Alamo Heights Roofing: Article About Structure Of Trusses
Trusses are an integral part of most roofs. The trusses in residences and small buildings are usually constructed from wood. Those used in industrial structures may be manufactured from steel. In either case, trusses are usually shaped like a triangle to provide maximum strength. Occasionally other geometric shapes are used.
The overall design of the building, the use for which it is intended, the kinds of materials from which it will be constructed and local weather conditions are all factors in engineering trusses. Alamo Heights Roofing professionals are familiar with the federal, state and county building codes that determine what kind and number of trusses is required for each building.
A truss is constructed of horizontal, vertical and diagonal beams joined in specific patterns. The primary vertical support is a post. A single centered beam is known as a king post. Two beams equally spaced to leave a center opening are called a queen post truss. The bottoms of the posts are perpendicularly attached to a tie beam. Rafters further join the ends of the tie beams to the posts. With a king post, the rafters attach to the tie beam at an angle and form a sharp peak at the top of the post. Rafters supporting queen posts join a horizontal beam, called a straining beam, to form a square. Struts further support all the beams. Other than the center square in queen post construction, all the components form various size triangles.
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Sometimes a king post and queen post are combined to form a compound truss. Compound trusses are also known as Palladian trusses.
Most buildings will have either a king post or queen post truss on each end of the roof. Particularly long buildings or those requiring more strength will have additional trusses spaced along the length of the roof. All trusses will be connected at the top by a long horizontal beam called a ridge board. Depending on the length of the roof and the weight of the final roofing material, the trusses may be connected by additional horizontal beams called purlins. There will be a purlin on each side of the roof connecting the tie beams.
Rafters will then be attached vertically from the bottom purlin to the ridge board. These are known as common rafters, to distinguish them from the principal rafters used for the trusses. Common rafters in wood roofs are generally made from 2 x 4 boards. Spacing between them is regulated by various building codes.
Contemporary terminology has changed the names of roof components so that rafters are now called top chords. Bottom chord is the modern name for a tie beam. Supporting struts are also called diagonals. The configuration of the rafters and internal beams is called the web.