How much rain can marine ply tolerate?

[Murph7355]

I have some marine ply boards outside that were covered by a tarp. The weather has, of course, turned sour before use and the tarp was not as waterproof as it needed to be...so they are damp.

Is it better to uncover to try and get them to dry out, with the risk of further rain? (Some due tomorrow but then shouldn't be much/any for a few days).

Or keep covered but with a better tarp?

[grumbas]

I had some builders grade ply under an awful tarp for months and it survived pretty well, so I wouldn't be overly concerned.

Biggest issue I had was surface mould from the lack of airflow.

[OutInTheShed]

They are probably ruined already, but the best you could do is store them covered on edge, with an air gap all around.
Obviously (?) the bottom edge needs to be off the ground.

Marine ply is quite variable stuff, the BS spec AFIAK is all about the glue, the wood can be ruined by a wet person looking at it.
The inner plies are often not great wood and may absorb water like a sponge.

Having said that, I've known some 'exterior' ply to be pretty good.

[PhilboSE]

Proper marine ply should be made exclusively from tropical hardwood veneers and have a waterproof glue. So if they look ok after a clean then they should be usable, but like all sheet materials they might cup if not stored flat, particularly in the presence of damp.

However, proper marine ply is also proper expensive! The next grade down, made only with hardwood exterior veneers and softwood internals (but still waterproof glue) is about half the price, and that might not survive so well. So it’s probably worth trying to determine what you’ve got.

TL;DR don’t have enough info to say if it’s ruined or not. Clean it up, dry it off and store it somewhere dry and flat. Ideally inside not outside as the damper air outside won’t do it any favours in the long term.