On Thursday, March 22, 2018 at 10:16:16 AM UTC-5, -MIKE- wrote:

That would be a real moisture trap, wouldn't it.
I think I would go for either a composite or at least rated for ground
contact.

I can't tell you how many decks there are around here that have rotted
joists where the decking boards sit against them and trap the water
run-off.
I've seen joists rotted out, half of their depth, in under 15 years.

Unless it is a pier sitting on a footing or poured in concrete, I don't so anything that has direct ground contact. Like you, I have found that ground contact causes the "treated" or "rated" wood to rot away quickly, but more importantly, become unstable. Also, if the building shifts, ground contact framing is almost impossible to repair after it conforms to the ground.

I don't know what the life expectancy is for treated wood these days, but it doesn't seem to be much more than regular untreated wood. The last fairly large deck repaired a few years ago, I painted all the substructure replacement members with a couple of coats of oil based paint before installation. Put the PT out in the customer's yard for month to stabilize, the painted, then used the wood. Much more satisfactory than my previous effort at which the treated wood shrank so much I was embarrassed and replaced it for free later on.

Don't trust pressure treated at all these days.

Robert