Roof Truss Calculator

A 2x4 roof truss typically spans 22 to 34 ft without interior support, depending on truss type, pitch, and spacing. King post trusses top out around 22-26 ft, fink (W-truss) profiles reach 28-34 ft, and common or attic trusses fall in the 26-32 ft range, all using standard No. 2 lumber grades at typical residential loads. Upgrading to 2x6 chords extends every one of these ranges by roughly 4-10 ft, and final allowable span always depends on lumber grade, spacing, and snow load zone, confirmed on a stamped truss drawing.

A 20 ft span typically works with 2x4 lumber across the six common timber profiles shown above, including the shorter-span king post design. At 20 ft, span isn't the limiting factor for any of those six, pitch, load category, and room use (attic vs. standard) matter more at this size. A king post truss keeps material cost lowest on a roof with no attic or vault plan; a fink truss works just as well and leaves room to extend the span later if the design changes.

Truss height equals half the span multiplied by the pitch ratio (rise divided by 12). For a 28 ft (8.5 m) span at a 6/12 pitch: height = (28/2) × (6/12) = 14 × 0.5 = 7 ft (84 in, about 2.1 m). This gives the rise from the top of the bottom chord to the peak, add wall height and heel height to get total roof height above the foundation.

Divide building length in inches by truss spacing in inches, then add one for the starting truss. A 40 ft (480 in) building at 24" on-center spacing needs 480 ÷ 24 = 20, plus 1 = 21 trusses. Round up on any fraction, and add extra pieces for gable-end trusses, hip sets, or girder trusses at openings, a manufacturer's quote accounts for these beyond the base count.

A 4/12 pitch truss costs less than steeper pitches because the shallower rise means shorter top chord members and less total lumber per truss. Moving to an 8/12 pitch or steeper increases top chord length and truss height, which typically raises material cost per truss by 10-25%, depending on span, even though the footprint and span stay identical.

Attic truss room size depends on span and pitch: a 30 ft span at an 8/12 pitch typically yields a usable room around 16-18 ft wide with 8+ ft of flat ceiling height at center, tapering to about 5 ft of headroom near the knee walls. Wider spans and steeper pitches both increase usable room size, run your own span and pitch through the attic truss selection above to see projected dimensions for your project.