Before you decide how to do the roofs you should determine what type of climate this is in. Northern climes tend to have steeper roofs made of lighter material with better insulation properties than clay tiles (like thatch or wooden shingles). Climes without too much rain tend to haves roofs as flat as the perception allows because then they get a another floor for use, in these places it is more about shade than drainage. The second consideration is cost, stone shingles are going to be expensive (especially if thieves run around on your roofs breaking them all). Clay tiles are relatively easy to make but are heavy as hell and are not going to be transported very far, so the area around must have the materials necessity for their manufacturing as they are not going to be cheap when transported any significant distance. By the same token in places where it takes trees longer to grow they are not going to waste wood on building short lived roof tiles, but when trees are in abundance it makes a lot of sense.