The color of slate is determined by its chemical and mineral composition. Because these factors differ in various regions, roofing slate can be obtained in a variety of colors. In addition, exposure to weather causes slate to change color. The degree of change varies depending on the slate. Slate exhibiting minimal color change is known as “permanent” or “unfading” slate. Slate that shows a more marked color change is known as “weathering” slate. Between unfading slate and weathering is “semi weathering” slate.
There are several classifications for slate roof systems. The first is standard slate, which refers to slate that generally is from 3/16 inch (5 mm) to 1/4 inch (6 mm) thick with uniform length. The category “standard smooth” refers to standard slate that has a relatively smooth surface, in comparison with “standard rough” or just “rough”. Rough slate has a rougher texture and generally is available in thicker pieces. Finally, there is “graduated/textural slate,” which is designed with varying lengths and thicknesses and generally is rougher than standard slates.