Roof and Floor Trusses

Roof trusses and floor trusses are support structures that consist of one or more triangular units that are made with straight, slender members and connected at the ends as joints. Roof trusses are used for roof framing and roof support. By contrast, floor trusses are used for floor framing and floor support. Typically, roof and floor trusses are made of wood or metal. They differ in terms of dimensions and may be sold as part of a framing package. In many locations, these engineered products must meet state, provincial or local building codes. In the United States, roof and floor trusses that meet standards from the National Evaluation Service (NES) and the International Code Council (ICC) use designs that meet the requirements of local code officials.

Roof trusses come in several standard configurations, including the W-truss, the M-truss, the scissors truss, and the gable truss. In residential construction, M-trusses are used over garages while scissors trusses are used in front rooms. Gable trusses are used at the ends of rooflines. Regardless of style, standard roof trusses have several basic parts. These include the bottom chord, diagonal, heel post, heel wedge, king post, gusset, steel connector plate, top chord, and vertical post. As the two principal members of a roof truss, the top and bottom chords extend from end to end are connected by web members. As the middle post of the roof truss, the king post or joggle post is a vertical support that transfers weight from the ridge beam to the end walls or joists.

Roof trusses may use designs that are categorized as attic, bowstring, double fink, double howe, dual pitch, fan, fink, flat, hip, howe, inverted, sloping flat, stub and studio. The attic truss forms the top story of a building without using internal members. The bowstring truss has a curved top chord and a curved bottom chord that meet at each end. The fink truss is a three-triangle, symmetrical truss for supporting large, sloping roofs. The flat truss or parallel chord truss has parallel top and bottom chords. A component of a hip roof system, the hip truss has an eave line which extends to the same level around all sides of the building and eliminates the use of gable ends. The inverted truss is used to provide a vaulted ceiling along a portion of the span. The stub truss (bob-tailed truss, studded gable) is a gable-end truss build as a wall.

Most floor trusses feature open-web or parallel flat chord designs; however, trimmable floor trusses are also available. There are two types of trimmable trusses. Hybrid floor trusses have steel webs and top and bottom chords made from lumber. For a short distance on each end, the web material is made from oriented strand board (OSB) to form an I-joist that can be trimmed. All-wood floor trusses are also available. Typically, these trusses do not use truss plates for connections. Instead, the chords and web are connected via finger-joining technology.

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