How to Use This Truss Span & Spacing Calculator
Enter the building length (gable end to gable end) and the span, which is the distance the trusses cross from the outside of one bearing wall to the outside of the other. Choose the roof pitch, chord lumber, spacing, your dead and live or snow loads, and the overhang, then press Calculate. You get the truss count, each truss position, peak height, rafter length, load per truss, and a span check that shows the widest spacing staying inside a conservative planning range. The diagram above redraws to match your span, pitch, overhang and spacing every time you calculate.
Why This Matters
Roof trusses are usually the biggest single framing order on a small building, and they are fabricated to your exact span, so a wrong count or a wrong spacing assumption is expensive to fix. Take a 28 × 40 ft garage: at 24" on center it needs 21 trusses, but at 16" on center it needs 31. At a typical $90–$150 per truss, that ten-truss difference is $900–$1,500 before delivery and crane time.
Snow country raises the stakes. A site designed for a 50 psf ground snow load carries far more weight than one designed to the 20 psf minimum, which pushes truss lumber up a size or spacing down. Span is the other half of the question. A 2x4 truss that works at 24 ft can be badly overstressed at 36 ft, and builders often learn this only when the fabricator's quote comes back with 2x6 chords and a surprise price. Running the numbers first lets you compare layouts, sanity-check a quote, and talk to your truss supplier already knowing the likely count, spacing and load per truss.
How It's Calculated
All results follow simple geometry plus a rule-of-thumb span check. Safety-relevant values are rounded the conservative way: truss counts, loads, areas and lumber round up, and the planning span rounds down.
Rise = (span ÷ 2) × (pitch ÷ 12) | Angle = arctan(pitch ÷ 12)
Rafter = (span ÷ 2 + overhang) × √(1 + (pitch ÷ 12)²)
Load per interior truss = (dead + live) × spacing(ft) × (span + 2 × overhang)
End truss load = ½ of interior | Bearing reaction = ½ of truss load
The live load used is never less than 20 psf, the minimum roof live load in IRC Table R301.6. If you enter less, the calculator raises it and tells you. Lumber is the bottom chord, two rafters and four web members of a typical Fink (W) layout, plus 10% waste. Heel height is ignored.
Span check. The 2021 IRC (section R802.10) requires wood trusses to be designed to ANSI/TPI 1 with truss design drawings, so no prescriptive span table exists for them. This tool therefore uses a planning rule of thumb, not a code table:
Base: 32 ft (2x4), 48 ft (2x6) | Pitch factor: 2/12 = 0.70, 3/12 = 0.80, 4/12 = 0.90, 5/12 = 0.95, 6/12 and up = 1.00
Rounded down to a whole foot and capped at 60 ft
Lighter loads never increase the limit above the 30 psf baseline. A span above 90% of its limit is flagged as near the limit, and above 100% as exceeding it. The "suggested spacing" is the widest standard spacing at or below 90%. Local codes vary, and the truss manufacturer's engineered design always governs.
Worked Example
These are the default inputs: a 40 ft long building, 28 ft span, 6/12 pitch, 2x4 chords, 24" spacing, 10 psf dead load, 20 psf live load and a 1.5 ft overhang.
- Count: 40 × 12 ÷ 24 = 20 bays, so 21 trusses at exactly 24.00" spacing.
- Geometry: rise = 14 × 0.5 = 7.00 ft, angle = 26.6°, rafter = 15.5 × 1.1180 = 17.33 ft.
- Load: 30 psf × 2 ft × (28 + 3) ft = 1,860 lb per interior truss, 930 lb per end truss and 930 lb at each bearing of an interior truss.
- Roof area: 2 × 17.33 × 40 ≈ 1,387 ft². Lumber: about 87.9 ft per truss × 21 × 1.10 ≈ 2,031 linear ft.
- Span check: the planning limit is 32 ft for 2x4 at 24", so 28 ft is 88% of it, within the planning range. At 32" the limit drops to 27 ft, which is exceeded. The suggested spacing is 24".
Tips & Common Mistakes
- Measure span bearing to bearing. Using the overall footprint including overhangs overstates the span, while using interior clear width understates it.
- Use real snow and dead loads. Get the ground snow load from your building department or ASCE 7 maps. Clay or concrete tile can push dead load past 20 psf, well above the 10 psf default for asphalt shingles.
- Spacing wider than 24" needs rated decking. Roof sheathing, bracing and ceiling finishes must all be rated for the wider spacing.
- Don't forget extras. Girder trusses, hip or valley sets, gable frames and bracing are not in this count. Ask your supplier.
- Treat "near the limit" as a stop sign. Ask the fabricator for a quote with 2x6 chords or tighter spacing rather than hoping a borderline design passes.
Frequently Asked Questions
What is the standard roof truss spacing?
Most residential trusses are spaced 24" on center, with 16" and 19.2" used for heavier loads or stiffer ceilings. Spacing of 32" or 48" is uncommon and needs engineered decking. Your truss design drawings specify the spacing you must use.
How far can a 2x4 roof truss span?
Fink trusses with 2x4 chords commonly span roughly 30–40 ft depending on load, pitch and spacing. This calculator uses a deliberately conservative 32 ft baseline at 24" and 30 psf. Your manufacturer's engineered design is what counts.
How much does a roof truss cost?
Simple residential wood trusses often run $75–$250 each depending on span, pitch and region. Delivery and crane fees can add several hundred dollars. Get written quotes from local fabricators.
Can I use this for spans over 60 feet?
The planning check is capped at 60 ft, and any span above that is flagged as exceeding the range. Longer spans need engineered parallel-chord, bowstring or steel solutions designed by a structural engineer.
What is the difference between dead load and live load?
Dead load is the permanent weight of shingles, sheathing, insulation, drywall and the truss itself, often 8–15 psf. Live load covers snow, maintenance workers and similar temporary loads, at least 20 psf under the IRC. Trusses are designed for both combined.
Do I need an engineer or permit for roof trusses?
Most jurisdictions require a permit and stamped truss design drawings from the manufacturer. Your building department may also want a site-specific snow load. Check locally before ordering.