A truss rafter, also known as a trussed rafter, is an engineered, triangulated roof support structure that is usually built off-site and then delivered to the construction location. Such a structure may consist of either metal or wood, but is most commonly made of kiln-dried lumber assembled with steel nail plates. The function of the truss rafter is to support the full weight of the building's roof and carry this load to the outside walls. These components differ from roof rafters, which are individual beams, in several ways.
Truss rafters generally use shorter lengths of lumber than are used in a conventional wooden rafter framework, resulting in a strong support system. Each truss is built to exact specifications using information provided by the home builder. The dimensions of the house are used to mathematically determine the number of trusses and the span needed to carry the load, which includes roofing materials. Most roof trusses are now pre-fabricated and factory-built, which allows for a more precise product than constructing roof joists in the field.
There are several advantages to using a truss rafter system over conventional ceiling joists and roof supports. Cost savings are achieved by the reduction of wood used as well as the factory-style production, which saves labor costs. Pre-fabricated units are assembled indoors, where materials such as wood are not exposed to elements that may cause warping. Contemporary and other non-traditional roof designs can easily be accommodated without greatly increasing building costs. These systems can also span greater areas than conventional rafters, allowing for buildings with fewer weight-bearing interior walls and resulting in a more open design.
One disadvantage of using truss rafters, at least from the homeowner's point of view, is that their design and construction leaves less attic space for storage or expansion. This problem can be solved by using room-size attic trusses. These truss systems are designed specifically to allow extra square footage, depending on the design of the home, with approximately 8 feet (2.4 m) of height. Another plus is that the bottom chords of attic trusses are specially designed to allow for extra weight, making them very sturdy. The additional cost of attic trusses is minimal, usually only a few dollars per square foot.
The two most commonly used types of roof truss styles are the pitched or common design, and the parallel chord, also known as the flat truss. The common truss is the one most familiar and used in the majority of home construction projects. Identifiable by its triangular form, it is the most common type of truss rafter used in roof construction. The parallel chord truss is mostly used in floor construction, and is so named for its parallel chords across the top and bottom.
Many variations of the truss rafter are available, depending on the type and style of home being built. For instance, a scissor truss is commonly used in homes with cathedral ceilings, and a raised heel truss is often the choice for energy efficient designs, because its shape allows for extra attic insulation and a tight moisture and vapor barrier. It is essential that the home builder knows exactly what style of roof truss framing system he wants before ordering these components. Measurements and specifications should be as exact as possible, and it is recommended that each roof truss be thoroughly inspected on delivery to the job site to ensure quality and customer satisfaction.