I am considering to build a patio with a roof. I am debating whether to use
16" squared patio stones, or using trated 1.25"x6" deck boards on
top of 2"x4" treated studs, flat layed. That will leave 1.5" space under
the deck board.

Eventhough I am going to put 4" of gravel underneath, I may still have some
drainage issue. Will mold grow under the deck board? If it does, will it
grow
to the otherside (deck surface)?

It will be a small patio/deck (16'x16'), covered with those pieces of clear
plastic roofs, outside the doors of the walkout basement.

Thanks. Y.