Using a real-life example of advice provided by F Ball’s technical service team, Andy elaborates on the increasingly common problem of lignite.

WHEN undertaking any flooring installation, it is important to ensure the subfloor is free of contaminants that may interfere with the adhesion of subsequently applied subfloor preparation products and floorcoverings. While mechanical methods, such as planing or grinding, are effective at removing most surface contaminants, other types of contamination may require additional preparations.

This is the case with lignite, a soft, low grade of coal, which is naturally present in some stone aggregates. Although historically it has been rare to find, it is becoming increasingly widespread because of the prevalence of pump-applied screeds and its relative buoyancy.

The main problem with lignite is that it is sensitive to moisture – particles absorb water and expand and protrude from the subfloor and subsequently applied smoothing compounds, affecting the appearance of a finished installation. When it’s present, it needs to be dealt with, either by removal or encapsulation.

However, the particle size and varying extensiveness where present mean it cannot always be mechanically removed, and you can never be sure of the likelihood of success before beginning.

Real-life situation
On a recent site visit to advise on subfloor preparation prior to the installation of luxury vinyl tiles (LVTs) in the downstairs areas of two domestic properties, one of F Ball’s technical service representatives noted that the sand/cement screed was contaminated with lignite. There were also several cracks in the screeds.

A routine moisture test also indicated that subfloor relative humidity (RH) levels in the subfloors of both of the properties were below the limit for installing resilient floorcoverings without a moisture management solution in place, with one giving a reading of 61% and the other 62%. BS 8203, Code of practice for installation of resilient floor coverings, stipulates subfloor moisture levels must be lower than 75% RH in these situations.

As the subfloor RH level was sufficiently dry, we could recommend a waterproof surface membrane be applied to isolate the lignite from the moisture in any subsequently applied floor smoothing compounds.

Initial steps
Prior to the application of a waterproof surface membrane, the contractor was advised to mechanically remove the majority of lignite and other contaminants from the screeds before vacuuming to leave a clean, dust-free, micro-textured surface.

Cracks in the concrete would need to be raked out and vacuumed to remove any dust and debris before being repaired with a suitable repair compound, such as F Ball’s Stopgap 400 Repair, providing that they were static, or using Stopgap F77 waterproof surface membrane filled with sand.

A single coat of Stopgap F77 waterproof surface membrane could then be applied over the whole area to create a barrier between the lignite and moisture in subsequently applied subfloor preparation products. It should be applied using a 1.5mmx5mm v-notched trowel, before rolling with a pre-coated roller to achieve a continuous pinhole-free finish.

Once the waterproof surface membrane had cured, it was recommended that the surface was primed using F Ball’s Styccobond P141 primer. It is specially designed to promote the application characteristics of smoothing compounds when applied to non-absorbent surfaces, including waterproof surface membranes.

When the primer was dry, contractors could apply a minimum 3mm thickness of Stopgap 1200 smoothing compound and allow it to dry to create a perfectly smooth base for floorcoverings.

Adhesive recommendation
Finally, we advised that LVTs were installed according to the manufacturer’s instructions using F Ball’s Styccobond F49 Hybrid PS temperature tolerant pressure sensitive adhesive, providing this is listed as compatible in our Recommended Adhesives Guide (RAG).

The use of a high temperature grade adhesive was specified because of the potential for large patio doors in the kitchens to cause solar gain. High temperatures or extreme temperature fluctuations as a result of solar gain can cause floorcoverings to expand and contract significantly, which may lead to unsightly tenting in floorcoverings and gapping at the edges of LVTs over time.

Styccobond F49 is a solvent-free, water-based adhesive that develops the ultra-high bond strength to hold floorcoverings firmly in place when exposed to temperature fluctuations from -20deg C up to +60deg C. Its pressure sensitive characteristics would also help with the positioning and alignment of LVTs.

Applying technical knowledge meant that we were able to provide a time-saving alternative to mechanical preparation.
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Andy Mees is technical service officer at F Ball and Co