what's the difference between a Shingle and a Slate? my dad (being equally uninformed on the same subject) wants to install slate just because his friends said it's cool. (My dad is in Asia, not here in the States) Now I've been to Home Depot and one roofing company and they say I will have to special order them because "most people don't use slates" or "slates are more expensive compare to other alternatives." So what are slates? just a better looking Shingle?