01
Use actual exposure—not slate length alone
Course count depends on the designed exposure after headlap is considered. Pieces per course depend on average slate width and side-joint layout. A nominal size does not automatically equal a fixed installed count on every roof.
02
Measure every roof plane
Calculate true roof area and account for dormers, towers, hips, valleys, sidewalls and penetrations. Steepness does not change the surface area once measured on the roof plane, but complexity increases cuts and handling.
03
Add project-specific allowances
Starter and top courses, thickness sorting, diagonal cuts, breakage, color blending and replacement stock all affect ordering. Historic matching or a product with long lead times may justify retaining a clearly labeled attic stock after completion.