Engineering and Construction

Roof trusses, and to a lesser extent floor trusses, are used in platform frame buildings because of their speed of erection, economy of material usage, and long spans. Though many floor  trusses are light enough to be lifted and installed by two carpenters, most truss assemblies are erected with the  aid of a small crane that often is attached to the truck on which the trusses are delivered (Figure 5.65).  Roof trusses are particularly slender in proportion, usually only 1½ inches (38 mm) thick and capable of spanning 24 to 32 feet (7.5 to 10 m). They must be temporarily braced during construction to prevent buckling or  the dominolike collapse of all the trusses until they are adequately secured permanently by the application of roof sheathing panels and interior finishes (Figure 5.66).

Figure 5.65 Roof trusses are typically lifted to the roof by a boom mounted on the delivery truck. This is one of a series of identical attic trusses, which will frame a habitable space under the roof.

Manufactured wall panels have been adopted more slowly than roof and floor trusses, and are used mostly  by large builders who build hundreds or thousands of houses per year. For  the smaller builder, wall framing can be done on site with the same amount of material as with panels and with  little or no additional overall expenditure of labor, especially when the building requires walls of varying heights and shapes.

Figure 5.66 A roof framed with prefabricated trusses. Approximately midway up the upper chords of the trusses, temporary strapping ties the trusses to one another for bracing. Other diagonal bracing, not visible in this photograph, ties the trusses to floor or ceiling framing to prevent the entire row from collectively tipping sideways.