I was playing around with Ascension's building shape brushes combined with various tiling textures. Adjusted the texture scale and bevel settings to come up with this. The image is very rudimentary right now for testing purposes. So what do you think? I found a decent array of styles on line that I can post if you think this method looks effective.

Click image for larger version. 

Name:	roof shingle test.jpg 
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Size:	151.3 KB 
ID:	29602