Enter your Roof Span, Overhang, Roof Pitch, Building Length, and Truss Spacing into the Roof Truss Calculator to get your Rafter Length, along with the Total Run, Rise, Roof Angle, and exactly how many trusses you'll need for the job.
Rafter Length
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Total Run
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Rise
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Number of Trusses
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Roof Angle
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Rafter length is calculated using the Pythagorean theorem: √(rise² + run²). The rise is determined by the roof pitch and run distance, while the run includes half the span plus any overhang.
On-center spacing is the distance measured from the center of one truss to the center of the next truss. Common spacings are 12", 16", or 24" on center, with 24" being most common for residential construction.
The number of trusses depends on your building length and truss spacing. Divide the building length by the spacing (in feet) and add one. For example, a 40-foot building with 24" spacing needs 21 trusses.
Roof pitch is the steepness of a roof expressed as a ratio. It's measured as the rise (vertical distance) over a 12-inch horizontal run. For example, an 8/12 pitch rises 8 inches for every 12 inches of horizontal distance.
Span is the total horizontal distance from wall to wall that the truss covers. Run is half the span plus any overhang, representing the horizontal distance from the peak to the end of the rafter.
Yes, roof trusses should be evenly spaced to ensure proper load distribution and structural integrity. Standard spacing options are 12", 16", or 24" on center, chosen based on load requirements and local building codes.
Rise is calculated by multiplying the run distance by the pitch ratio. For example, with a run of 14 feet and an 8/12 pitch: Rise = 14 × (8/12) = 9.33 feet.
Key factors include roof span, pitch, overhang requirements, snow and wind loads, building codes, and the intended use of the space below. These determine the truss size, spacing, and reinforcement needed.