A truss is a prefabricated rigid framework used for roof and floor construction. Designed by an engineer, it's factory-built, and shipped directly to the building site. Most new homes are built with roof trusses because it's efficient and expedient. But plan ahead--you need to order them weeks in advance from your local building company or truss manufacturer.
Parts of a Roof Truss
Wood members, formed by rafters and ceiling joists, form a roof truss. Think of a triangle--the three sections forming the triangle are the chords. The two upper chords, the rafters, form the upside-down V, and are attached to the bottom chord, the ceiling joist. The structural members, rafters and ceiling joists, of the truss are generally 2-by-4s or 2-by-6s. The diagonal connecting pieces that transfer the weight from one wood element to the next are connected by gussets, a flat metal or plywood plate.
Roof trusses are designed to carry the load of both the roof and ceiling to the outside walls. Placement is spaced at 24 inches on center. Many types of trusses are available and can be fabricated to fit different roof styles. The W-type truss is the most common. As the name implies, the interior portion of the truss, or the webbing, is shaped in a W-pattern. Also popular: the king-post truss, (which has a limited span,) and the scissors-type truss used for architectural designs that call for sloped ceilings.
Advantages of Roof Trusses
Using a pre-assembled roof truss is a good framing option if you want to avoid the cumbersome task of cutting rafters and joists from scratch. It saves both time and labor. Trusses span greater distances than standard rafters, allowing for more flexibility in the interior planning of the home because fewer load-bearing walls are needed.
In lieu of framing a floor with common floor joists, prefabricated floor trusses are available. Generally made from 2-inch-thick composite wood material, they are extremely strong and lightweight. They also span a wider distance than a traditional floor truss. The initial cost may be more per lineal foot, but you'll save from faster installation and less wasted material.
It is possible to build your own trusses, but it's not recommended. Trusses for the simplest house can demand complex angles. If you err by even an inch, it can result in a great loss of time and money. Truss manufacturers are set up to produce quality products with precise engineering calculations. Note: Trusses cannot be altered in any way--all the parts are calculated to interact precisely with each other.
- Housebuilding: A Do-It-Yourself Guide; R.J. DeCristoforo; 2007
- Building Your Own Home: Everything You Need to Know About Home Construction from Start to Finish; Robert Roskind; 2000
- Truss Technology in Building: Floor Truss Fact Sheet
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