Roof Truss Calculator

A roof truss is the triangular structural framework that supports your roof, and sizing it correctly is critical before ordering materials or starting construction. Enter your roof span, overhang, roof pitch, building length, and truss spacing into the Roof Truss Calculator to get the rafter length as the primary output. Secondary results include total run, rise, number of trusses, and roof angle in degrees.

ft

Total width of the building from wall to wall

in
ft

Distance roof extends beyond the wall

in
/12

Rise over 12 inches of run (e.g., 8/12 pitch)

ft

Length of the building for truss count

Distance between trusses (on-center spacing)

Results

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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Frequently Asked Questions

How do you calculate rafter length?

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.

What is on-center spacing for roof trusses?

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.

How many trusses do I need for my roof?

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.

What is roof pitch and how is it measured?

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.

What is the difference between span and run?

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.

Do roof trusses have to be evenly spaced?

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.

How do I calculate the rise of my roof?

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.

What factors affect roof truss design?

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.