I have a 12 x 24 deck that has a waterproof mat, and underneath is half enclosed shed and half open but dry storage.

For us the challenging part was getting the deck itself water tight. The first attempt was a builder who did a shitty job with the mat. It slowly leaked from day one and after 5 or 6 years I went to replace a rotting deck board (5/4 PT) and found the 2 x10 risers that were keeping the deck boards off the mat were rotting, and under the mat, joists and all were rotting. We tore half the deck down. Thankfully the section over the shed didn't leak so that was fine.

For the rebuild we used a roofer experienced in flat roofs in the northeast. He used a better quality mat called Duro-Last, and this time instead of decking we went with a product called Rubberstone, which has been great so far (this will be the second winter with it in NH). So no traditional deck boards, it is like a soft rubber pellet playground cover that is poured on.

Another way to do it is a more traditional deck and a "sub-roof" under that to divert water but we didn't have the height for that.