Porphyry Paving
Porphyry is a dense natural stone, extremely strong and hard-wearing. It is frost resistant, and suited for both pedestrian or vehicular use. Named after the Greek word for purple (porphyra) it is an igneous rock containing large crystals of plagioclase. Harder than marble, it is difficult to cut, with the majority on sale in the form of cropped setts, ‘chopped’ into smaller sizes suitable for laying as detailed paving, featured banding courses or in fish-scale or fan shapes as driveways.
It is a volcanic rock, similar in many ways to granite, comprising over 70% silica, 14% alumina and 8% alkali, with traces of iron, calcium and magnesium, it is very versatile as a product for use in garden schemes.
Laid in a similar way to cobbles and setts (see Cobbles Set into Mortar), over a solid base of MOT Type1 or concrete, and jointed in a non-permeable jointing material. Traditionally laid with 13mm (average) joints and pointed using sand and cement slurry, it is a rigid finish material, requiring expansion joints (see Expansion Joints in Concrete) when laid over larger areas.
Cropped sided porphyry or cropped setts may also be laid unbound, each sett loosely touching its neighbour, and jointing aggregate brushed into the joints.
Photo credit – Dave Stanger, Tecknotix.
