Roof Truss Calculator to estimate roof truss count, truss spacing, roof rise, pitch angle, top-chord length, material cost, and total project cost for preliminary roof framing plans.
Home & Construction Calculators
Calculate truss layout and basic roof geometry from building dimensions, pitch, and on-center spacing.
The count includes a truss at each end. This calculator assumes a centered, symmetrical gable roof and does not design structural members or connector plates.
About the Roof Truss Calculator
This roof truss calculator provides a preliminary truss count and simple symmetrical-gable geometry. It divides the building length into equal bays that do not exceed the selected maximum spacing, adds both end trusses, and estimates roof rise, pitch angle, top-chord length, and project cost.
Formula and assumptions
Required bays = building length in inches ÷ maximum spacing, rounded up. Truss count = bays + 1. Actual spacing = building length in inches ÷ bays. Horizontal run per side = building span ÷ 2. Roof rise = run × pitch ÷ 12. Pitch angle = arctan(pitch ÷ 12). Top chord per side = √(run² + rise²). Material cost = truss count × price per truss. Project total = material cost + labor, delivery, bracing, and extras.
Worked example
A 40 ft long building with a 24 ft span, 6:12 pitch, and maximum 24 in spacing requires 20 bays and 21 trusses including both ends. Actual spacing is 24 in. The roof rises 6 ft from wall plate to ridge, the pitch angle is 26.57°, and each theoretical top-chord side is about 13.42 ft. At $160 per truss plus $1,200 in other costs, the estimated project total is $4,560.
Measurement checklist
- Measure building length along the direction the trusses repeat.
- Use the outside-to-outside bearing span specified for the truss order.
- Confirm maximum spacing with plans and local code.
- Identify overhangs, raised heels, attic rooms, scissor profiles, and load requirements separately.
Manual calculation steps
- Convert building length to inches.
- Divide by maximum spacing and round bays upward.
- Add one truss to include both ends.
- Divide length by bays for actual equal spacing.
- Calculate rise, angle, and top-chord geometry.
- Multiply truss count by price and add related costs.
Frequently asked questions
Why is one added to the number of spaces?
A row of equally spaced trusses has one more truss than the number of bays because a truss is placed at both ends.
Does this calculator design a safe truss?
No. It estimates layout and simple geometry only. Truss member sizes, webs, plates, reactions, and bracing require engineered design for project-specific loads.
Does top-chord length include overhang?
No. The geometry runs from bearing to ridge. Add engineered heel and overhang details from the approved truss drawing.
Common mistakes
- Using full span instead of half-span for one top chord.
- Forgetting the end truss when converting bays to truss count.
- Allowing rounded spacing to exceed the permitted maximum.
- Ordering before confirming loads, heels, overhangs, and bearing details.