DIY Doctor

12mm WBP exterior plywood - how much rain can it stand ?

Postby seanxkelly » Thu Jun 05, 2008 10:25 am

In an earlier post I mentioned my mission to affect some sort of roof repair to my garage/utility room roof.

I want to "reboard" the roof as the current one has a few small dips which collect water and if I just overlay some new felt (or something) these might continue to collect and eventually lead to new weak points.

So (part of) my plan is simply to screw some new 12mm WBP exterior plywood down on top of the old roof just to level stuff out and then apply the new felt (or whatever) on top of that.

Problem is that we're having frequent spells of bad weather here in the UK and I probably can't get the entire job done in one run of dry weather !


If I get the WBP ply down first, will that survive a few days of rain before I get the chance to apply the layers of felt (or whatever I'm using) ?


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Postby Tralfamadore » Thu Jul 17, 2008 9:27 am

I am thinking of doing the same thing (I've been thinking about this for over a year now!). I propose to use sheets of WBP ply 3050mm x 1500mm x 18mm, so that I can walk on them.

My idea is to paint two or three coats of marine primer over the edges and sides of the ply before screwing to the roof joists. By this means, the ply should be fully weather proof before I expose it to the elements. The ply will be long enough to go from the house wall (where the garage joins) to the outer wall of the garage.

I don't propose to cover the ply with anything. There will, of course, be a join between the sheets of ply. I propose to use the plastic/bitumen wide tape (looks like lead) to tape over the joins. This means that if there is a leak I should be able to locate it and repair it easily and quickly.

I personally would not cover your existing roof with ply. I would go back to the existing structure, presumably joists, repair where necessary and rebuild.

I would be grateful to hear of any thoughts about the above process.
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Postby Alph » Tue Sep 30, 2008 12:52 pm

I ran a roofing business for years. There was a trend a few years ago on big commercial projects to indeed leave the old felt on and cover with new over the top as this saves a lot of time and money. But most reputable manufacturers wont guarantee their systems unless all the old stuff is removed or completely isolated from the new felt system. The problem is trapped moisture which, if it has no where to go, will produce bubbles which will at least weaken the felt if it doesnt burst it. If the ply and first layer of felt is nailed rather than fully bonded, the moisture (or vapour) can at least even itself out and find, eventually, a way out, perhaps at the perimeters.
On sand and cement screeded roofs, the screed always takes months to fully dry out and so vapour vents are fitted, small plastic tube things, to allow the vapour to escape at predetermined regular points- you could put these through your ply but I suspect it would be too much trouble for just a small domestic job

Bear in mind even wbp ply will rot, one job I did there was a large indoor cactus in the room below the leak, which must have been a moisture source and the ply rotted from below, it was quite pretty, loads of blue green fungus.
So nail/screw the ply down, making sure the edges are fixed together and not just with sticky tape either - a batten (nogging) of some kind - there must be no differential movement or the felt will tear through as they move. Get the roof as dry as possible first.
Then nail the first layer of felt, stick the rest. It may be easier to use a sheet material like onduline or pvc if you have got at least 7½ degrees of slope
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