What is the best kind of roof for keeping your house cool in hot weather?

Clay and concrete tiles are great for hot weather because they can circulate air under the tiles but they are not optimal for colder climates.

Metal roofing is a great choice to help regulate your home's temperature. They reflect sunlight in warm weather & absorb UV heat in the winter to help melt snow.

Sod -- but the house must be constructed with that in mind, as a sod roof with living grass and irrigation system is quite heavy.

There is a great article that gives you the low down on multiple roofing materials on ServiceWhale's blog. The post is called Choosing the Best Roofing Materials to Use on Your Home.