Closing in the shed under the stairs

by Janice
(USA)

My two-storey house is built on a cement slab and the shed is built at the back of the enclosed carport. It is attached to the house and is under the upstairs deck.

The problem is that the original builder stopped the shed wall where the top of the stairs started and just plywooded in a big open box under the stairs. The walls have rotted from the water gathering under the stairs and have now been gutted out.

We are left with a good cement base under the stairs and no rotting in the main support beams at the top of the stairs or under the deck. My question is: How do we build a "roof" or slanted wall under the stairs so that it will drain off to the cement at the base of the last three stairs?

This base is about 4 inches down from where the house and shed wall stop (the stairs carry on about four feet past the end of the house and shed---it is an extra cement square and will drain nicely onto the lawn).

Do we slope a piece of plywood, cover it with roofing paper, shingle it and put flashing down the sides?

We thought of building a cinder block row where the raised cement pad meets the lower cement pad--allowing the water to run down and fall on the cement pad that is under the last few stairs.

Or do we cover that piece of plywood with some of the heavy duty decking material that we have left over and then caulk it into some flashing that is attached to the house and the shed wall?

Or is there a better and more waterproof way?

We like the idea of this extra space under the stairs but really want to make sure we never have this kind of water damage again!