Truss Calculator

Truss geometry reduces to a right triangle. Pitch is expressed as rise per 12 inches of run, so the rise equals pitch divided by 12 times half the span, and the top chord length is the hypotenuse of half-span and rise. A 24-foot span at 6/12 gives a 6-foot rise, a 13.42-foot top chord, and a roof angle of arctan(6/12), or 26.6 degrees. Adding overhang extends the chord past the bearing point, and the bottom chord runs the span plus the overhang at each end. Truss count is building length divided by spacing plus one, so 40 feet at 24 inches on center takes 21 trusses. Triangulation is what makes the assembly efficient. A properly configured truss loads its members largely in axial tension and compression rather than bending, which is why relatively small 2x4 chords and webs can clear spans that would demand a much deeper solid rafter. Residential trusses are typically spaced at 24 inches on center and commonly clear-span 24 to 36 feet without interior bearing walls. Two limits deserve emphasis. Truss capacity depends on the specific engineered design, including ground snow load, wind uplift, and roofing dead load, so geometry alone never confirms adequacy. And no chord or web may be cut, notched, or drilled in the field without written approval from the truss designer, because every member and connector plate is sized for a computed force.

Triangulated timber roof framing is old. Roman basilicas used trussed roofs, and Andrea Palladio documented timber truss bridges in his Four Books of Architecture in 1570. The nineteenth century American railroad boom turned the truss into a calculated engineering product and attached names still in use today, including William Howe's patent of 1840, the Pratt truss patented by Thomas and Caleb Pratt in 1844, and the Fink truss developed by the German-born engineer Albert Fink while working for the Baltimore and Ohio Railroad in the 1850s. The Fink or W configuration remains the standard residential roof truss. The prefabricated wood truss industry dates specifically to the metal connector plate. A. Carroll Sanford patented a toothed steel plate in the early 1950s in Pompano Beach, Florida, which allowed truss joints to be pressed together in a plant rather than nailed, bolted, or gusseted on site. Manufacturers organized as the Truss Plate Institute in 1960 to standardize plate design and testing, and the Structural Building Components Association, formerly the Wood Truss Council of America, publishes the BCSI guidance on handling, installing, restraining, and bracing that governs field practice. Plated trusses displaced site-built rafters across most American residential construction over the following decades because they arrive pre-engineered, shorten framing time, and free the floor plan from interior bearing walls.