Roofing over porch enclosed area?

I have had a porch yjat is about 8 years. We have a growing family and feel a quick way to gain 240 sq feet of livable space would be to simply enclose he area under the porch. This area is located on a slab & I don't forsee any prolems there. My major consern is what to do to the roof. How to keep the water from entering the room and causing water damage. Do we have to rip up the existing decking an place exterior plyscore. Does it have to be pitched? Do we then hace to redeck over the plyscore? It is improtant that we maintain the deck upstairs as it looks out on a beautiful lake.

 

It depends upon how much heighth there is from under the deck to the slab. I would guess that you need a minimum of 10', inn order not to have to remove the deck.
You may be able to build a room under it with a pitched roof of corrugated plastic or aluminum sheet roofing (that you won't have to maintain) by building it and lifting it into place. You will have to remove deck boards next to the side of the house in order to flash the roofing properly.
I would measure the clearance, and call the local Building Inspector to see what you can do?, permit?, etc.?
Good Luck!