The roof of building is generally supportd by the walls below and is made up of two sections:
Cast iron, steel, reinforced concrete and timber are the rigid materials generally used. More commonly, strong timber beams often give an aesthetic as well as a practical function and are now often fashionably left exposed. The other advantage of timber is that it lends itself to various roof shapes that are long and of strong, fairly rigid material such as timber, and since the mid 19th century, cast iron or steel. In countries where bamboo is readily available, it can be used to create a dinstincive curved roof line, due to its stength and flexibility. This type of roof can often be seen in Oriental architecture.
In some major ancient buildings, the roof structures were dominated by stone arches or vaults - such as St Paul’s Cathedral. This type of construction was used during the time of the Ancient Romans, but stone lintels cannot be used safely over a long distance - not far beyond 40 metres. The Industrial Revolution brought iron as a material with great strength, flexibility and length. It was used in the design of Crystal Palace, completed in 1851. The success and continual improvements in strengthening the steel girders led to their use on major roof constructions and in turn for houses.
Reinforced concrete beams are constructed using encased metal rods which give greater strength under compression. Many of the modern villas built in the Mediterranean often favour this type of construction.