Roof Truss Calculator
Enter the roof span and pitch to find the rise, rafter length and how many trusses you need.
Geometry only — not a structural design. This tool gives the geometry of a simple, symmetrical common truss for planning and estimating only. It is not an engineered structural design. Any load-bearing truss must be designed or approved by a qualified engineer and meet your local building code.
For a simple symmetrical truss, the rise equals half the span times the pitch (rise per 12 inches of run). The top chord, or rafter length, is the square root of half-span squared plus rise squared. The number of trusses is the building length divided by the spacing, plus one.
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How to Use This Calculator
Enter the span — the full width the roof covers — in feet. Enter the pitch as the rise per 12 inches of horizontal run, the way roofers quote it: a 6/12 roof rises 6 inches for every 12 inches across, so you enter 6.
Add the building length and the truss spacing, given in inches on center (24 inches is typical). The calculator returns the peak rise above the wall, the length of each top chord (the sloping rafter), and how many trusses the roof needs.
How the Calculation Works
A common truss is a symmetrical triangle. The horizontal run to the peak is half the span. The rise is that run multiplied by the pitch expressed as a fraction — pitch divided by 12 — because pitch is defined per 12 inches of run.
The top chord is the hypotenuse of the right triangle formed by the run and the rise, so the Pythagorean theorem gives its length: the square root of run squared plus rise squared. The truss count divides the building length by the spacing and adds one for the truss at the far end.
rise = (span ÷ 2) × (pitch ÷ 12); rafter = √((span ÷ 2)² + rise²)
Worked Example
For a 24-foot span at a 6/12 pitch: the run is 24 ÷ 2 = 12 feet, and the rise is 12 × (6 ÷ 12) = 6 feet. The top chord is √(12² + 6²) = √180 = 13.42 feet before any overhang.
For a 40-foot building with trusses 24 inches (2 feet) on center: 40 ÷ 2 = 20, plus 1 = 21 trusses.
Assumptions and Limitations
- Covers a simple symmetrical common truss only — not hip, scissor or attic trusses
- Rafter length is the bare top chord, excluding any eave overhang
- Geometry for planning and estimating; not an engineered, load-rated design
- Any structural truss must be engineered and meet local building code
Who Uses a Roof Truss Calculator
Builders, framers, and DIYers planning a roof, shed, or deck cover use this to find the rise, rafter length, and number of trusses from the span and pitch. Enter your dimensions before ordering or cutting to lay out the roof, estimate lumber, and confirm the pitch gives the headroom and drainage you want. It turns a roof plan into concrete cut lengths and a truss count you can buy from.
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Open calculator →Frequently Asked Questions
What does a 6/12 pitch mean?
It means the roof rises 6 inches for every 12 inches of horizontal run. Enter the first number — 6 — as the pitch. A 4/12 roof is shallower, a 12/12 roof is a 45-degree slope.
How do I find the rafter length of a truss?
It is the hypotenuse of the run and rise: the square root of half-span squared plus rise squared. For a 24-foot span at 6/12, that is √(12² + 6²) = 13.42 feet before overhang.
How many trusses do I need?
Divide the building length by the truss spacing and add one for the end truss. A 40-foot building at 24 inches on center needs 40 ÷ 2 + 1 = 21 trusses.
What spacing should roof trusses be?
Twenty-four inches on center is the most common for residential trusses. Heavier loads or certain roofing materials may call for 16 inches; always follow the engineered design.
Is this calculator an engineered design?
No. It gives the geometry of a simple common truss for planning and estimating. Any truss that carries load must be designed or approved by a qualified engineer and pass local code.
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