European Standard Is Only The Beginning
Jim Coulson on what makes a ‘good’ plywood
LAST month I wrote about plywood used for overlayment on old floors. I tried to dispel the myth of so-called WBP. (If you recall, that has, for a long time, been superseded by plywood which has at least the glue bond characteristics of the European Standard EN 314, Class 3 and which is therefore suitable for exterior use, or situations where the plywood may become wetted in service.
But as I warned you, there is much more to specifying (and buying) plywood than merely asking for Class 3 Exterior to EN 314 – although that is a jolly good start!
Once you’ve sorted out the correct glue bond (and please remember to ask for some form of certification – or other guarantee. This proves that the plywood really is as good as it claims to be. You will also need to consider certain other characteristics that you ought to be asking for when you next specify or buy plywood for overlayment – or any other purpose, for that matter.
The layup of the plywood is also highly important.
So what do I mean by that term, ‘layup’? It is the way the veneers are put together. It can drastically affect the overall quality of any plywood, even if the glue-bond is OK.
Problems with layup can include core gaps, overlaps, splits and irregular veneer thickness. These are, essentially, variations on how well – or how badly – the veneers are fitted together in the centre (or core) of
If the sheets of core veneer are split and therefore separate, a gap can occur. This then causes the adjacent veneers to ‘compress’ as the plywood is finally pressed. This gives a slight hollow on the plywood’s surface which then tracks through to the final floor finish. This process is called ‘telegraphing’.
By the same token, if partial sheets are used, which are not closely butted up against one another, gaps will result, which will then have the same ‘telegraphing’ effect of causing depressions and hollows on the
Overlaps are, essentially, just the opposite: Where two core sheets of veneer sit one upon the other (instead of being laid properly side-by-side). Thus they form a hump or bump, which then telegraphs through to the final surface as a thicker area. This can also show up very badly on a glossy floor finish.
Gaps and overlaps can also give rise to localised areas of poor adhesion, which may also show up as ridges or blisters. This is where the surface veneers have expanded if the plywood gets wet during use.
You can see therefore that the overall quality of your plywood is more than just the fact that it should not delaminate.
Next month I will discuss the question of the quality of the plywood’s surface appearance.
Jim Coulson is the director of TFT Woodexperts, Ripon, North Yorkshire
T: 01765 601010
This article has been reproduced from the Contract Flooring Journal website. You can find them online at www.contractflooringjournal.co.uk.