Building on recent technical advice articles that use real-life examples, Andrew Mees offers guidance on installing floorcoverings over calcium sulphate screeds
Continuing the format of recent technical advice articles drawing on real-life situations as examples of best practice in flooring installation, here I’ll provide guidance on installing floorcoverings over calcium sulphate screeds.
Calcium sulphate screeds have become an increasingly common choice of subfloor to install in newbuilds, 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 smoothing compounds over calcium sulphate screeds can result in a chemical reaction that causes the formation of ettringite, a crystalline material that can cause the smoothing compound to debond from the substrate.
F Ball’s technical service department was recently called for advice on a flooring installation in 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 smoothing 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 smoothing 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 was not 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 floor coverings 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 smoothing 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.
Recommendations
If not removed, both laitance and contaminants, such as paint, grease and oil, can cause subsequently applied smoothing compounds to de-bond from the substrate, potentially causing floor failure. Therefore, it was advised that the laitance was mechanically removed, along with other contaminants and cement-based smoothing 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 that subfloor relative humidity levels are below 75%.
To accelerate this process, contractors can ensure that areas are adequately ventilated, by opening windows, using a dehumidifier or turning on underfloor heating systems, if installed. However, it’s important underfloor heating 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, a 50/50 dilution with water followed by a neat coat. Priming a calcium sulphate screed is essential for the overall appearance and performance of the floor.
It helps promote the adhesion of the subsequent smoothing compound to the screed and prevents the unacceptably rapid drying of the smoothing compound. Stopgap P121 is specially formulated to promote the application characteristics of compatible smoothing compounds when applied over calcium sulphate screeds.
Final steps
Finally, it was recommended that Stopgap 1100, F Ball’s calcium sulphate-based smoothing compound, was applied to create a smooth base for floorcoverings. The application of a calcium sulphate-based smoothing compound, rather than a cement-based one, eliminates the possibility of ettringite forming and causing floor failure.
Once the smoothing compound had cured, contractors could proceed with installing floorcoverings using a compatible flooring adhesive.
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Andrew Mees is technical service officer at F Ball and Co