Roof trusses are custom-built items, engineered to the specific size, needs and building codes of a house. All the elements can vary and affect the final design. There are common types -- the Fink or W is most widely used style -- and known span limits for roof widths. But roof loads vary widely with locale, weather conditions, size of the house and other factors. A few builders stock some common widths, such as 24 and 30 feet, in frequently used pitches or slopes, but most trusses are customized using computer design.
The most basic truss designs will span roofs up to 30 or 36 feet wide, with average loads. Roof loads include dead load, the weight of the roof itself and a ceiling below, and live loads, mainly accumulations of snow and ice and the force of wind against a roof surface. These vary with locale and the pitch or slope of the roof sides. Most roofs have medium pitches, 5/12 or 6/12, slopes of 5 or 6 inches per foot of roof.
Trusses up to about 34 feet are usually built with 2-by-4-inch lumber. Longer spans and severe snow climates require 2-by-6-inch boards. Spacing of trusses affects spans; most trusses are set 24 inches apart, but closer spacing, such as 16 inches, will increase the allowable span. The pitch also affects truss design. Steep roofs shed loads better and so can have longer spans; low-slope roofs, under 3 inches per foot, have shorter spans or require closer spacing or bigger material.
Most trusses are made of pine, with southern yellow pine the preferred lumber, but some use Douglas fir or a spruce wood. Prefabricated trusses have joints fastened with metal gussets. The strength and integrity of the truss depends on these gussets. These are stamped from light structural steel and coated with zinc to resist rust. They typically are fastened with eight metal teeth per square inch, usually 5/16 to 9/16 inch long, which are driven into the two boards being joined.
Span and Pitch
A typical truss design starts with the width or span and the pitch, which determine the length of the horizontal bottom chord and the length and angle of the rafters. Internal bracing is added, depending on the load, type and size of lumber and specific stress placed on components. Top and bottom chords (rafters) are usually made of lumber that has been specifically stress-rated for those loads.