I’m in the process of putting a new deck on the back of my house. I’ve going to use rubber pool liner to make the deck waterproof, creating flow channels in the joist bays that run to a gutter on the support beam. That’s pretty straightforward. 

What I’m trying to figure out is how to do it with the stairs. I want to make the area under them dry for storage. I plan on flashing the inside stringer to the house, and creating some sort of channels between the stringers that run down to a french drain. Anyone ever done this or have ideas on a good design? I’d really like the stair structure to be fully covered, as we get a lot of rain (80 inches a year) on the Oregon coast. I don’t want to run a plastic roof panel on the underside of the stair assembly, but mimic the over-the-joist channel method.

Also: Over the T-1-11 in the picture, I’m putting two layers of #30 felt, then cedar breather 10mm thick mesh, and cedar shingles. So lapping siding to the flashing is not an issue.