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.
A simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall.The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths, battens, or purlins are fixed to the rafters; or boards, plywood, or oriented strand board form the roof deck (also called the sheeting or sheathing) to support the roof covering. Heavier under purlins or purlin plates are used to support longer rafter spans. Tie beams, which may also serve as ceiling joists, are typically connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength. The rafters, tie beams and plates serve to transmit the weight of the roof to the walls of the building.
The basic shapes of roofs are flat, mono-pitched, gabled, hipped, butterfly, arched and domed. There are many variations on these types. Roofs constructed of flat sections that are sloped are referred to as pitched roofs (generally if the angle exceeds 10 degrees). Pitched roofs, including gabled, hipped and skillion roofs, make up the greatest number of domestic roofs. Some roofs follow organic shapes, either by architectural design or because a flexible material such as thatch has been used in the construction.
A more recent design is the installation of a roof deck with foil-backed foam along with a second deck that is air-gapped away from the foil-backed foam to allow air to flow vertically to a ventilation outlet at the peak of the roof—it is a double deck design with an air gap. This design improves efficiency.
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