Roof Truss Span Calculator

How the Roof Truss Span Calculator Sizes a Truss Order

This roof truss span calculator answers the two questions a truss plant or lumberyard will ask you on the first call: how many trusses do I need, and is my clear span inside the typical range for the truss profile I want? Enter the building length, clear bearing-to-bearing span, on-center spacing, and the truss type — Fink, queen post, mono, scissor, or attic — and the tool returns count, total truss length including overhangs, and a feasibility flag.

Truss types and where each one fits

• Fink (W-web) truss — the standard residential roof truss with a W-shaped web pattern. Most economical between 20 and 40 ft, engineerable up to about 60 ft.
• Queen post truss — short to medium spans with vertical posts. Used where a center ceiling cavity is wanted without going to a full attic truss.
• Mono truss — a single sloped chord, used on shed roofs, lean-tos, and asymmetric two-pitch assemblies.
• Scissor truss — vaulted ceiling profile, with the bottom chord pitched up. Reduces effective depth and limits achievable span vs. a Fink.
• Attic truss / room-in-roof — designed bonus room inside the trusses. Span is limited because interior chords carry live load from the floor.

Truss spacing and what drives the choice

24 inches on center is the residential default for stock truss orders, matched with 5/8-inch or thicker sheathing. Drop to 16 inches OC under tile, slate, or concrete tile dead load, under heavy snow (verify with the roof snow load calculator), or when the architect specifies it for deflection. 48-inch OC is only possible with purlins or panelized roofs and is engineered, not selected.

Roof Truss vs Site-Built Rafters: A Real Trade-Off

Trusses ship as engineered assemblies with shop drawings stamped by a P.E. — every joint, plate, and chord size is calculated for your specific load case. They install in a fraction of the time of stick framing and use less lumber for the same span. The trade-offs are delivery logistics, restricted attic storage volume (the web pattern fills the attic), and a longer lead time. Stick framing with the rafter calculator still wins on tight access, custom architectural shapes, and cathedral ceilings without a structural ridge beam.

Common truss order mistakes

• Confusing clear span with building width. Clear span is bearing to bearing. If you have an interior bearing wall, only span between bearings — not the full outside-to-outside dimension.
• Forgetting the gable-end truss. One gable-end truss at each end of the run is not the same as a common truss — it carries the gable wall sheathing and the overhang lookouts. The count returned by this tool counts a truss at each gable.
• Adding an interior bearing wall mid-run. A standard simple-span truss is not designed for a mid-span support and may be overloaded by adding one. Always ask the truss manufacturer for a multi-bearing design if the floor plan needs it.
• Storing trusses flat on the ground. Trusses must be supported at their bearing points until lifted into place. Stacking flat warps the chord and can void the design.

What the Truss Manufacturer Needs From You

Once your truss count and span feasibility are confirmed, the truss plant will need: roof pitch, top and bottom chord overhangs, soffit width, heel height, snow load (Pg from the snow load calculator), wind speed and exposure, dead load including roof finish (use the roofing weight calculator), and any concentrated loads from mechanical equipment or solar arrays. Bring those numbers to the first call and your shop drawings turn around in 7 – 10 business days instead of 3 – 4 weeks.