My neighbor has a slate roof. It costs a lot more, but they tell me it is supposed to last forever. Why don't more people install slate roofs?

Slate roofs do last much longer than asphalt shingle roofs, though not forever. Depending on the quality of the slate, the roof may last 50 years or more. Slate does cost a lot more initially, and because it is brittle, may require more repairs over time. There are very few people skilled in slate repair work, so it can be a problem to get repairs done. Among those that opt for slate, most do so because it is perceived to be a higher-end product. Among new homes, you're only likely to see it on the most expensive. If you already have slate, you're usually better off repairing it rather than replacing it.