I built a 10x20 composite deck a few years ago. It is beautiful, but i am kicking myself in the arss for where i put it. And i went with the pvc railing system, and living on the tug with a metal roof in the winter, sure does kick the pvc railings butt. Replaced it twice. Now im taking it down every year. If you make a good vapor barrier, you wont need to worry about the mold and stuff. Half of mine is on the ground then slopes out a lil bit. No mold yet.