‘Beehive’ shaped, corbelled roofs have been used since the Stone Age to span voids and create roofs in buildings. One of the earliest examples of a part corbelled roof can be found at the Knap of Howar on Papa Westray, Orkney - the oldest preserved stone house in Northern Europe - dating to around 3500BC. Corbelled roofs can also be found in the Great Pyramid of Giza, the Roman Parthenon and many later castles and structures. This photo is of the 16th century beehive doocot at Aberdour Castle, Fife, which was once home to around 600 birds. It shows how a corbelled roof is formed. Each successive ring of stone is overlapped slightly to narrow the aperture of the void. They form a harmonic stack, with the shortest overlap at the bottom. This can be described by the equation x=1/2n, where x is the overlap and n is the number of rows from the top. Using this equation, you can see that the top row can overlap by 1⁄2, the second by 1⁄4, 3rd by 1⁄6 and so on. The lower down the corbel you go, the less the overlap – or in building terms, the higher you go the more you can overlap. This self-supporting building method is an efficient way to span a void without the need for significant extra mass.
