Slate Roofing Calculator – Slate per Square / Slate Weight

Need to find out how many slate per square?  (A square is the equivalent of 100 square feet) 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.  When calculating how much slate you’ll need for a project, don’t forget to add additional slates for waste!

How much does slate weigh per square?  It depends on the size of the slate.  On average, slate (measured at approximately 1/4 inch thick) weighs between 700 and 800 pounds per square.

Per square, smaller roofing slates actually weigh more than larger slates and come in at around 800lbs/square, while larger slates (12×24 inches) 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!)