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Feeling Groovy

Peter Kaczmar on ‘stress relieving grooves’

OCCASIONALLY I like to dust off and revive old hobbyhorses. They serve, I hope, to draw attention to issues of concern or areas of ambiguity which remain unclear seemingly ad infinitum and which continue to frustrate and bemuse in the absence of a definitive explanation.

One such issue is the practice of machining so- called ‘stress-relieving’ grooves – those channels machined into the backs of solid woodstrip flooring.

These grooves can take the form of shallow recesses about 1mm in depth, ranging from a few millimetres to greater than 1cm in width. Alternatively, they can be little more than saw cuts a few millimetres deep, or V-profiled slots cut into the back of the board. Most commonly they occur either as channels or slots throughout, but can also occur in combination.

The apparent explanation, we are told, for incurring extra handling operations and labour costs to produce these grooves, is to relieve stresses which are built up in the wood as a consequence of moisture uptake or loss from the floor during service. However, the manner in which they do so does not always stand up to technical scrutiny.

Take the function of stress relief. In the past, deep grooves were machined into the backs of floorboards to minimise cupping. The theory was that stresses, resulting from the combined effects of drying from the upper surface and moisture pickup from the subfloor, would be dissipated and cupping minimised.

However, in order for the grooves to work in this way, they would need to be deeper than at least half the thickness of the board to be effective. This as we know is seldom the case.

The conundrum here is that the practice of machining deep grooves into the backs of boards would make them more vulnerable to other types of distortion such as twist, because the effective depth of the board would be more than halved, whereas its width would remain unchanged.

Essentially, this would increase its aspect ratio and would lead to an unstable floor covering susceptible to a variety of dimensional movements. Put simply, as the depth of the groove increases, the board behaves as if it were progressively thinner meaning that it will react more readily to changes in the environment. Machining grooves into floorboards can therefore create contradictory dynamic effects.

Contemporary flooring can typically carry stress- relief channels of one millimetre depth in boards 20mm or more in thickness. In these cases, it is extremely difficult to see how such features can fulfil a stress-relieving role. The fact that engineered flooring less commonly possesses ‘stress-relieving’ channels would support the notion that grooves in solid wood floors are there to improve dimensional stability.

Equally, one can also provide examples of solid wood flooring without stress-relieving features used successfully, suggesting that in today’s industry they may not be needed.

Indeed, one can make a valid technical case that current flooring practices have obviated the need for stress-relieving details; floor seals provide an effective barrier against moisture ingress and subfloors are isolated either through the use of thick gauge polythene or a damp proof membrane However, commercial logic suggests that if manufactures did not feel these features played an important role in the service performance of the floor they would not waste time and money in machining them.

Could there be other drivers at work? The problem here is finding an authoritative and reliable source of information. Most manufacturers will have differing views on the purpose of the grooves and the function(s) that they fulfil.

It is possible to develop alternative arguments suggesting that the grooves, particularly the wide shallow channels, may play a useful role in stick-down flooring systems. In these cases the adhesive is applied to
the subfloor in the form of transverse beads.

Inserting woodstrip with channels into the adhesive may help the movement of adhesive along the woodstrip and effect a better bond by increasing the contact surface of adhesive.

One can equally make a case that by interposing a layer of air between the woodstrip and subfloor sound transmission may be reduced, or for that matter thermal insulation properties may be enhanced.

The truth is, that whilst all sorts of claims may be made concerning the function of these grooves, there appears to be no clearly defined functional stratagem which requires their presence.

In the absence of any compelling technical rationale the presence of these features may simply boil down to a commercial predisposition of added value. Addition of grooves and channels may be more to do with the consumer’s perception of quality in the form of a fully machined product than with any technical necessity.
The flooring market provides us with a variety of different floor systems and profiles.

Perhaps it is time for a little more transparency to establish what the motives and rationale for the inclusion of par ticular profile features are and in par ticular whether they are based on tradition, hearsay or, dare I say it, actual research.

If anyone out there can offer a robust rationale please relieve my frustration.

Peter Kaczmar is the flooring expert at TRADA

T: 01494 569601

This article has been reproduced from the Contract Flooring Journal website. You can find them at