Been working on this script for quite awhile and am happy to finally release it. For the moment the script focuses on barrel tile roofs only, however future updates will include other types of tile, metal roofs, and shingles. It works by creating the materials and displacement maps and then applies proper UV coords to your existing roof geometry. After the script has run you can interactively change the tile color, contrast, and tile size as well. There are a couple video demos on Youtube.
The script can be accessed here (you will need to register to download it):
And the videos are here (you may need to turn up the volume):