exterior grade plywood doesn't like rain

Sounds to me like you didn't get what you asked for. I've had some bits of WBP 20mm ply outside, uncovered, leaving in a wall with bottom edge directly on the soil and they are still in good condition after a year or two. Not perfect but not falling apart...

The message from ernie mendoza contains these words:

Only if you're very lucky.

Sounds like your "exterior grade" was not WBP. IME WBP will last for years exposed to the elements without delaminating.

I recall a Groundforce show where Mr Titch left a sheet of alledged WBP out in the rain overnight and it delaminated.

What does WBP stand for anyway?

Wickes describe it as: 'Ideal for exterior joinery'.

Water and Boil Proof I thought - God knows why anyone would want the boil proof feature though.

Nick

is it to tell you cannot steam and bend it?

According to Wikipedia; -

Plywood meant for indoor use generally uses the less expensive phenol formaldehyde glue (which has limited water resistance), while outdoor and marine grade plywood are designed to withstand rot and use a water resistant phenol-resorcinol glue to prevent delamination and retain strength in high humidity.

But;-

Uses: -Sheds and cladding are made from this material.

Weather boiled proof plywood requires paint or varnish to protect the outer veneer from the elements.

So; one may use WBP for exterior work ...but the outer 'plies' aren't weather proof!

Good info Brian, thanks - but why would anyone want to boil plywood?

The message from "Nick" contains these words:

Wash, not water, I believe.

It takes quite a time to tenderise it, it's not good roasted or grilled.

I would take a large pinch of salt with anything coming from wikipedia. Best to verify their "facts" from a makers or similar site...

And thats it.

It will stand a shower and boiling water poured on it, but not continuous soaking.

You need marine ply for that..

I presume it is talking of the possibility of it being deformed or delaminated by blisters caused by boiling of trapped moisture or thermoplastic glues. So if you leave a damp bit sat in the sun it should not be damaged.

I thought that the only thing extra marine gave you was a guarentee of no voids in the board?

I guess you didn't / couldn't / wouldn't recognise the reference to a _second_ source?

My understanding, which may be completely wrong, is that when left out in the sun, some particles of water within the board will boil in local pockets, causing delamination from the expanding gas.

Christian.

And a guaranteed big void in your wallet...

Though for the OPs application shuttering ply might be best.

cheers, Pete.

The message from "Christian McArdle" contains these words:

Sounds very unlikely. More likely is that water penetrates in some areas only, and the grain swells only in those places. Since the rest of the board can't move the grain pops up in those characteristic long blisters, which would fit in with it being the wood swelling as the blisters align across the direction of expansion.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember ernie mendoza saying something like:

Seems to be pot luck these days. Much supposed WBP is crap or simply mis-sold non-WPB, whereas a sheet of WBP I used as an exterior workshop door extension some 8 years ago is perfectly fine in all weathers and has shown no signs of de-laminating at all without a scrap of paint on it. Only thing I did was seal the top edge.