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Focus on floors: why smoothing compounds need a minimum thickness

In a focus on floors feature, Stephen Boulton, F. Ball’s technical service manager, explains why a floor smoothing compound is designed to work at a minimum thickness and what can typically go wrong if you try to install it too thinly.

Installation instructions for floor smoothing compounds normally include a minimum and maximum thickness at which the smoothing compound should be applied. The upper limit will ensure that the compound is able to dry out correctly and retain its integrity. It is relatively rare to come across problems caused by smoothing compounds being applied too thickly, as most contractors are conscious that the thicker the material is applied, the less coverage is achieved.

In contrast, the temptation to spread a smoothing compound too thinly is more prevalent and this can and does give rise to problems in the performance of the floor.

We’re talking millimetres
The minimum thickness advised for many smoothing compounds is only 2 or 3 millimetres (some require a minimum of 5mm).

And whilst even a millimetre less than the prescribed minimum might not seem that significant, it can cause problems. Smoothing compounds are carefully formulated to perform in a certain way at the specified thickness and display properties that will ensure a finished floor will perform up to the required standard.

Formulated to perform
These properties include:

  • specific flow characteristics that help the smoothing compound to create a smooth finish at the correct depth.
  • the ability to absorb the moisture present in a floorcovering adhesive.
  • sufficient thickness to attain the necessary strength required.

Modern day smoothing compounds are designed to flow easily with the minimum amount of trowel work to encourage the flow. If a trowel is being used to ‘drag’ the compound into position or the trowel is being used in the same way as it would be to apply adhesive (i.e. with the trowel in contact with the subfloor), it is a sign that there is not sufficient smoothing compound to work with.

What can (and does) go wrong
Without applying the recommended minimum thickness, problems can occur, particularly if the subfloor is non-absorbent or has limited absorbency. Some of these problems will be heightened if a resilient floorcovering is being installed, such as vinyl/PVC, lino and rubber.

Potential issues that could arise include:

  • Insufficient bond strength between the adhesive and the smoothing compound – this can occur if the adhesive doesn’t have a sufficient ‘body’ of smoothing compound to dry into.
  • Blistering or discolouration of the floorcovering – this can be caused by moisture from the adhesive collecting between the floorcovering and the non absorbent subfloor, especially as nowadays, many adhesives are water based.
  • Surface imperfections showing up through the floorcovering – as well as providing a sound surface for the adhesive to tie into, the smoothing compound is designed to create a perfectly smooth finish, free from blemishes and imperfections. Without sufficient coverage, the likelihood of surface imperfections or trowel lines showing through the floorcovering is hugely increased.
  • Substandard strength of the smoothing compound, especially if it is located in areas that experience high levels of foot traffic or heavy equipment. This will shorten the lifespan of the installation, which will show signs of aesthetic deterioration and/or the floorcovering becoming detached from the sub floor.

All of these problems have been seen by the F. Ball technical department a number of times over the years. All of them have been caused by not applying the advised minimum thickness of floor smoothing compound.

And don’t forget to prime…
Priming is essential for almost all flooring installations. One exception is when using products that have been developed for use without a primer, such as F. Ball’s Stopgap 1200 floor smoothing compound, which is for use over old adhesives residues. Otherwise, priming promotes good adhesion between the subfloor and the smoothing compound and, importantly, prevents the smoothing compound from drying too rapidly, which could compromise its integrity.

…or check for moisture
As with every flooring installation, the subfloor must be dry, with a relative humidity level of below 75% (or below 65% if wood floorcoverings are being installed). If it is not sufficiently dry, a waterproof surface membrane must be applied.