The Westmorland green slate quarries in Cumbria differentiate between block (‘clog’) that will rive, which is called ‘metal’ and can be used to produce riven, hand split, roofing slates, and clog that won’t rive and is used for architectural and monumental purposes.
Not all block will rive, but it can all be sawn. When we saw block we saw with the cleavage, which means the inherent strength of the slab, its enormous flexural strength, is maintained, exactly as if it was hand split. As such, it is possible to make use of all the block the quarry produces by making calibrated roofing slates which are sawn to thickness, have the face textured to give a riven appearance, and the edges dressed to produce a traditional looking roofing slate.
Unlike Westmorland green riven slates, as the slates are machined to size, it is possible to produce Westmorland green calibrated slates in set sizes and fixed lengths x random widths, as well of course as genuinely random sizes for laying in traditional diminishing courses.
As the back of the slates are sawn flat, the slates are easier to lay than traditional riven slates.