When the warm, moist air from below rises into the cold roof space; condensation begins as the air temperature drops to the 'dew point' or as the warm air comes into contact with any of the cold surfaces in the roof.
The durability of a roof is a matter of concern because the roof is often the least accessible part of a building for purposes of repair and renewal, while its damage or destruction can have serious effects.
Most building materials are permeable to water vapour; brick, concrete, plaster, wood and insulation all can fall victim to interstitial condensation, this is why UK Building Regulations require roofs to be ventilated, either by the use of soffit vents, ridge vents, or replacement ventilation slates or tiles.
Stone lintels have been used to support roofs since prehistoric times, but cannot bridge large distances. The stone arch came into extensive use in the ancient Roman period and in variant forms could be used to span spaces up to 140 feet (43 m) across. The stone arch or vault, with or without ribs, dominated the roof structures of major architectural works for about 2,000 years, only giving way to iron beams with the Industrial Revolution and the designing of such buildings as Paxton's Crystal Palace, completed 1851.
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