Roof deck question

There is two-story shed attached to the back of my house. The furnace, water heater and electrical panel reside on the ground floor of the shed. The upper floor is now used for storage.
I'd like to turn the upper floor into a screened in porch. Since the walls will now be open, I suspect that I will need to waterproof the floor, since that will now be acting as a roof over the furnace, etc.
Any suggestions for how to do this? I will leave the original roof (over the second story) intact, but I know that won't keep rain from blowing in.
The floor of the second story (where the porch will be) is nearly level with the first floor of my house. Thus, I don't have enough clearance to put a "roof" on the floor and then a deck over it, since that would require a step up from the inside -- which I'd like to avoid.
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