Run a string up the edges of the roof to give yourself a straight edge to follow when laying the slate tie the string to temporary nails or chalk lines up the roof edge to align the inside edge of the slate in order to leave a one inch overhang on the outside.
How to set out a slate roof.
Alternatively you could use a 12 inch high starter slate thereby guaranteeing a 3 5 inch headlap on the starter course and an 8 5 inch exposure on all field slates.
In order to maintain your investment and make sure it lasts a slate roof will need regular maintenance.
Although a slate roof is initially quite costly it will often last for a lifetime and beyond.
The exposure on the first course of slate must instead be dropped to 7 inches in order to create the 3 inch headlap on the starter course.