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.

The boat was afloat 12 months of the year in fresh water.

Ah. That's still less demanding than the top 6" of soil, though.


Regards,
Nick Maclaren.