How to Find the Load of a Roof Truss

Calculate dead load by finding the weight of the truss lumber and roofing material; asphalt shingles, for instance, weigh about 2 pounds per square foot. Refer to lumber industry tables to get the weight of various boards and the maximum span for each type. Figure the weight per square foot and use that as a basis for determining the type of truss to use and the internal bracing required.

Get snow loads from U.S. Forest Service, U.S. Department of Agriculture or similar tables, based on Weather Bureau statistics of the average and maximum snowfall and accumulations. Look up your state to find the pounds per square foot loads and the related International Building Code specifications for trusses to handle that weight.

Check Weather Bureau and Federal Emergency Management Agency tables for wind loading, both shear forces striking an upright surface and upload, pressure from under a roof. Consult FEMA tables for recommendations in high-wind areas subject to hurricanes or tornadoes. Vary truss design to see how angles affect these forces; flat or low-slope roofs, for instance, will have less horizontal wind load that steeper pitches with more vertical area.

Take advantage of computer programs and online calculators to help determine truss load factors (see Resources). Determine roof widths or spans, pitch or slope angles, square footage of roof area and similar data. Enter these figures into a program template and get a recommended truss style, design, spacing and other construction details. Consult truss manufacturers, most of whom will provide such calculations free.