Roof Truss Calculator

Roof truss calculator — what this returns

This tool calculates everything you need to order or compare prefab roof trusses for a residential or light-commercial build:

• Top chord length (the rafter portion of the truss, per side)
• Bottom chord length (the ceiling joist portion)
• Web member total length per truss, by truss type
• Truss count based on building length and spacing
• Total lumber linear footage for cost-comparison against site-built
• Per-truss and total prefab cost in USD, delivered

All values use the X/12 pitch convention standard in US residential framing per IRC 2024.

How the math works

A roof truss is a triangulated assembly of timber members held together with metal connector plates (gang nails), engineered to TPI 1-2014 (Truss Plate Institute, National Design Standard for Metal Plate Connected Wood Truss Construction). The geometry is fully determined by three numbers: span, pitch and depth.

Top chord (rafter portion). For a symmetric truss, each top chord runs from the wall plate to the peak. The horizontal distance is half the span; the actual length is half-span × slope factor:

slopeFactor = √(1 + (rise/run)²)
topChord    = (span / 2) × slopeFactor

For a 30 ft span at 6/12 pitch, that’s 15 × 1.118 = 16.77 ft per side. The mill orders the timber at the next standard length up — typically 18 ft 2x6 SPF #2 — and trims on the saw before plating.

Bottom chord. Equal to the span, plus a small allowance for the bearing notch at each end. Manufacturers usually leave the bottom chord as one piece (no splice) up to 32 ft, then go to a finger-joint or two-piece splice with a hardware plate.

Web members. The internal diagonals depend on the truss type. For the common Fink (W-truss), the four web members add up to roughly 0.62 × span in total linear footage. King-post (one vertical) is much lighter at ~0.30 × span; scissor trusses (vaulted ceiling) need extra material to break the bottom chord, ~0.70 × span; attic (room-in-roof) trusses with vertical side webs and a flat ceiling chord are the heaviest at ~1.10 × span.

Truss count. Trusses sit on the wall plates at fixed centers (typically 24” OC in the US), with one truss at each end wall. Count = ceil(building length ÷ spacing) + 1. A 40 ft house at 24” OC needs 21 trusses.

Truss types — when to use which

The calculator above covers the five most common residential cases. For long-span Howe trusses (warehouses, agricultural), you’ll get a custom-engineered quote rather than a typical-pricing estimate.

US pricing — 2026 reference data

Lumber for roof trusses runs on the Random Lengths Composite, which has averaged $440/MBF for SPF #2 KD through Q1 2026. Translated to delivered prefab Fink trusses on a 30-mile delivery radius, mid-Atlantic and Midwest yards are quoting $4.00–$6.50 per linear foot of span for residential 4/12–8/12 pitches under 40 psf snow load.

Worked example (the calculator’s reference test): a 30 ft span × 40 ft long single-family build at 6/12 pitch, 24” OC:

• 21 Fink trusses at $4.50/lf × 30 ft span = $135 each → $2,835 truss order
• Delivery: included up to 50 mi for orders above 15 trusses
• Crane + 3-person crew for set day: $850–$1,400
• Total prefab roof structure: $3,700–$4,200

By comparison, stick-framing the same roof with site-cut 2x6 rafters and a doubled 2x10 LVL ridge runs $2,400 in lumber + $1,800 in labor (16 hrs × 2 carpenters at $55/hr) = $4,200. The trusses cost the same delivered, but go up in 6 hours instead of 16 — most of the savings are framing time, not material.

Code and load requirements (US)

• IRC R802.10 — Wood trusses must be designed in accordance with TPI 1, with a sealed truss design drawing for each truss profile delivered to the site. The sticker on the truss shows the design loads (top chord live, top chord dead, bottom chord live, bottom chord dead) and the deflection criteria.
• ASCE 7-22 / IBC 2024 — Snow load (Pg from the snow load map), wind speed (Vult from the wind map), and seismic category drive the truss design. Manufacturers default to 30 psf snow / 90 mph wind unless you specify your jurisdiction.
• IRC R802.10.2 — Trusses cannot be cut, notched, drilled or otherwise modified in the field without engineer approval. Run plumbing and HVAC through web bays designed for it.
• IRC R802.10.3 — Permanent and temporary bracing per Building Component Safety Information (BCSI) B1, B2, B3 series. Prefab manufacturers ship bracing diagrams with each load.
• IRC R802.10.4 — Roof sheathing nailed to top chord per Table R602.3(1). 8d common at 6” edge / 12” field for 7/16” OSB at 24” OC.

Buying tips

1. Get three quotes — truss pricing varies more than lumber on the same MBF feedstock because mill scheduling and plate-press utilization vary. A 15% spread on a $20K order is real money.
2. Order overhang and tail design with the truss — having the manufacturer cut the rafter tails at the soffit profile saves a half-day of sub-fascia work.
3. Specify the heel height for ventilation. A 12” energy heel turns a code-minimum 1” baffle into a real 4” of insulation continuity at the eave — see the roof rafter calculator for the supporting member math.
4. Ask for the engineering — truss companies will email the sealed PDF design drawings before manufacture. Forward to the inspector to clear the rough framing inspection in one visit instead of two.
5. Schedule the crane in advance — small mills will help arrange a one-day boom truck rental for $750–$1,200 in metro areas. Lifting trusses by hand is allowed for spans under 24 ft; above that it’s a safety hazard and most yards won’t release the load without a crane on site.

Pair with these calculators

• Roof pitch calculator — convert between rise/run, degrees and slope factor before you order.
• Roof rafter calculator — sizes individual rafters with snow + dead load checks, useful when comparing site-built to prefab.
• Roof replacement calculator — full re-roof project, including tear-off, decking allowance and crew duration.

When you change any input above, the output updates immediately. Save or print the page once you’re happy with the numbers and bring them to your truss manufacturer for a comparable quote.