Rag slate is unique to Cornwall and Devon, often present on old and listed buildings. Slating with rag is entirely different from any other kind of slate, given there irregularity in size.
Traditionally rag slate would have been nailed directly to the rafters (hence why traditional rag slate roofs, have rafters which are smaller yet closer together then battened roofs), however it has become more common over the years, to nail them to roofing batten, which helps speed up what is a labour-some process.
Unlike regular slate, rag slate is all different sizes and often second hand slate, sorting the rag slate has to be done before any roofing, as the different size categories determine the number of courses the roofer can do in the select category.
Again unlike, normal roof slate, roofs, can’t be pre-stacked out with slate as different size slate are required to achieve the coverage needed for a water-tight roof. As such rag-slating usually requires a minimum of two-people.
The two things to be wary off when rag slating is the coverage i.e the degree to which the next slate and/or course cover any holes in the slate. This should always be a minimum of 3’inches and the kick. If adequate kick isnt provided (where the eaves slate and bottom course meets gutter/wallplate) this will result in the next courses of slate “kicking” which can allow wind blown rain to enter the roof.
Below: Recent rag slate roofs done by Bristow & Reeve