01
Convert using the actual layout
A roofing square represents 100 square feet of coverage. The number of slate pieces required depends on the slate width and the exposure in the specified layout. For conventional uniform double-lap field slate, an estimating relationship is 14,400 divided by width times exposure, with both dimensions in inches. This does not include cutting losses, starters, or special details.
02
Compare equal scopes
For example, 16-by-10-inch field slate at a specified 6.5-inch exposure gives about 222 pieces per square before allowances. Multiply the per-piece price by the appropriate field count, then compare freight, sorting, accessories, and quantity allowances separately. Do not use this conventional calculation for proprietary interlayment systems without their coverage instructions.