Roof battens and gauge the gauge is the space between the roof battens this will set the overlap of your chosen slate in most cases.
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.
This can be carried out cheaply and it s work you can comfortably undertake yourself.