How to Figure the Length of Roof Trusses

Determine the width or span of the roof, the distance from one outer wall to the other, and the pitch or angle of slope, from a house plan. Decide on a bearing point or spot at which the truss will sit on the outer wall; either the bottom chord or the sloped rafter chords may be the bearing point, depending on style of truss. That bearing point will help determine the length of the truss.

Pick a truss style depending on the span required and the pitch of the roof. The length of the bottom chord will depend on the width of the span and the pitch; a steeper pitch will have a greater allowable maximum span or length of bottom chord because steep roofs shed snow and ice more effectively. Use a truss design manual or an online span table to calculate the type and span of the truss. A truss of 2-by-4-inch lumber, for instance, might span 46 feet at a pitch of 5 inches per foot with a medium snow load, but 47 feet at a rise of 7 inches per foot.

Understand how truss style or internal bracing affects the length. A simple king post truss, a triangle of two rafter chords and a bottom chord with a vertical king post in the center, can span only up to 16 feet. Replace the king post with two diagonals in a W shape and you have a Fink or W truss that can span up to 33 feet. A Howe style, which retains the center post and has two diagonal webs on each side, can be up to 36 feet long.

Combine triangular elements of basic Fink or Howe trusses for longer spans. Double Fink or Double Howe styles double the number of internal triangular webs to make trusses that can be from 40 to 60 feet long. Triple the number of Fink or Howe triangle webs and you can make a truss up to 80 feet long, with 12 triangular web members.