Slate splits readily into thin slabs which have great tensile strength and durability. This makes it a roofing material of unequalled quality, impermeable to wind, rain and storm. Varying in length from 10 to 36 inches, and in width from 5 to 24 inches, slates can be cut to produce distinctive decorative shapes. As a general rule, the steeper the pitch of the roof, the smaller the slate.
When laid in roofing, the upper surface of the slate is known as the ‘back', and the under surface is known as the ‘bed'. Slates are fixed by nailing, either at the top or near the centre. The distance between the lines of nailing depends on the length of the slate, and is called the ‘gauge'. The area where there is more than one thickness of slate is called the ‘lap'.
Slates are either laid upon open battens fixed to the rafters; or on close boarding; or upon battens fixed to boarding. Slates must be laid with due consideration to ventilation, otherwise they are liable to decay and crumble. Slates are suitable for use on roofs with a pitch steeper than 25 to 30 degrees from the horizontal.
Slate is a quintessentially Georgian building material. There is no naturally occurring slate in Sussex; the slate used in construction in Brighton and Hove was shipped down from Wales. The concealment of slate roofing behind parapets was a popular Regency architectural device. Slate has great longevity: when constructed correctly, a slate roof can last two or three hundred years.
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