Not all slates are created equal. Slates are rated on 3 levels by the American Society for Testing and Materials (ASTM): S-1 has an estimated lifetime of 75-100 years, S-2 with 50-75 years, and S-3 is so poor that it's not used in roofing. What gives this Metamorphic rock longevity is long term compression. S-2 slates are usually compressed between 200 - 300 million years, and trust me that's still pretty good, it's much better than asphalt shingles. But the true Cadilla