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Do I need to check the subfloor?

Alec Stacey explains the importance of checking subfloors before installation.

I WAS recently reminded that issues relating to damp subfloors are still frustratingly common in the wood floor industry. A recent enquiry involved a homeowner being told that following the failure (lifting) of her newly installed oak floor, a moisture test hadn’t been done beforehand because, according to the contractor, the property didn’t smell of damp. I’m a fan of indicative moisture tests, but this was a new one to me. As it’s often stated; ultimately the floorlayer is responsible to ensure the subfloor is suitable for the planned installation.

Often when a flooring contractor visits a site for the first time to install a timber floor, some work has already been carried out on the subfloor. Perhaps the site is a new extension in a home, or similar ‘new build’ scenario where a new subfloor has been constructed or a screed or levelling compound has been applied. Invariably the customer wants the flooring installed straight away and the contractor is expected to comply. Of course, the first thing to do is to take moisture readings using a box hygrometer. On a recently laid screed/levelling compound it’s possible the readings will exceed the maximum permissible level, 65% RH for most ‘stick-down’ installations. This would invariably be the point where the contractor reaches for a liquid moisture barrier to allow them to proceed. However, a degree of caution is necessary; what’s the presented subfloor comprised of? It sometimes surprises people to learn that not all levelling compounds etc are suitable for use below a moisture barrier. If moisture is trapped in a material which is moisture intolerant or indeed soluble, there’s a real risk the material will begin to break up and soften, rather than fully harden, as the moisture becomes chemically bound in the cement, such as the case of a material which can be used under a moisture barrier.

Identifying the screed/levelling compound can be difficult. Products produced by the same manufacturer may appear identical once dried; some of which can go under moisture barriers, others not. Information may be available from the customer or their builder etc, but with no additional information the subfloor should be allowed to dry naturally until suitable moisture readings are obtained. If the specific product used can be identified, a simple call to the manufacturer will give the required information.

This is one of the reasons Bona always recommends the subfloor is protected from moisture first, and then a levelling compound applied afterwards. This ensures moisture intolerance isn’t an issue.

In some situations, an anhydrite screed may have been used. At Bona, we’d always recommend this should be allowed to dry until RH readings are below 65%. This is owing to the solubility of this type of screed. Although some manufacturers recommend the use of a moisture barrier on damp anhydrite screeds, we would take the view that it’s important the material becomes as physically sound as possible. This is particularly important if timber is going to be glued to the screed which will later exert considerable stresses on the subfloor as it responds to changes in ambient humidity. If moisture is trapped in an anhydrite screed it won’t reach the same strength.

Irrespective of the type of screed/levelling compound, it’s important the surface is free from contamination and especially laitance. This is the weak layer of fine particles which reside on the surface. This can simply be removed by buffing the surface with a coarse disc or grinding plate. If this weak layer is allowed to persist and timber is glued to it, the bond strength will be seriously compromised, and the flooring may lift as soon as any stresses occur. It’s particularly important to abrade the surface of an anhydrite screed to remove the brittle surface layer. This also serves to optimise the conditions required for the drying of the screed. Following the abrasion of a subfloor, also consider the use of a suitable/compatible primer which will serve to bind any residual dust which hasn’t been removed by vacuuming.

With the launch of the Bona Flexisander for example, a buffing machine, with various ‘heads’ available, including a diamond disc for subfloor preparation. It can be employed through all stages of a floor’s installation from the subfloor prep, aggressive sanding of the timber along with finishing sanding.
01908 525150
Alec Stacey is technical manager region South and West Europe at Bona

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