Continuing the format of recent technical advice articles drawing on real-life situations as examples of best practice in flooring installation, Jason Tatton provides guidance on installing floorcoverings over calcium sulphate screeds.

CALCIUM sulphate screeds have become an increasingly common choice of subfloor to install in new builds, especially where underfloor heating is incorporated within the subfloor. This is because of advantages over concrete bases, such as greater thermal conductivity, the speed with which they can be applied over large areas, less shrinkage, and fewer joints required within the subfloor.

However, these screeds necessitate a few special considerations when it comes to subfloor preparation, and not undertaking certain procedures when working over them is a common cause of floor failure. As a result, they’re a frequent topic of question for F Ball’s technical service department.

Common concerns
For example, the application of cement-based levelling compounds over calcium sulphate screeds can result in a chemical reaction that causes the formation of ettringite, a crystalline material that can cause the levelling compound to de-bond from the substrate.

F Ball’s technical service department was recently called for advice on a flooring installation in a large residential development, consisting of multiple apartment blocks, with calcium sulphate screeds. The flooring contractor who sought our advice was tasked with installing floorcoverings in apartments on a number of floors in three different buildings (over 120 residences in total), and the cement-based levelling compound that had already been applied had begun to detach from the screed in many areas.

Closer inspection
A closer inspection by an F Ball technical representative of areas where a levelling compound hadn’t yet been applied revealed that, although they appeared to be sound, the calcium sulphate screeds were contaminated with paint overspray at the edges.
Laitance (the crust of cement and fine aggregates formed on the surface of the screed as it dries) was also found. The screed installer should have removed the laitance, but this wasn’t carried out, as is often the case.

Moisture check
As recommended as part of any floorcovering installation, the F Ball technical representative also undertook a moisture test to see if subfloor relative humidity levels were low enough for floorcoverings to be installed. In the first instance, a handheld radio frequency moisture meter was used to check for the presence of subfloor moisture, giving readings of 230-299, where above 200 is considered ‘at risk’.

Therefore, a digital hygrometer was used, (in accordance with BS5325 installation of textile floorcoverings Annex A, and BS8203 code of practice for installation of resilient floorcoverings Annex B) to accurately measure subfloor relative humidity levels, which were found to be above the 75% maximum threshold for installing floorcoverings.

Apart from attacking adhesives and causing floorcoverings to de-bond, excess subfloor moisture can also promote the production of ettringite. Given the presence of multiple different cement-based levelling compounds which had been applied to the calcium sulphate screed and the high levels of moisture within the screed, the presence of ettringite was a potential contributing factor to the failure.

If not removed, both laitance and contaminants, such as paint, grease, and oil, can cause subsequently applied levelling compounds to de-bond from the substrate, potentially causing floor failure. Therefore, it was advised the laitance was mechanically removed, along with other contaminants and cement-based levelling compounds, before proceeding with the flooring installation.

Calcium sulphate screeds gain their strength through moisture loss. Therefore, they need to be allowed to dry naturally to the point where a test with a digital hygrometer confirms subfloor relative humidity levels are below 75%.

To accelerate this process, contractors can ensure areas are adequately ventilated, by opening windows, using a dehumidifier, or turning on underfloor heating systems, if installed. However, it’s important underfloor heating (UFH) is off for at least 48 hours before and after, as well as during, an installation.

Once the screed was sufficiently dry, the surface was primed using two coats of F Ball’s Stopgap P121 primer. Priming a calcium sulphate screed is essential for the overall appearance and performance of the floor.

It helps promote the adhesion of the subsequently levelling compound to the screed and prevents the unacceptably rapid drying of the levelling compound. Stopgap P121 is specially formulated to promote the application characteristics of compatible levelling compounds when applied over calcium sulphate screeds.

Final steps
Finally, it was recommended Stopgap 1100 Gypsum, F Ball’s calcium sulphate-based levelling compound, was applied to create a smooth base for floorcoverings. The application of a calcium sulphate-based levelling compound, rather than a cement-based one, eliminates the possibility of ettringite forming and causing floor failure.
Once the levelling compound had cured, contractors could proceed with installing floorcoverings using a compatible flooring adhesive.