plywood deck

Have purchased building with a large plywood porch. the porch has no roof, it is covered with rolled roofing nailed down to the floor it looks awful the building is a 1906 hospital and has a large bay window and door on the building side that comes all the way to the floor, there is about a 3" slope. What can I replace the roofing/flooring with that will protect the plywood and fit under the window and look like it belongs on the building? Any help please