As with all slate quarries, different grades of slate are produced and Burlington blue/grey and Westmorland green slates are supplied in three grades:
Best grade slates are the thinnest and flattest slates that the quarries can produce, typically a 7-8mm nominal thickness. While this is the thinnest slate that we produce, it is still appreciably thicker than many other slates from elsewhere in the UK and the rest of the world. This thickness, aligned with the broad whittled edge of the slates, helps to pick out the shape of each individual slate on the roof, producing a roof covering that looks like it is comprised of a collection of individual natural components that makes a connection with the building, rather than a more homogenous, uniform, one dimensional roof covering that sits atop of the building as a disparate, artificial looking element, as with imported slates.
Strong grade slates are thicker than bests but still relatively flat with a typical nominal thickness of 10mm.
Extra strong grade slates are thicker and often subject to a cross rive leaving a protrusion on the surface, and can be subject to slight bow and twist (which looks great when well slated).
Strongs and extra strongs accumulate as a ‘by-product’ of quarrying bests and are mostly available in random sizes.