Mate ive 19 years experience in roof slating and i can assure you when am doing historic slate work here in Scotland when using ballachulish slate i cant use any slate under 150mm if i do i have to take them out.In times gone by when we was doing random slate we could use inbands and then on...
so in other words you dont put in half cuts you get slate and half if your cuts are going below 150mm wide at abutments etc or generally anything you use a slate and a half
Dosent matter what size the slates are the minimum width of slate clerk of works etc want you to use is 150mm wide they dont want people using inbands, Bachelor slates any more its bad practice ive seen may skipping hundreds of slate because of this on historic work or large sites 150mm is the rule.