Roofing slate is a dense, durable, naturally occurring material that is essentially nonabsorbent. Two properties of slate are cleavage and fracture. It has natural cleavage, which permits it to be easily split in one direction. Fracture, usually occurring at right angles to the cleavage, is called the grain. Roofing slate commonly is split so the length of the slate runs in the direction of the grain. The surface texture of slate after being split for commercial use derives from the characteristics of the rock from which it was quarried. Some slate splits to a smooth, practically even surface, while other yields a surface that is rough and uneven.
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.