I would like to know what is a good long term protection (20yr+) for the underside of plywood flooring sheets 19mm - 2400x1200mm (NZ made)
The application is a 6x6m demountable building. Initially it will be close to the ground 125mm, then 250~300mm but ultimately will be on steel stumps.maybe 700mm.
Location is coastal qld,
Ground moisture & termite eating are the main concern.

Can ply boards be chemical treated 1 side only

The top side I will probably oil, maybe linseed as the surface has some nice patterns for a wood floor.

Thanks