Manufactured roof trusses offer economies of scale in labour and materials, making them less expensive than traditional rafters. Lumber retailers should have a salesperson available to assist with selection of trusses by manufacturers whose designs have been reviewed by structural engineers for added value and safety.
The width of the structure -- measured to the outside of each framed wall -- determines the span of the roof truss. Outside of the overall width or span is the overhang, beyond the plane of the outside wall, referred to as the soffit. Its depth can vary and will be depicted on the architectural design.
Railroad engineer Albert Fink invented the Fink truss in the 1800s. Used primarily in residential construction, it has the added bonus of light storage capabilities because of its open web structure, which also allows space for mechanical systems, such as air conditioning units. Other designs exist, but the Fink truss serves as a fair representation for average home construction.
Roof pitch or slope appears on the design plans as well. Roof pitch is the dimension of rise compared to run, with geometry that appears similar to a right triangle; for example, 8 inches of rise on 12 inches of run is referred to as an 8/12 pitch.
A structural engineer calculates loading requirements -- based on forces such as wind, water, snow and roof shingling -- to meet local building codes. The truss manufacturer should offer certification of local code compliance for perspective buyers.
The average cost of a Fink truss will vary from market to market. For example, this type of truss installed in the greater Boston region will average £2.50 per square foot of building area, according to the 2011 RSMeans "Residential Cost Data" book. The same truss in New York City costs about £2.8, compared to £1.60 per square foot in Raleigh, North Carolina. These prices don't include shingles or additional required framing materials such as furring, roof sheathing, soffit and fascia board. RSMeans bases its average on a truss span of 26 feet with a 1-foot overhang, 8/12 pitch, 24-inch spacing and a 40-pound loading factor.