Wood Roof Truss Calculator — ft, lumber
How to use this calculator
Pick a preset or punch in span, pitch (rise per 12″ run), spacing, and building length. The output is your lumberyard order — truss count for crane scheduling, board feet of chord and web stock for the PO, and rafter length so you can size your top-chord material to standard 8/10/12/14/16/20 ft lengths without a wasteful splice.
Truss types — King Post (simple, short spans), Queen Post (medium spans), Fink/W (most common residential), Howe (heavier loads).
Lumber — board feet = width(in) × depth(in) × length(ft) / 12.
Saved Calculations
| Time | Type | Span | Pitch | Trusses | Total BF | Cost |
|---|
How to size a wood roof truss
Set the truss type (King Post for spans up to ~20 ft, Queen Post 20–30 ft, Fink/W for typical 24–32 ft residential roofs, Howe for heavier snow or storage loads), enter the wall-to-wall span, roof pitch in rise per 12″ of run, truss spacing on-center, and the building length along the ridge. Chord and web sizes use nominal lumber with NDS / IRC actual dimensions (2×4 = 1.5×3.5″, 2×6 = 1.5×5.5″, and so on); board feet are computed from the nominal dimensions per the lumber industry convention.
Buy 10–15 % extra lumber on top of the calculated quantity for cuts, defects, and bracing. The number is a takeoff for stick-built or shop-built trusses; pre-fabricated MPC (metal plate connected) trusses are usually quoted per truss instead. For sheathing on top of the trusses, run the plywood / OSB sheets calculator. To price the framing lumber itself, see the board feet lumber calculator.
Formulas
Rise = (span / 2) × (pitch / 12). Rafter length Lr = √((span/2)² + rise²) + overhang / cos(θ), with θ = atan(pitch/12). Number of trusses N = ceil(building length / spacing) + 1 (end trusses included). Board feet per piece = (nominal width in × nominal depth in × length ft) / 12. Total BF = 2 × rafter BF + bottom-chord BF + Σ web BF, then × N trusses.
Truss types — when to use which
King Post — one vertical web, simplest geometry, light loads to ~20 ft. Queen Post — two vertical webs plus a horizontal tie, 20–30 ft with central attic space. Fink / W — the standard residential truss, very efficient for 24–36 ft spans at 24″ o.c. Howe — verticals in compression, diagonals in tension, preferred for higher snow loads. For floor framing inside the building, cross-check the floor joist span calculator.
FAQ
16″ or 24″ o.c.? 24″ o.c. is standard residential spacing and is what most pre-fab Fink trusses are designed for. Drop to 16″ o.c. only for high snow loads, long spans, or to match joist spacing for deck or porch framing — for which see the deck calculator.
Why does my total BF look high? Top chord BF doubles the single-rafter length because the truss has two top chords. Heel cuts and seat cuts are not subtracted — the small overrun covers waste.
Are nails, plates, and bracing included? No. The total is structural lumber only. Add metal connector plates (factory) or plywood gusset plates plus 8d/10d nails on top of this estimate.
On install day
The number this calc drives is your purchase order: total board feet of #2 SPF/SYP chord and web stock, plus the truss count for crane scheduling. Three things estimators miss. Order length — lumber ships in 8/10/12/14/16/20 ft lengths, so a 28 ft span needs 16 ft top chords with a splice (or 20 ft stock for a clean cut); round up at the yard. Bracing — permanent lateral bracing per BCSI/IRC R802.10.3 (1×4 or 2×4 rows along the bottom chord and webs) is not in the takeoff; add ~5% of total BF. Pre-fab vs. site-built — for spans over ~24 ft, factory MPC trusses ($150–350 per truss delivered in 2025 USA) almost always beat stick-built once you add plate connectors, labor, and the engineer's stamp the inspector wants. Run a board-feet check to confirm yard pricing before you commit.
