Subfloors & screeds - F Ball & Co

FASTER drying times and the speed at which they can be applied over large areas has seen the rise in popularity of calcium sulphate (or anhydrite) screed subfloors, especially in newbuilds. When installing new floorcoverings over such screeds, it’s essential to take the time to assess the condition of the subfloor and suitably prepare it, before choosing a compatible levelling compound.

Subfloor preparation

As with any screed, a moisture measurement test should always be conducted to determine whether the calcium sulphate screed subfloor is dry enough to install floorcoverings immediately. If the relative humidity of the subfloor is above 75%, contractors will need to allow further time for the screed to dry.

It’s also essential to remove any laitance (a crust of fine aggregates and cement formed on the surface of the screed as it dries), as failing to do so will increase the risk of floor failure once the levelling compound has been subsequently applied.

After ensuring the subfloor is suitably smooth, dry and free of contamination, the contractor can then proceed to prime the subfloor. Priming helps promote the adhesion of the levelling compound to the screed and prevents unacceptably fast drying, which can weaken the levelling compound. F Ball’s Stopgap P121 acrylic primer is specially formulated to promote the adhesion of Stopgap levelling compounds over calcium sulphate screeds.

Selecting the right levelling compound

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 when drying, limiting the amount of stress at the bond interface and minimising the risk of floor failure.

Another reason 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 can produce a chemical reaction resulting in the formation of ettringite. 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 use over calcium sulphate screeds, such as anhydrite,’ says the company. ‘It can be applied at thicknesses between 2-10mm, has excellent self-levelling properties, and can be pump-applied up to 1,500sq m per day. Stopgap 1100 Gypsum is also low-odour and protein free, making it suitable for use in biologically sensitive areas, such as hospitals.’

Case study

F Ball products including Stopgap 1100 Gypsum were used to install floorcoverings in an upstairs function room in the newly-built clubhouse at CB Hounslow United Football Club in Middlesex.

Contractors from Middlesex Flooring used a rotary sanding machine to remove laitance from the newly-installed anhydrite screed subfloor to prepare the surface ahead of the application of the levelling compound. A moisture test was also carried out, and it was determined that the subfloor was dry enough to receive floorcoverings.

‘Stopgap 1100 Gypsum was then applied to create a smooth surface ahead of the installation of the floorcoverings. As a self-levelling compound, the compound was poured onto the prepared substrate, once mixed, and allowed to flow and achieve a thickness of 3mm. The compound was ready to receive floorcoverings 24 hours later,’ says the company.

Craig Gargan, of Middlesex Floors, said: ‘We received great support from F Ball throughout the job, including with the site survey and moisture testing. As usual, using their products meant that it was straightforward and there were no problems.’

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