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.

Thanks,

John