Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rockcomposed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock.
Slate can be made into roofing slates, a type of roof shingle, or more specifically a type of roof tile, which are installed by a roofer. Slate has two lines of breakability – cleavage and grain – which make it possible to split the stone into thin sheets. When broken, slate retains a natural appearance while remaining relatively flat and easy to stack.
Slate roof tiles are usually fixed either with nails, or with hooks as is common with Spanish slate. Nails were traditionally of copper, although there are modern alloy and stainless steel alternatives.
Areas of weakness on the tile are fewer since no holes have to be drilled
Roofing features such as valleys and domes are easier to create since narrow tiles can be used
Hook fixing is particularly suitable in regions subject to severe weather conditions since there is a greater resistance to wind uplift as the lower edge of the slate is secured.
The primary means of failure in a slate roof is when individual slates lose their peg attachment and begin to slide out of place. This can open up small gaps above each slate. A secondary mode of failure is when the slates themselves begin to break up. The lower parts of a slate may break loose, giving a gap below a slate. Commonly the small and stressed area above the nail hole may fail, allowing the slate to slip as before. In the worst cases, a slate may simply break in half and be lost altogether. A common repair to slate roofs is to apply 'torching', a mortar fillet underneath the slates, attaching them to the battens. This may applied as either a repair, to hold slipping slates, or pre-emptively on construction.
Where slates are particularly heavy, the roof may begin to split apart along the roof line. This usually follows rot developing and weakening the internal timbers, often as a result of poor ventilation within the roofspace. An important aspect to slate roofs is the use of a metal flashing which will last as long as the slates. Slate shingles may be cut in a variety of decorative patterns and are available in several colors.