I have a small deck, roughly 4.5' tall and 5.5' deep and I would like to convert all that unused space underneath to create some sort of storage facility.

I apologize if I'm not using the right "deck terms" :)

WATERPROOFING

I was looking at Trex Rain Escape waterproof membrane between the joists, but a) it's awfully expensive b) it would require taking off the existing deck boards and c) it's not readily available at any retailer near me.

So now I'm considering corrugated asphalt roof panel which is considerably cheaper and also seems easier to install. I did a bit of research on the installation, but I had a few questions:

1) Is this an appropriate solution? I was also looking at corrugated polycarbonate sheets (?)

2) There are 2 ledger boards at each end of the deck with a ~3" gap between them - am I limited to installing the panels within the interior of the ledgers or is there an alternative solution that I'm not thinking about?

3) The deck is not attached to the house structure, so there is no flashing in place, so what would be the best solution for waterproofing that gap?

4) What could I use to stop sideways rain and snow from blowing in through the sides? Currently there is only lattice.

FLOORING

For the floor, I was leaning towards building a raised platform using some of those same concrete blocks with 2x6 joists and then a panel of PT plywood on top, because trying to level that whole area with tamper would prove quite challenging giving the limited space.

Thanks for reading through all of this and I appreciate any help!