Riven paving is made from rough-finished slabs of stone that result when large blocks are quarried and split along natural cleavage planes. Commonly called flagstone, natural riven stone generally is hewn from sandstone, which is sedimentary; or from quartzite, a hard, metamorphic version of sandstone that has been subjected to the Earth’s heat and compression. Riven paving most often is found in decorative applications such as terraces, garden paths and driveways. As an economical alternative to a material that is labor-intensive and costly to ship, man-made paving cast from concrete or reconstituted stone that features a similar rugged appearance is also often marketed as “riven.”
A material appreciated for its rustic look, riven paving fares especially well in exterior applications because of its low water absorption, resistance to abrasion, strength and overall durability. Stone's inherently substantial compressive strength — how much compaction the material can stand — makes relatively thin riven pavers suitable for most applications. Where exceptionally heavy, eccentric loads are expected, such as a vehicle in a driveway, a much thicker slab is needed to provide adequate flexural strength, which is how much the material can bend before breaking. Unlike smooth stone, the surface of weathered riven paving will remain rugged, making it less likely to become slippery in wet conditions.
Although the large blocks of stone are quarried and cut with machinery, the riving must be done by hand, because different pressures are required to split the various layers. With a block of stone standing on edge, a quarryman forces small metal wedges and chisels between the readily visible strata. The resulting pavers can be left irregularly shaped, but the availability of modern diamond-disc saws means the slabs also can be cut into rectangles or other shapes for standard architectural uses.
China and India are the primary exporters of riven stone. Indian Kota stone has grown in popularity because its layers are relatively uniform, and it is easy to split. Also gaining in international stature is stone from northern Norway. English Yorkstone, much of it reclaimed, has been a traditional favorite in Great Britain, although it is more difficult to cut and lay than imported pavers.
Regardless of origin, riven flags vary in thickness and shape, and although riven paving is meant to look imperfect, its installation demands precision. Of particular importance is the type of bedding or underlayment used, which depends on the thickness of the stone and its intended use. Garden paving and patio paving are among the least stressful uses for flagstone, so a simple, compacted bedding generally suffices. Even so, because each paver is of a different thickness than its neighbor, care must be taken as the work progresses to ensure there is no vertical misalignment from stone to stone and that the overall surface appears to be level. The riven paving for a driveway application, on the other hand, calls either for a concrete underlayment so the flags won't break or a stone that is thick enough to handle the vehicular loads.