Roof Truss Calculator
Enter building length and truss spacing to calculate total truss count. Works for standard gable roofs at 16-inch and 24-inch OC spacing.
Get your truss count instantly. Enter building length and spacing below to calculate the number of trusses needed.
Total exterior building length along the ridge direction.
24-inch OC — modern residential standard.
Enter building length
to count trusses
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Truss count formula
Truss count is a deceptively simple calculation. The formula divides the building length by the truss spacing to find the number of gaps (bays), then adds one for the starter truss at the beginning of the run.
Truss Count
Count = (Building Length in inches ÷ Spacing in inches) + 1
Worked example. Building = 32 feet, spacing = 24 inches OC. Length in inches: 32 × 12 = 384″. Gaps: 384 ÷ 24 = 16. Total trusses: 16 + 1 = 17 trusses. The +1 accounts for the truss that sits flush at the starting gable wall — without it, you have 16 spaces but nothing to hold up the first gable end. This is the single most common ordering mistake on truss deliveries. Use the roof area calculator before ordering to confirm your total sheathing requirement based on the truss span and spacing.
Truss spacing and sheathing
Truss spacing determines both the structural rating and the sheathing requirement. 24-inch OC is the modern residential gold standard — it reduces lumber volume while maintaining structural performance when paired with minimum 1/2-inch OSB sheathing with H-clips. 16-inch OC is specified for heavy snow load zones or when the roof carries heavy tile or slate.
| Spacing | Min sheathing | Application | Rating |
|---|---|---|---|
| 24″ OC | 1/2″ OSB + H-clips | Standard residential, asphalt or metal | Recommended |
| 24″ OC | 5/8″ OSB or plywood | Tile or slate roofing (dead load premium) | Recommended for tile |
| 16″ OC | 7/16″ OSB | Heavy snow load zones (40+ psf) | Spec as required |
| 12″ OC | 3/8″ OSB | Structural specialty applications | Rare / engineered only |
Never cut, drill, or notch any web member of a prefabricated truss on-site. Pre-engineered trusses are designed as complete load-path systems — modifying any member instantly voids the engineering stamp and creates a structural failure risk. Any field modification requires a Registered Engineer’s repair design before work continues.
Truss cost benchmarks
Truss costs vary by span, complexity, and design. A standard Fink gable truss for a 30-foot span runs $150 to $250 per unit in 2026. Hip roof conversions using step-down hip trusses at the gable ends cost an additional $50 to $150 per set. For the full roofing material picture, the roofing material calculator covers squares, bundles, and underlayment. Use the roof pitch calculator to confirm the pitch your trusses will be engineered to before placing the order.
Frequently asked questions
Why do I add 1 to the truss count formula?
Dividing building length by spacing gives the number of gaps or bays between trusses. But a roof also needs a truss at the very starting point — position zero — to hold up the first gable end wall. That starter truss is the plus-one. Without it you have 16 gaps but no beginning, so you actually need 17 physical trusses to span the 32-foot building.
What is the difference between 16-inch and 24-inch OC truss spacing?
24-inch OC is the standard for most residential applications and uses fewer trusses, reducing material cost. 16-inch OC is required in heavy snow load zones (over 40 psf) and for very heavy tile or slate roofs. Using 16-inch OC adds approximately 50 percent more trusses to the order.
Can I modify a prefabricated roof truss on site?
Never. Prefabricated trusses are engineered as complete systems — every web member and connector plate is calculated. Cutting, notching, or drilling any member destroys the load path and voids the engineering certification. Any modification requires a written repair design from a Registered Engineer before the roof can pass inspection.
What is lateral bracing and why is it included in the estimate?
Lateral bracing is temporary and permanent 2x4 lumber nailed perpendicularly across the top chords and web members during and after erection. It prevents individual trusses from buckling sideways before sheathing is applied. SBCA guidelines recommend approximately 15 percent of total top-chord linear footage as lateral bracing material.
Do hip roofs use the same truss count formula?
The common truss count formula applies to the main field trusses running the length of the building. Hip roofs additionally require step-down hip trusses and jack trusses at each gable end. These are typically specified by the truss manufacturer based on your hip geometry and are not included in the basic count formula.
What size truss do I need for a 30-foot span?
Most residential truss manufacturers can engineer a standard Fink truss (the most common W-shape) for spans up to 60 feet. For a 30-foot span at 24-inch OC with a 6:12 pitch and standard 40 psf live load, a 2x4 chord truss with 3.5-inch connector plates is typically sufficient. Your local truss supplier will provide engineered drawings confirming the design for your local snow and wind loads.
How long does it take to install roof trusses?
A crane can set all trusses on a standard 2,000 sq ft residential home in 4 to 8 hours. Manual crane rental runs $500 to $1,500 per day. Framing crews brace and sheath after setting — another 1 to 2 days. The full framing phase from crane set to sheathed deck is typically 2 to 3 days on a single-story residential project.
References
Structural Building Components Association. (2024). Truss Handling, Installation, and Bracing Guide (HIB-91). SBCA.
International Code Council. (2021). International Residential Code, R802: Roof-Ceiling Construction. ICC.
MiTek Industries. (2024). Engineering Design Reference Manual. MiTek.