Measure the distance to be spanned across a house or other structure. The span is the distance from one outer wall to the other. Determine the type of lumber to be used. Truss strength will vary among lumber types, such as white pine, yellow pine, cedar or fir, which are the most common woods. It also will vary with lumber grade, such as No. 1 or No. 2 white pine.
Determine the spacing of trusses. Trusses spaced 16 inches apart will carry greater loads than those spaced at 24 inches; 24 inches is the normal spacing for roof trusses in a house but conditions may dictate a narrower spacing. Figure a roof pitch or slope; that angle will affect the length of the truss from peak to outer wall, which in turn will affect the strength needed. A truss of 2-by-4-inch lumber for a steep 6/12 pitch that rises 6 inches in 12 feet will cover almost twice the span as on a 2/12 roof, which slopes only 2 inches in 12 feet.
Calculate the load for the truss. This includes the weight of the roof decking and the shingles. Shingle weight can vary with the type of material and thickness used. Figure a "snow load," using Weather Bureau information about the maximum amount of snow a roof might be expected to bear and wind loads, maximum wind force on a roof. Consult truss tables for variations according to the size of lumber used in the truss; a truss framed with 2-by-6-inch lumber will span about a third more distance with the same load factor than one with 2-by-4-inch lumber.
Consult local building codes. Most jurisdictions have codes that regulate the size of lumber, the length of truss, the type of internal bracing required and other factors that affect strength of a structure. The codes also will cover floor trusses, which are used in raised floor systems such as pier and beam foundations or on concrete stem walls or basement walls.