensive and must be used in strict accordance with the manufacturer’s instructions, but the benefits may be more than worth it where time is money.

An uncoupling layer is sometimes mistakenly considered to provide a barrier to moisture. Such layers will prevent moisture flow to some degree, but the mositure will still have to escape, either through joints in the layer or some other location, potentially leading to problems elsewhere.

Generally, trapping moisture is not a good idea. It usually emerges somewhere with undesirable consequences.

In some cases the worry of problems developing has led to the drying times recommended by BS 8204 being complied with but laying still not being allowed until the floor is considered to be dry, which can only be determined by taking humidity readings. If you regularly install screed floors, I would recommend purchasing a moisture meter with a hygrometer where disposable sensors can be set into the screed and realistic readings obtained. British Standard allows for various methods of measuring moisture, but they are mostly impractical.

So, yes, there are ways to speed up the time between installing a screed and laying a stone floor but these must be considered carefully. Waiting remains the best policy if there is the time to do so. If you do speed things up, give the screed a chance and give it a month’s grace.

Reference

BS 8204-1: 2003. Screeds, bases and in situ floorings – Part 1: Concrete bases and cement sand levelling screeds to receive floorings – Code of practice. Published by the British Standards Institution, London, England.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646. Email: info@ibis4u.co.uk

The advice offered in answer to readers’ questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought with all stone-related issues as each problem will inevitably have its own unique set of circumstances.