Flagstone and slate tiles

Flagstone is not a scientific rock name but an everyday name for stones that can be split along their natural layers into flat, thin slabs. They are used for garden paths, patios and terraces, wall cladding and roofs. Slate in the scientific sense is a very fine-grained metamorphic rock formed when mud rocks such as shale are transformed by pressure and heat, and it is best known as a roofing material. In the trade, however, the names “flagstone” and even “slate” are also used for schist, gneiss, sandstone, quartzite and limestone that split into slabs.

Paths and patios: Garden paths, terraces, courtyards, steps and squares are paved with flagstones. Roofing: Thin slates make a watertight, long-lasting roof covering; in mountain regions thicker stone slabs cover roofs too. Wall cladding: It is used to face buildings, garden walls and fireplaces. Other uses: Slate was once used for school blackboards and writing slates, and today for billiard table beds, gravestones and worktops. In some regions stone slabs set upright have been used as fences and boundary markers.

Looking at the edge, you can see thin layers stacked on top of each other; the stone splits along these layers into flat slabs. True slate is very fine-grained with a dull or slightly silky sheen; a sound slab gives a clear ringing sound when tapped. Sandstone flags have a gritty, sandpaper-like surface with visible sand grains; schist and gneiss flags show mica flakes that glitter in the light and wavy bands. “Slate-look” roof coverings made of fibre cement or plastic are common; they are all the same thickness and pattern and show no natural layering at the edges.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Flagstone is a building stone sold by the square metre. Its value depends on how thin, flat and evenly it splits, the size of the slabs, the evenness of colour, low water absorption and frost resistance. Roofing slate must be free of defects such as pyrite and carbonates that cause deterioration. In Europe, natural stone slabs for outdoor paving are tested to the EN 1341 standard. Because the stone is heavy, transport costs and the labour of laying it by hand make up a large share of the total cost.