The Load Capacity of a Residential Home Roof Truss

Measuring Capacity

• The load capacity of a residential roof truss represents the maximum safe load that the truss, or a design consisting of many trusses, can support before breaking and threatening severe damage to the home. Engineers and builders measure this capacity in pounds per square foot, or PSF. The total PSF for a roof includes the permanent, or dead, load, which is the weight of the roof itself, plus the weight imposed by snow, wind, and rain.

Typical Load Capacities

• Load capacities for residential roof trusses vary, but are typically within the range of 40 to 60 PSF. Snow alone can account for up to 20 PSF, which is why lightweight roofing materials are important in residential construction. The square footage refers to the entire area of the roof that the trusses are designed to support. This means that a large home uses more trusses with the same PSF rating to support its roof, rather than the same number of trusses as a smaller home but with a higher PSF rating per truss.

Factors

• The load capacity of a specific residential roof truss varies based on several factors. One is the design of the truss, with different designs distributing the load differently across the lumber and joints. Different types of wood also have their own resistances to compression, bending and sagging under a load. A truss' span, or overall length, and its pitch, or angle, also affect load capacity.

Considerations

• Most residential roof trusses are fabricated, with builders able to select trusses that meet their design needs and have adequate PSF ratings based on regional snowfall, roof material and the number of trusses included in the home's design. Truss builders use advanced computer models to complete logarithms that result in designs with desired PSF ratings and other characteristics such as space-saving shapes.