Need to find out how many slate per square? (A square is a 10×10 foot section) Use the calculator below. The answer depends on the length and width of the slate, and also the headlap that is used when installing. The normal headlap is 3 inches, but this may need to be increased when installing on a lower slope roof. How much does slate weigh per square? It depends on the size Roofing slate (measured at approximately 1/4 inch thick) weighs between 700 and 800 pounds per square. Smaller roofing slates actually weigh more than larger slates and come in at around 800lbs/square while larger slates (12×24) weigh about 700lbs/square.
Slate sizes can start as small as 6×12 inches and be as large as 16×24. (Some thicker slates can be much larger—and heavier!)