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Use OSB when marine ply is too dear

NOW and then, I get phone-calls directly triggered by one of my technical articles in CFJ. Recently I had an enquiry about a suitable wood-based board for use under ceramic shower trays.

The question arose because a company had won a contract to supply and fit showers for a house-builder; and they had been told that they had to fit them onto moisture-resistant chipboard flooring as a base.

However, they usually fitted them onto ‘marine plywood’ (as they told me). And apparently, their agreed contract price didn’t run to the use of marine plywood. Hence the house-builder’s decision to use MR chipboard (or, more correctly, Type P5 Particleboard).

I immediately knew that the enquirer had very little knowledge of wood products (though they presumably knew a lot about showers), because he asked me if the green colour in the chipboard meant that it had been treated with preservative. I presumed he meant Tanalith, which indeed gives wood a greenish colour).

However, I soon disabused him of that idea; I explained that P5 chipboard is simply given a green dye during the manufacturing process, which then shows it to be ‘moisture resistant’. That helps it to stand out from ‘normal’ chipboard, which is pale straw-coloured, of course.
I also explained that P5 chipboard is only – as the name indicates – moisture resistant – and not fully waterproof.

So, although it can resist spillages and minor plumbing leaks for a time – so long as they are fairly quickly detected and remedied – it cannot resist long-term wetting without losing its integrity and hence its strength. That is the reason why many plumbers and installers prefer to use ‘marine plywood’ underneath baths and showers.
But there is a problem with that ‘solution’...

Genuine marine plywood is quite hard to find – and it generally costs considerably more than builders would like to pay for it.

Of course, there is a lot of so-called ‘marine plywood’ out there, but almost none of it conforms to BS 1088 – which is the British Standard for ‘proper’ marine plywood.

And the risk is, if you use something described as ‘marine ply’ when it isn’t, you will end up with similar problems to having used P5 chipboard, if there is a serious leak: in other words, your ‘marine ply’ will simply come unstuck – and then so will you!

So I suggested to my enquirer (as I now suggest to you, Dear Reader) to think of using OSB, which is ‘Oriented Strand Board’.

OSB is a highly useful material in the sort of situation as described here. It is made (as the name suggests) from narrow strips or strands of veneer, which are lined up (ie, ‘oriented’) in three distinct layers, laid more or less at right-angles to one another – in a manner very similar to 3-ply plywood.

This gives it very good strength and hence the ability to act as a structural flooring board. But the really good part about it is that it is glued with a waterproof adhesive, exactly the sort of glue that is used in ‘proper’ Marine Plywood.

Therefore, OSB is a much better proposition than using P5 chipboard.
And it is much, much cheaper than genuine marine plywood.

So OSB is, in my view, a better and more affordable alternative to chipboard since it is much better at resisting long-term moisture problems than P5 chipboard would be.

Then why not use it all the time? You might well ask! Well, it has one drawback; it tends to have a very uneven surface and so it will ‘telegraph’ through sensitive floor finishes such as vinyls.
But used underneath a shower tray, OSB’s ‘rougher’ texture is not a problem at all, especially when ceramic trays are usually set onto a mortar bed anyway, which soon smooths out any unevenness.

So, if your flooring budget won’t run to Marine Plywood and you are worried about future moisture problems, then give OSB a try!

Jim Coulson FIMMM FFB is director of TFT Woodexperts n T: 01765 601010