Roof Truss Span & Spacing Calculator
Instantly calculate truss count, spacing, peak height, load, and lumber requirements.
| Truss # | Position (ft from end) | Bay Width (in) | Note |
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How to Use This Roof Truss Calculator
Enter your building length and width (span), select a roof pitch and truss spacing, then add your load values and overhang. Click Calculate to instantly see the number of trusses needed, their exact positions, total load per truss, and a full spacing layout table.
The diagram updates dynamically with your values — showing the peak height, span, and pitch angle at a glance.
Why This Matters
Getting the truss count right saves money and prevents structural failures. Too few trusses leads to undersized roof systems that can't handle snow loads or wind uplift. Too many wastes lumber budget and adds unnecessary weight.
For a typical 40×30 ft house with 24" on-center spacing, you'll need 21 trusses — but that number changes significantly based on pitch and spacing. A 16" spacing (common in high snow-load regions) on the same building requires 31 trusses. At $80–$200 per truss, that difference is $800–$2,000.
Contractors use this type of calculation when bidding jobs, ordering materials, and scheduling crane delivery. DIYers use it to double-check framing plans or verify quotes from lumber yards. Architects use it for preliminary estimates before detailed engineering.
How It's Calculated
Number of trusses:
- Trusses = ⌈(Building Length × 12) ÷ Spacing⌉ + 1
- The +1 accounts for the final truss at the end of the building.
Peak Height: Height = (Span ÷ 2) × (Pitch ÷ 12)
Rafter Length: Rafter = √((Span/2)² + Height²) — this is the hypotenuse of the roof triangle, plus the overhang added to each side.
Tributary Area per Truss: Area = Spacing (ft) × Span (ft). This is the roof area each truss supports.
Total Load per Truss: Load = (Dead Load + Live Load) × Tributary Area (lbs). This is the design load each truss must carry.
Tips & Common Mistakes
- Always add gable-end trusses — the first and last truss are structural end members, already included in this calculator's count.
- Don't confuse span with width — span is the horizontal distance between bearing walls, not the overall building footprint width including exterior walls.
- Snow load varies by region — use your local ground snow load from ASCE 7 or your local building code. 20 psf is a common starting point, but mountain areas can require 70+ psf.
- Overhang adds rafter length — even a 1.5 ft overhang on each side adds 3 ft to the total rafter length, which matters when ordering lumber or pre-engineered trusses.
- Always get engineered drawings — this tool is for estimating only. Structural trusses for permitted construction require stamped engineering drawings.
Frequently Asked Questions
What is standard roof truss spacing?
The most common spacing is 24" on-center for residential construction in North America. Some builders use 16" for extra strength or to meet specific snow load requirements. Spacing wider than 24" (32" or 48") is sometimes used with heavy timber or commercial trusses and requires specially engineered roof decking.
How much does a roof truss cost?
Pre-engineered wood trusses typically cost $75–$250 per truss for a standard residential roof, depending on span, pitch, and your region. Complex designs (scissors, attic, cantilevered) cost more. Delivery and crane fees add $200–$600. Always get quotes from local truss fabricators since pricing varies widely.
Can I use this for pole barn or commercial buildings?
Yes — enter the building length, width (clear span), and your specific load values. Commercial buildings typically use 4 psf dead load for steel roofing and site-specific snow loads. For spans over 60 ft, you'll need engineered parallel chord or bow-string trusses; a structural engineer must be involved.
What's the difference between dead load and live load?
Dead load is the permanent weight of roof materials — shingles, sheathing, insulation, and the truss itself (typically 8–15 psf). Live load includes temporary loads like snow, workers, or stored materials (typically 20–40 psf for residential). Both are used together to determine the total design load your trusses must safely carry.