Alamo Heights Roofing: Article About Truss Designs
Roof trusses provide the framework on which a roof is built. They may be constructed from either timber or steel, although the latter is generally used only in industrial buildings such as warehouses. Many factors determine how a roof truss will be engineered. Alamo Heights Roofing experts know the local conditions, cost of materials and building codes to help you choose the best roof for your building.
Trusses include rafters, also called top chords, which are vertical supports, and purlins, which run horizontally the length of the roof. Other components are tie beams, also called bottom chords, posts and a ridge board. All must meet certain requirements in order to support the weight of the final roof and to keep the roof from shifting sideways. The design must also transfer some of the roof weight to the studs in the walls of the building.
A truss commonly used in residences and small commercial buildings is the pitched truss. The framework is constructed in various sizes of triangles, resulting in a very stable roof. Pitched trusses are available in numerous configurations. Houses often made with a fink configuration, which is advised for roofs that have a steep pitch and short span. Others have names like bowstring, double bobtail and scissors.
Bowstring trusses allow for an arched roof and are frequently used in military facilities. Bobtail refers to a short piece, or stub, that's fitted in for additional vertical support.
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When one side of a roof is longer than the other side is, a bobtail may be inserted near the outer edge of the long side. A double bobtail is a stub installed on each side. Scissors reflects the shape of the supporting diagonals. The truss does not have a tie beam connecting the rafters. They are often used in buildings that include a vaulted ceiling.
Commercial buildings, which tend to be long, are generally built using Pratt or Warren trusses. Airport hangars are a good example of long span roofs. These trusses feature more diagonal beams to support the longer and heavier roof. Newer industrial buildings can be built with north light trusses if they don't have a long span. As the name implies, the roof is designed to let in natural illumination with the goal of reducing the cost of lighting the room. The saw-tooth truss is a variation of the north light configuration.
A closed truss means that the building was constructed using a tie beam and included a ceiling that conceals the entire roofing framework. Structures built with a scissors truss or in some other way without a complete tie beam and do not have the framework concealed are designated as open truss construction.