🏗️ Roof Truss Calculator
Find out how many roof trusses you need and their basic chord lengths, based on your building’s dimensions, truss spacing, and roof pitch.
What this calculator does
The roof truss calculator estimates how many prefabricated trusses you need to frame a roof, plus the basic top and bottom chord lengths for each truss.
The formula
Trusses are typically spaced a fixed distance apart (commonly 24 inches on center, sometimes 16 or 12 inches for heavier loads). Dividing the building’s length by the spacing, rounding up, and adding one extra truss for the starting end gives the total truss count. Each truss’s bottom chord spans the full building width, while the top chord (rafter member) is found using the same rise-and-run geometry as a common rafter: half the span as the run, and (pitch/12) x run as the rise, combined with the Pythagorean theorem.
This gives a general layout and rough chord-length estimate. Actual truss design — web configuration, load ratings, and exact chord lengths — must be engineered and stamped by a truss manufacturer or structural engineer for your specific building and loads.