Making plywood waterproof?

[Ruggers]

I'm looking for any suggestions and products that I can buy to paint or lay onto 8x4 ft sheets of ply wood to make it waterproof. the sheets will be used for the back outer walls of a summerhouse which is fitted in the corner of a garden and not much gap at all to maintain it so I want it to last.

one option was to fibreglass them with the same UV resistant product I use on the roof but this seems expensive. I've 6 sheets to seal.

The two sides are facing away from the elements so 9 times out of 10 won't take much weather.

 

[foxhole]

Thompsons water seal, not intended for that use but works well.

 

[Ruggers]

Is it not just for brick though and would need re done every few year

 

[foxhole]

It's intended for brickwork, used it on shed base timbers , ten years later dismantled shed and timbers were as clean and undamaged as when fitted, Other timbers not treated had rotted.

 

[AronSearle]

Are the sheets not clad with something?

Are they viewable, if not, and there is no UV exposure, why not stable a breather membrane over them.

It is important to seal the end grain of plywood against long term wetting.

 

[Ruggers]

the two faces are north East & north west facing so only getting minimal UV and the wind and rain mainly comes from the south direction here.

It's 3 m high but I have a fence and wall to each side of it so shields a lot of weather too. The ply going on does not have any cladding that's why I need to add some plastic sheeting like polycarbonate twin wall used on conservatries roofs, pond liner but quite expensive too both £120

or possibly this cheaper option? bitumen http://www.ebay.co.uk/itm/Everbuild...-5-Litre-/200704918831?_trksid=p2054897.l5659

 

[JohnD]

the edges will fail first from water penetration. How are you protecting them? Are you flopping the roofing felt down with a bargeboard and gutter or something?

The bottom needs to be spaced off the ground by 150mm to prevent watersplash during rain. It should not stand on anything or water will collect on whatever's underneath and soak in. If there is grass or other vegetation it must not touch the wood as it will hold water.

Cladding spaced off the ply on battens might do the trick, though it could be damaged by lawnmowers or something.

 

[AronSearle]

Here's the question.

Do you want to do this properly.

Or do you want to knock together a ramshackle shed that will last you 5-10 years.

If the latter, then yes, just slap on some paint or cheap plastic cladding/wrapping.

 

[Ruggers]

I want to do it properly as every part of it so far has been done to a high standard and is costing a few grand, all treated wood etc, there is no reason if I keep ontop of the wood treatment every few years it shouldn't last forever. I don't want the back sides letting me down.

The eco sheet I originally wanted to put on has been stopped and stokboard is a rip off at £350 for 6 sheets

 

[AronSearle]

Do it properly then.

Cover the plywood sheathing in breather membrane, create a drained and vented cavity with battens, and then install some rainscreen cladding (cement board is cheap and durable, if you can't see it and don't care what it looks like).

Tyring to seal the plywood with various paints or sealants is a bodge. That is not to say you can't make it work well, and last a long time, but a bodge is a bodge.