How to calculate rafter length and truss count
Rafter length is the true sloped distance from the ridge to the eave tail. Run is half the span; rise is run multiplied by pitch ÷ 12. Then use the Pythagorean theorem and add the overhang.
Example: a 30 ft span, 6/12 pitch and 12 in overhang has a 15 ft run, 7.5 ft rise and a rafter of about 17.9 ft with the overhang.
Truss count comes from building length and spacing:
A 40 ft building needs 21 trusses at 24 in and 31 at 16 in. Price them with the cost calculator or check quantities with the truss count guide.
What is on-center spacing?
On-center (O.C.) spacing is measured from the center of one truss to the center of the next. Common values are 12, 16, 19.2 and 24 inches. Tighter spacing means more trusses and less load on each; 24 in is standard for engineered trusses. Compare in truss vs rafter.