THE DEFINITIVE GUIDE
Stephen Boulton, Technical Service Manager at F. Ball and Co. Ltd., explains how calcium sulphate screeds should be prepared to ensure an optimum flooring installation is achieved.
The speed with which calcium sulphate or screeds can be applied over large areas combined with quicker drying times compared with concrete have resulted in them becoming an ever more popular choice of subfloor in new build construction.
The basic principles that apply when creating a base to install new floorcoverings over other subfloor types are applicable when preparing calcium sulphate screeds. It’s essential to take the time to assess the condition of the subfloor and prepare it appropriately, and then to choose a compatible smoothing underlayment to install over it. There are just a few unique considerations:
DRYING TIME The first thing to do when working with calcium sulphate screeds is to ensure that the subfloor is suitably sound, smooth and dry. Just as when installing floorcoverings over any other subfloor type, a moisture measurement test should be conducted to ascertain if the subfloor is dry enough to proceed directly to installing floorcoverings. Recently installed calcium sulphate screeds may still contain high levels of residual construction moisture. If a moisture test indicates that the relative humidity of the subfloor is above 75%, contractors will need to allow further time for the screed to dry.
It’s important to remember that, unlike concrete, calcium sulphate screeds dry from the bottom up, so a moisture test may deliver a positive reading right up until the point the subfloor is completely dry. Depending on how long ago the screed was installed, there may only be a short time to wait. Therefore, contractors should persevere in allowing
the subfloor to dry sufficiently. This can be speeded up by ensuring areas are adequately ventilated, by opening windows or using a dehumidifier.
It is also essential to remove any laitance, a crust of cement and fine aggregates formed on the surface of the screed as it dries. If laitance is left untreated, the application of subsequently applied levelling compound will have a high risk of failing. Laitance is normally removed using rotary sanding or abrading machines. Any contaminants that may prevent adhesion, such as wet trade waste or debris, should be cleaned off as well.
PRIMING After ensuring the subfloor is suitably smooth and dry, the contractor can then proceed to the next step involved in properly preparing calcium sulphate screeds: priming. Priming a calcium sulphate screed is essential to the overall appearance and performance of the floor. It helps to promote the adhesion of the smoothing underlayment to the screed and prevents the unacceptable drying of the levelling compound, which can result in floor failure.
F. Ball’s Stopgap P121 primer is specially formulated to promote the application characteristics of compatible levelling compounds when applied over calcium sulphate screeds. It is designed to be applied in two coats; the first diluted with one part water for every one part primer, followed by a second coat applied neat.
COMPATIBLE LEVELLING
COMPOUNDS Calcium sulphate-based levelling compounds, rather than cement-based ones, offer maximum compatibility with calcium sulphate screeds. This is because calcium sulphate-based levelling compounds will contract at a similar rate to the calcium sulphate screed underneath, limiting the amount of stress at the bond interface and minimising the risk of floor failure.
Another reason that F. Ball recommends the use of a calcium sulphate-based levelling compound is that the application of cement-based levelling compounds onto a calcium sulphate screed in damp environments can result in a reaction occurring that leads to the production of ettringite at the interface between the levelling compound and subfloor screed. Ettringite is a crystalline material that can cause floor failure.
Stopgap 1100 Gypsum is F. Ball’s calcium sulphate-based levelling compound, designed especially for application over calcium sulphate screeds. It can be applied at thicknesses between 2–10mm, has excellent self-levelling properties and can be pump-applied up to 1500m2
per day, dependent
upon manpower, thickness applied and equipment used. It is also low-odour and protein free, making it suitable for use in biologically sensitive areas, such as hospitals and schools.
www.f-ball.com 34 | SCREEDS & RESINS
www.tomorrowscontractfloors.co.uk
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