Use map objects of buildings
Go to Dundjinni forums and find 3D building map objects that are appropriate to what you're looking for and use them.
Or, if you use 3D software, just create some roof structures, maybe some with chimneys. Create the variety you need, use photos of roofing tiles for texture materials and generate your own 3D roofs. Its fairly easy to do.
Depending on the variety you'd need, there would be no more than 10 to 20 building styles, then just load copies, rescale, rotate and place to get your volume of buildings in seeming variety.
Once you create your building roofs, of course, you can use them again and again in all future maps, as your own map objects. (That's how I do it!) 8)
I was thinking more like these...
I was thinking more like these type of 3D buildings...
Here's a couple of simple house transparent PNGs that I've created for use in some of my maps... 8)