On what type of substrate can I attach Rustic Slates?
They can be installed on a nailable substrate such as plywood or OSB that is 18 mm thick. Your old roofing does not need to be removed. It can serve as an underlay. Mind the length of your screws in this case. Longer screws should be used so they penetrate the wooden substrate by at least 1.5 cm to 2 cm.