I could probably have painted something but I had just come up with the idea to make the texture in Blender. To do that I made five tiles and gave them five slightly different materials, next I copy pasted them until I had enough tiles, threw in a black background, changed the camera off perspective to orthographic (to avoid any distortion) and this is what I ended up with:
Then it was time to hit F12 and wait for the render to come in.
It wasn't bad, but it was very clean and didn't look like it had seen any use. With that in mind I grabbed a photo of a grungy concrete slab and another of some old bricks I had taken ages ago and set them to overlay, this warped the colours a bit but a grey layer between them on about 50% opacity brought the saturation levels back over to something tasteful. I also planned to make this a generic "roof" texture that I could use with all of the buildings instead of textureing each one. Of course this meant that I couldn't have OA baked on, but there would be very little on the roof and it wouldn't be all that visible anyway due to the texture. By selecting one line of tiles and rotating by 90 degrees I had a center tile ... thing. By now the texture was looking much more realistic.
All that was left now was to throw it on the roof of my buildings which was done easily enough by placing the center of the roof with my center tile thing on the texture. It was looking good at that point, but I went ahead and made a low poly object set to the same texture that would look like the single row of tiles in the middle of the texture except raised above, that will make it look more convincing when viewed from something other than directly above.
And there's the finished roof, it'll be pretty dark in that house until some windows are put in, but it's looking good