01
The field formula
Use exposure = (slate length − headlap) ÷ 2. An 18-inch slate with 3-inch headlap has a nominal field exposure of 7.5 inches. This establishes the repeating course spacing for the main roof field.
02
Why the answer is not the entire layout
Eaves, top courses, changes in slate length, roof breaks and decorative courses may require separate planning. The installer should calculate the available run and distribute any necessary adjustment without reducing required lap.
03
Confirm the governing requirements
Roof pitch, climate, applicable standards and the specified slate system can affect required headlap or minimum slope. Never increase exposure simply to save material without confirming the resulting roof remains compliant and water-shedding.