Slate roofs are one of the most durable, beautiful and expensive of roofs. Even the maintenance of the roof is expensive.
It is very risky to walk on a slate roof, you may crack some slates, and repairing one in the middle of the field is a real job -- I know I grew up in a house with real slate on the roof.
How about a new product that looks exactly like slate from the ground, but is flexible. Ok, it won't last as long as slate, only guaranteed for 50 years, but then again it doesn't cost nearly as much as real slate. And maintenance is as easy or easier than with asphalt shingles. It is made out of polymers, the same stuff they use to make sports car bodies. One more very good alternative for your roof.