A percentage of semi-weathering slates, when exposed to the elements will change to brown, buff, and tan over time. The mineral and chemical composition of the slate contributes to the weathering process. As roofing slate is a natural product, the weathering can vary from quarry to quarry as well as the stone extracted from within a quarry. When ordering a “semi-weathering” slate it’s important to keep this in mind. The installer should also take the time to blend and sort from various pallets to avoid unsightly blotches of browns.