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.
Although a slate roof is initially quite costly it will often last for a lifetime and beyond.
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.
In order to maintain your investment and make sure it lasts a slate roof will need regular maintenance.
What roof pitch do i need for what slate size generally speaking 20 degrees is the lowest advisable pitch for a slate roof depending on the size of the slates and how much that the roof is exposed to moderate weather conditions.