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Why You Have To Strike A Balance

Jim Coulson on maintaining a balanced approach when installing wood

IT IS a while since I wrote about the technical properties of wood-based flooring products, and a recent case pointed out to me the fact that wood is still a very misunderstood material.
Of course, wood is loved for its warmth (literally as well as figuratively) and its infinite variety of surface patterns, colours and textures, but it is also used for its technical properties as well.
Wood is unusual in that it is comprised of longitudinal fibres which run (mostly) along the length of a piece of timber and that unique structure influences many of its working properties, including movement: which I have written about before.
Wood moves only across the grain and thus has almost no change in dimension along the grain. And this characteristic is exploited in plywood, where alternate ‘plies’ (veneers) are laid at right-angles to one another, so that one ply will restrict the movement of the adjacent plies above or below it.
The reason for this is because each alternate layer only wants to move in one direction but not in the other and so they more or less ‘cancel each other out’, so to speak. And that’s fine – so long as things are kept in balance.
You may notice, the next time you look at a sheet of plywood, that it has an odd number of plies, or veneers: 3, 5, 7 or 9 being typical numbers in most plywoods used as flooring overlays.
And that means that the face veneers – front and back – will have their grain directions parallel to one another (think about it!).
That is a vitally important characteristic because it means that any potential movement in those opposing veneers will be ‘working’ in the same direction as each other and thus the panel will be more stable in use.
However, if the front and back veneers were to be at right-angles to one another, then their individual movements would be in opposition by 90 degrees; and the result would be bowing or curling up, at 45 degrees across the plane of the panel. That means that every extra veneer added to a panel on one face must be ‘balanced’ with an additional veneer on the other face.
By the same token, those front and back faces must be of an equivalent thickness to one another, because any significant differences in thickness will result in differential amounts of movement on each face – hence the need for another type of ‘balance’ in the panel’s construction.
And that is where the problem occurred, that came to my attention recently. Some very expensive wood-based flooring had been laid – quite properly, I must add – but it then gave problems of unexpected movement and also cupping. But this was downwards (ie, so that the middles of these laminated-type strips were higher than their edges). This was different from what is usually seen, where the middles tend to ‘dish’, with the edges being higher.
When I examined these strips, I found that they were made from a thick oak layer (too thick to be regarded as a ‘veneer’), glued onto a 9-ply plywood backing material. And so this product had contravened not one, but both of my ‘rules of balance’.
It used an extra layer added on the top, without a corresponding ‘balancer’ on the underside; and it also had the top layer about six times thicker than the bottom layer, so that its movement characteristics were also seriously out of balance. No wonder it moved and cupped.
So the next time you choose a flooring material for overlayment or as a finished floor surface have a good look at it and make sure everything is properly in balance: then it won’t give you anything like as much trouble!
Jim Coulson FIMMM FFB is the director of TFT Woodexperts T: 01765 601010
www.woodexperts.com

This article has been reproduced from the Contract Flooring Journal. You can find them at www.contractflooringjournal.co.uk.