This month, Simon Jenkins gives advice on preparing subfloors in the basement of a property constructed prior to 1960.
We were recently called to advise on subfloor preparation prior to the installation of floorcoverings in the basement of a property in an old building in Knightsbridge, London, where there were a multitude of factors to take into consideration, including different subfloor types in varying conditions.
Initial inspection
The total area of the subfloor was approximately 250m2, comprising a mix of ceramic tiles and new and old sections of sand/cement screed.
The ceramic tiles in the corridor of the property appeared to be well bonded to the base. The subfloor in the kitchen was a sand/cement screed with infills where services had been installed; the feathered edges of which were found to be weak.
Standing water, caused by ongoing plumbing works, was observed in the stores and smaller rooms, where old adhesive residues and the remains of previously applied levelling compounds were also present. There were also several manholes and cracks in the sand/cement screed. Under the staircase was an exposed brickwork floor.
Moisture Test
A moisture test conducted using a digital hygrometer gave a reading of 93% relative humidity (RH). In accordance with British Standards BS8203 (Installation of resilient floorcoverings) Annex B and BS5325 (Installation of textile floor coverings) Annex A (Dampness Testing), floorcoverings should not be installed if subfloor RH levels exceed 75% without the application of a moisture management solution.
It was evident that the central core of the building was not weathertight and subject to rain ingress. The age of the building and high levels of subfloor moisture meantthat it was reasonable to conclude that the base is not protected by a structural damp-proof course.
Recommendations
Based on our assessment, we recommended that contractors undertook the following steps once all areas were made weathertight and plumbing leaks were rectified, providing any cracks are static.
It would first be necessary to mechanically prepare the base to remove all adhesive residues, old levelling compounds, weak sections of screed and any other contaminants to leave a suitably sound and smooth surface for the receipt of subfloor preparation products. The ceramics and brickwork must also remain sound and well bonded to the base and a micro textured finish attained by mechanical means. All areas should then be vacuumed to provide a dust free environment throughout. Any static cracks/joints and divots could then be repaired using Stopgap 460 Exterior Repair mortar, a fast-setting repair mortar that can be used to fill non-structural cracks between 3-30mm. It is specifically designed for use in damp conditions and has excellent freeze/thaw resistance when used in external environments.
If the subfloor is still rough or uneven after the initial subfloor preparation, contractors may wish to apply a layer of F. Ball’s Stopgap 1200 Pro levelling compound, not less than 3mm in thickness, to improve the coverage rates of a subsequently applied waterproof surface membrane. Stopgap 1200 Pro is a fast-setting, fast-drying levelling levelling compound that is suitable for use over old adhesive residues without priming beforehand. It is also moisture tolerant, meaning it can be used under waterproof surface membranes. Where old adhesive residues are not present, it will first be necessary to prime areas with Stopgap P131 primer diluted with seven parts water to one part primer.
Final steps
Contractors may then proceed to apply F. Ball’s Stopgap F77 two-component waterproof surface membrane using a 1.5mm x 5mm trowel before using a roller that has been pre-coated with the product to obtain a continuous barrier to impede the passage of subfloor moisture. The product will isolate excess subfloor moisture where RH values are up to 98%.
A final skim of Stopgap 1200 Pro can then be applied directly to the cured membrane providing this is carried out within 24 hours of the membrane curing. If this time is exceeded, the surface will need to be primed with neat Stopgap P141, F. Ball’s primer for non-absorbent surfaces, including waterproof surface membranes, and allowed to dry prior to the application of the levelling compound.
Floorcovering installation
Once the levelling compound has cured, contractors can proceed with the installation of floorcoverings using compatible flooring adhesives. To check the compatibility of F. Ball adhesives and chosen floorcoverings, flooring professionals can consult F. Ball’s Recommended Adhesives Guide or the floorcovering manufacturer’s guidelines.
Simon Jenkins is southern area sales manager.
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.