In article ,
Martin wrote:

Marine ply is very uncommon - they are likely to be either one of the
ordinary internal grade ply or WBP (water-boil-proof) which is OK for
external applications.

But not standing on damp soil! Depending on lots of things, the soil
underneath such a greenhouse might or might not be dry enough.

Outdoor quality/WBP ply lasted 15 years without delaminating when used
as bottom boards in a Wayfarer dinghy. Most of the time the boards were
wet. The ply had a 20 year guarantee.

Damp soil is actually a MUCH more demanding environment! It's the
bioactivity that causes the damage, and most of the relevant fungi
can't tolerate seawater, so the main decomposition there is chemical.


Regards,
Nick Maclaren.