[Cob] Cob over wood deck

[michael montagne]

I have a cedar deck built close to the ground.  There is ventilation under
the deck but not good ventilation.  The area will be covered but is likely
to get damp thru the course of the winter.  I'd like to put a cob bench on
it.  The base will be rock and rubble.  Should I fear moisture related
issues rotting my decking?  What are the recommendations to facilitate air
flow at the base?  Am considering some low sleepers under P.T. plywood under
waterproof membrane.  But that may be overkill.....

-mjm