Roof Truss Calculator — Span Tables by Truss Type & Pitch

A roof truss span table runs on three coordinates: span, pitch, and total load (snow + dead). Find your ground snow load first — it sets the load column — then your span row, then read across the pitch you are building. Unlike rafter tables, truss tables are printed by each fabricator because the plate joints and web pattern are proprietary: two suppliers can quote different depths for the same 32 ft span. The number that matters for ordering is the quoted depth (heel height) at each bearing, since that is what the walls below must accommodate.

Most residential fink-series trusses in 2×4 chord lumber carry clear spans of 24–40 ft at standard snow loads — larger than almost any house needs — and heavier series or scissor/attic designs push beyond that with engineered chords. Compare that to site framing: a 2×10 rafter pair tops out near 16–18 ft even with a ridge beam. The practical limit on a truss roof is rarely span; it is wall height (the heel grows with span) and delivery access for 40 ft members. Your fabricator's chart, not a generic table, gives the final depth for your exact span and load.

Trusses run across the 24 ft span, spaced along the 40 ft length: n = ⌈(40 × 12) ÷ 24⌉ + 1 = 21 trusses at 24″ o.c., plus two gable-end trusses closing the ends — 23 total. At 16″ o.c. under heavy snow the count rises to 31. The calculator above does this count live, including the off-by-one rule that catches most back-of-envelope layouts.

A 12 ft span mono-pitch (shed) truss is the classic small-building answer. Unlike a symmetrical gable, a mono rises across its full span: at 4/12 the rise is 12 × 4/12 = 4 ft and the top chord runs √(12² + 4²) ≈ 12'-8″; a 10 ft shed at 24″ o.c. needs ⌈120 ÷ 24⌉ + 1 = 6 trusses. Low pitches (3/12–4/12) keep the shed profile friendly; snow country pushes the load to 50–60 psf and may add a web or two to the pattern. Any fabricator quotes 12 ft monos from stock patterns — this is the most commodity truss in the industry.

Trusses win on labor and schedule, rafters win on lumber price per stick and attic freedom. A truss roof frames a weather-tight roof plane in days with one small crew — no ridge beam, no collar ties, no on-site compound cuts — but each truss costs engineered-labor dollars and the attic becomes unusable webs. Stick framing uses cheap dimensional lumber and keeps open attic space, but takes several times the labor and demands real rafter-geometry skill. Industry experience: for an average production house the truss package lands at or below the stick-framed total once labor is counted; for a small cabin with a usable attic, rafters can win.

No — never cut, notch, or remove any truss member, including the webs that make the attic look 'wasted space'. Every web is a designed compression or tension member; removing one converts the truss into an unengineered frame with no load path. The legal route to attic space is ordering room-in-attic (attic) trusses from the start, where the fabricator designs the open room into the pattern, or a full re-engineering of the affected trusses before any cut. Most jurisdictions treat field-modified trusses as a structural violation, not a handyman project.

Three kinds, and the fabricator's drawing specifies all of them: temporary bracing that holds the first trusses plumb during installation (the most dangerous minutes of the whole build), permanent top-chord bracing usually provided by the roof sheathing itself, and permanent bottom-chord lateral bracing — rows across the bottom chords at roughly 10 ft maximum spacing to stop them from buckling sideways under ceiling load. Missing bottom-chord bracing is the most common truss-roof defect inspectors flag: the roof sheds snow for years, then the ceiling line develops waves.