Impact of using a system factor in the design of light-frame roof assemblies

A light-frame roof assembly was analyzed assuming system factors ranging from 1.0 to 1.5. Different assembly configurations, obtained by varying the assembly span, truss spacing, and the dimensions of the lumber in the top and bottom chords, were analyzed. For each configuration, the minimum lumber grade required to satisfy design requirements was determined for the entire range of system factor values. The application of system factors resulted in significant reductions of the quality of the material required to satisfy a specific structural configuration. This may be translated to an increase of permissible truss spacings or assembly spans, or a combination of the two, and thus more efficient use of material. However, the inclusion of system factors in design will indirectly imply a reduction of the redundancy built into the current design of light-frame roof assemblies. It is thus recommended that the use of the system factor should be accompanied by concerted efforts to improve quality control in the production and handling of the trusses.