Wood flooring can add so much to the interior of a building. The choice of flooring type and species obviously needs careful consideration, but specifiers must also give due consideration to installation and maintenance issues.

One of the major areas to be addressed is moisture content. Whenever I am asked to give advice on appropriate moisture values for wooden flooring, it is all too easy to reach for BS 8201, the flooring code of practice, and to quote the appropriate value for the environmental conditions which best describe the job in question.

For the majority of cases this is a perfectly reasonable approach, capable of giving realistic moisture recommendations. However, it is intended as a specification system where complex issues are distilled down to basics, to make them more understandable to users. The problem is that “idiots’ guides” are seldom foolproof and that moisture specification “by numbers” can, in some instances, be over-simplistic, overlooking factors which may eventually lead to failure.

I once listed as many different variables as I could think of that were important in regulating moisture uptake and which would have a bearing on the moisture dynamics of installed wooden flooring – and there was no shortage of factors which individually or cumulatively could conspire to cause failure of one kind or another.

It goes without saying that wood flooring should be installed on a flat, level and dry sub-floor (≤5% moisture content or ≤75% relative humidity), but how many of us are as conversant with the need to consider the movement characteristics of the wood species? A beech floor will move far more (>4.5%) than an identical floor made from oak or merbau (3-4% and <3% respectively).

Linked with this is the configuration of growth rings and the laying pattern of the flooring. Tangentially cut floor elements will have greater movement and distortion characteristics than radially cut boards of the same species. Similarly, woodstrip aligned across a long run of flooring such as a corridor will move disproportionately more than an axial or diagonal arrangement, to say nothing of the relative impacts that board dimensions, profiling and expansion detailing may have on movement.

Specifiers must also give due consideration to installation and maintenance issues

The type of sub-floor and the intimacy of contact of the boards with it will also play a major role in determining the level of moisture-related movement, despite the fact that an effective vapour check may be fitted. This is mainly due to its influence on “cold-bridging” or “heat sink” effects. Contemporary floor systems are increasingly stuck to the sub-floor with an adhesive, and flooring manufacturers argue the case that the glue-line acts as a damp proof membrane (dpm), although the fact that adhesives are commonly applied with a toothed trowel or as interspersed beads would suggest that this function is only partially achieved. Similarly, the use of water-borne adhesives may also contribute to dimensional movements.

Less obvious factors such as the nature of the floor seal may also have a direct bearing on the moisture dynamics of the floor in terms of the vapour transmission characteristics of the lacquer system, be it as a consequence of its formulation characteristics or film continuity. This is an aspect which is understated in the context of interior coatings, being overshadowed by factors considered to have further reaching consequences on aspects of dimensional stability – such as the propensity of certain seals to induce the phenomenon of edge-bonding, which results in “rafting”, particularly on parquet floors.

Rafting is a term applied to cumulative shrinkage where individual floor components become stuck together by the seal and the floor behaves as a single unit or “raft”. Consequent shrinkage results in cumulative gaps opening along joints where the adhesive strength is weakest.

A common sense and holistic approach by the design team can help prevent many disasters

The issue of rafting opens up still further opportunities for failure as a consequence of inappropriate materials selection, in the sense that the floor seal should be extensible and capable of some degree of elastic movement to accommodate dimensional movements between adjacent flooring elements when in service. This has, in the past, been associated with water-borne lacquers, although it is just as likely to be a combined consequence of the viscos-ity and adhesive characteristics of the seal and its apparent ability to percolate between adjacent flooring members.

A common sense and holistic approach by the design team can help prevent many disasters. Simply, don’t install new floors at a moisture content inappropriate to the intended in-service conditions without any attempt at pre-conditioning, or while wet trades are finishing off, or indeed during periods of cold, wet weather with no interior heating.

These are just some of the factors which can disrupt the dynamic equilibrium of a flooring system and ensnare the unsuspecting into a web of remedial action. The question one instinctively asks against such a backdrop is why are there not more instances of observed failure?

The answer lies in the fact that the majority of failures are likely to be the consequence of a combination of faults. Increasingly, the specifier must be able to look beyond the basic guidance offered by existing Standards and to appoint a contractor with a proven track record.

Wood flooring has many advantages, but it requires a far more technical and skilled approach than is often adopted.