In this issue, Gary Kenworthy provides an alternative to completely removing and replacing a screed with weak patches, saving considerable time and expense.
Sometimes, upon removing old floorcoverings as part of a refurbishment, flooring contractors are faced with a weak screed surface that will be unsuitable for the application of subfloor preparation products and the installation of floorcoverings. This is usually the result of poor installation or curing techniques, environmental factors or age.
In these situations, it will often be necessary to completely remove and replace the weakened screed. However, there are sometimes less costly and time-consuming solutions available to make the surface suitable to receive floorcoverings.
This was the case when we were asked for advice on subfloor preparation prior to the installation of rubber floorcoverings over two floors, a total area of approximately 300m2, of a university campus building that was constructed in 1970 and extended in 2000. It was also subject to flooding in 2020.
Site visit
Both the ground floor and first floor had concrete substrates, and the first floor also
had a small section of raised access timber flooring. A structural engineer confirmed that the concrete bases were structurally sound.
During our visit, it was noted that the base was extremely rough, and some weak
sections had already been removed, leaving about 75mm thickness to be made up. New P5 chipboard had been affixed to the wood subfloors with screws and glued at
the joins.
Moisture tests indicated subfloor relative humidity (RH) levels in the concrete subfloors were between 60% and 78%, suggesting a variable drying rate throughout the building. In accordance with BS8203 (Installation of resilient floor coverings) Annex A Dampness Testing, floorcoverings should not be installed where subfloor relative humidity levels exceed 75% unless a moisture management solution is in place. Otherwise, unmanaged excess subfloor moisture, whether residual construction moisture or rising damp, may attack floorcoverings and adhesives, potentially causing complete floor failure.
Our solution
Based on our assessment, it was possible to recommend the following course of action to negate the need to completely remove and replace the concrete screeds.
Firstly, any remaining weak spots in the base should be removed by mechanical means along with any surface contaminants to create a clean, sound micro-textured finish. Any dust and debris should also be swept away.
Contractors would then need to apply a single coat of F. Ball’s Stopgap F77 solvent-free epoxy waterproof surface membrane and use a roller pre-coated with the product to obtain a continuous, pinhole-free barrier against subfloor moisture over the entire area.
Once the waterproof surface membrane has cured, it would be required to prime the surface using F. Ball’s Stopgap P141 primer, which is designed to create a textured finish to enhance bond performance between levelling compounds and non-absorbent surfaces, such as epoxy waterproof surface membranes. It is particularly recommended where levelling compounds will be applied at a thickness greater than 20mm; the strong bond created when using Stopgap P141 is better able to withstand the increased stress that thicker depths of levelling compound exert when drying.
Raising the level
It would then be necessary to raise the level of floors where weak patches of screed had been removed. This could most quickly and easily be achieved using F. Ball’s Stopgap 600 Base deep section levelling compound. Most levelling compounds can be applied up to a maximum thickness of 20mm, whereas Stopgap 600 Base can be applied at a thickness between 5–50mm, meaning only two applications would be needed to achieve the required 75mm thickness. The product can also be pump applied, as the contractors opted to do, saving further time. Once the first layer had dried, it was primed using F. Ball’s Stopgap P131 general-purpose primer, diluted with four parts water to one part primer, before the second application.
Last steps
It was also recommended that the chipboard over the raised access floors was checked to ensure that the surface was clean, secure and undamaged before applying a minimum thickness of 0.2mm of F. Ball’s Stopgap Fill and Prime over the entire area and leaving to dry. Stopgap Fill and Prime is a flexible cement-based primer, which has a thixotropic consistency, meaning it flows freely when stirred but sets to a gel-like consistency on standing, enabling it to fill joints between panels, preventing indentions that could transfer to the finished installation when a levelling compound is applied.
The whole subfloor area could then be primed using Stopgap P131 prior to being capped with a layer of a suitable floor levelling compound to create a perfectly smooth base for the receipt of new floorcoverings.
The final phase of the refurbishment was to install Nora rubber sheet floorcoverings
using F. Ball’s
Styccobond F48 PLUS in accordance with F. Ball’s Recommended Adhesives Guide (RAG®). The adhesive develops the high bond strength required to hold floorcoverings in place when exposed to high temperatures, which are expected in heavily glazed areas of the university building. It is also fibre-reinforced to provide high initial grab to prevent floorcoverings moving when working.
Gary Kenworthy is F. Ball technical representative for the North West.
F. Ball’s regional technical representatives provide advice to flooring contractors on how to get the most out of F. Ball products, including conducting site visits and producing case-specific reports advising on the best course of action for particular flooring projects. Find out who your technical representative is here.