For the roof I used a piece of 0.030" styrene. I made it overlap the exterior dimensions of the building by about 6 scale inches. Here I am using weights to put pressure on the styrene sheet being glued to the building. I used 5-minute epoxy to do that, for strength, and also to fill any gaps there might be between the roof and building.
I decided to put shingles on the roof. The prototype photos don't show the roof clearly, so I am guessing here. I made mine out of some black stiff paper, the kind you get with structure kits to put in as diagonal dividers to prevent people from seeing clear through the building's windows. I made them 1' x 2' in size.
The paper is a bit too thick, but I am just going after the overall effect here. I glued individual pieces down in the first row, and then overlapped the second row on top of it. Much like you would do in the real world.
I had a hard time getting a good photo of the building with all the shingles in place. The shingles look red, but the paper is a very dark gray.