Anhydrite screeds: Handle with care

FOLLOWING my November column in CFJ on subfloor moisture, I received a call from a reader asking for advice on dealing with moisture in calcium sulphate (ie. anhydrite or alpha-hemihydrate) screeds. When it comes to calcium sulphate screeds, the most common question I am asked is about drying times and what to do if a quick installation of a floorcovering is needed.

Although calcium sulphate screeds are sometimes sold and applied as a fast track option, their drying times are in fact similar to sand/cement screeds: typically 1mm per day up to a thickness of 40mm, and then an additional two days per mm over this.

British Standard Code of Practice 8203 for installing resilient floorcoverings says the subfloor Relative Humidity (RH) should be below 75%. RH should be measured using a floor mounted digital hygrometer. If the hygrometer indicates an RH level of 75% or more, and the customer demands quick installation over a calcium sulphate screed, the flooring contractor is faced with a
potential problem. Whereas with typical

sand/cement systems, excess moisture can be suppressed by applying an epoxy resin waterproof surface membrane, I do not advocate using these products with calcium sulphate screeds. This is because high levels of moisture trapped at the inter face between a calcium sulphate screed and a waterproof surface membrane will become soft and may result in failure when one of the sur faces becomes stressed.

Alternative isolating systems can be used, but only in specific applications, and up to a maximum moisture reading of 85%. Advice should be sought from the manufacturer of such systems before use.

The only other solution is to allow these screeds to dr y out naturally, which can be a lengthy process. Fortunately, dehumidifiers and forced drying via underfloor heating can be used to assist dr ying.

Additionally, grinding or sanding of the surface, in accordance with manufacturer’s instructions, can take place soon after a calcium

sulphate screed is applied. As well as producing a laitance-free surface, this process also helps the screed to dry, especially if a hard, dense finish is achieved.

Another question I am often asked is the type of smoothing underlayments or levelling compounds that can be applied over calcium sulphate based screeds.

A professionally installed screed should not need covering, but there may be instances when either the sur face is not quite smooth enough or when the correct floor level/height has not been achieved.

Special care needs to be taken when applying cement-based products over calcium sulphate screeds to avoid the formation of ettringite crystals that may lead to disruptive expansion and subsequent failure of the smoothing/levelling compound. With this in mind, I offer the following advice before laying a new floorcovering:

1. Ensure that the RH is always below 75%. Take measurements at various points across the floor
using a digital hygrometer. Alternative means of measuring construction moisture can be used, such as the carbide bomb or oven drying at 40degC where readings below 0.5% should be achieved before fitting resilient floorcoverings.
2. Ensure the surface is suitably sound and no sur face laitance remains. Sanding and vacuuming is recommended.
3. Apply an appropriate acrylic or water dispersible epoxy penetrating sealer according to the supplier’s recommendations and allow it to dry.
4. Using a proprietary self- levelling/smoothing compound, such as the fast dr ying systems now available is recommended. Flooring contractors should seek specification advice from the manufacturer.
By applying these general guidelines you can successfully install new floorcoverings over calcium sulphate screeds.

Neil Sanders is technical manager at F Ball and Co
T: 01538 361 633 I www.f-ball.co.uk