Since the internet ended traditional ways of buying and selling, anyone with a smart-phone and a credit card can buy slate. There is far more to understand about slate and quarries than this, however.
Slate quarries have the most daunting task of producing a consistent product. Consider that 95% of what is extracted from the ground ends up on the waste pile, unfit for use. Their battle is with nature- the characteristics of the rock and how it is situated in the ground. Their expertise is applied to extract it and maximize the amount of usable product.
Availability of Slate
Very little is known about a slate seam until the expense and effort has been made to open it up and begin quarrying. One end of the seam to the other is an unknown. A seam that began as Unfading Purple slate may suddenly become Unfading Green, with a transition of Mottled Purple. Given the very high operational costs, the quarrier is constantly faced with difficult decisions. Inevitably it is to make the most of what is available, but there’s no saying this matches the orders he is receiving. Sometimes a color becomes unavailable; the high presence of joints in the seam may force the production of only smaller slate, so larger slate become unavailable. It is no exaggeration to say that predicting the availability of slate is a day to day event.
Thickness, planarity, size, quantity and specific coloring, are often specified as though slate were a man-made product. Slate is most definitely a natural product, however. The goal of the quarrier is to make the most consistent slate possible, but it is not an exact science. Variations abound and should be considered part of the aesthetic quality of a natural slate roof.
A walk around an older neighborhood with slate roofs demonstrates how, in former times, architects, tradesmen and quarriers combined forces constructively, to make the slate industry work well.
Roof styles, while they look (and are) carefully designed, are also lessons in efficient material use. When slate of one width was not available for the whole roof, a “Random Width” slate roof was offered. If there were not enough slate of the same length, a “Graduated” slate roof could be built. When the quarry was forced to cut slate in different thicknesses, a “Mingled Thickness” slate roof was proposed. Or as an old time Vermont quarrier put it, “Everything coming out of the ground should land on a roof.”