01
Establish course exposure
With 3-inch headlap, the nominal field exposure is 7.5 inches. Different approved headlap changes that figure, so the proposal should state both measurements.
02
Size fasteners for thickness
Length follows slate thickness, hole position, deck and required embedment—not the face dimensions alone. Thick or textured 18×12 material may need different fasteners than thin calibrated slate.
03
Order for layout and breakage
Theoretical field count excludes starters, top courses, cuts, sorting and repair stock. Complex roofs and blended colors need larger, deliberate allowances.