A roof structure is the load-bearing framework, usually timber and sometimes steel, that carries a building’s roof and gives it its shape.
Characteristics
The roof structure is the skeleton beneath the roof covering. It is made up of several members that work together to carry loads down into the walls:
- Trusses and rafters, which run up the slope of the roof.
- Purlins, which run horizontally and support the rafters or roof covering.
- Wall plates, which spread the load onto the tops of the walls.
- A ridge at the apex, with collar beams, tie beams and king posts adding stiffness.
Depending on how these parts are arranged, roofs are broadly grouped into rafter roofs, purlin roofs and truss roofs. Each type suits a different span and layout.
The structure has to carry more than just its own weight. It supports the roof covering and must resist snow loads pressing down and wind loads pushing and lifting. All of these forces are collected by the framework and transferred safely into the load-bearing walls below.
Application
A sound roof structure is essential to any pitched-roof building, from a small shed to a large hall. It is set out on a roof plan before construction, and the size of each member is chosen to suit the span and the expected loads. Good design keeps the roof rigid, watertight and durable for decades.
Related terms
- Roof truss
- Purlin
- Rafter
- Ridge
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