Common rafter anatomy — run, rise, length, overhang tail, and the ridge, seat, and tail cuts

A rafter is more than a hypotenuse: the run stops at the ridge face (½ the ridge thickness comes off the span), the overhang tail is cut at the same slope, and the plumb and seat cut angles come from the pitch.

A common rafter drawn to scale at a 6:12 pitch. The run is measured horizontally from the top plate to the ridge FACE — half the ridge board thickness is subtracted from half the building width, so the rafter does not reach the building centerline. The rise is the vertical height over that run, and the common rafter length is the sloped distance, equal to the run times the square root of one plus the pitch over twelve squared. Beyond the wall the rafter continues as an overhang tail cut at the same slope, with a plumb cut at the ridge, a birdsmouth seat notched over the top plate, and a plumb cut at the tail. — Source: IRC R802.3 (ridge) + right-triangle geometry

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A common rafter drawn to scale at a 6:12 pitch. The run is measured horizontally from the top plate to the ridge FACE — half the ridge board thickness is subtracted from half the building width, so the rafter does not reach the building centerline. The rise is the vertical height over that run, and the common rafter length is the sloped distance, equal to the run times the square root of one plus the pitch over twelve squared. Beyond the wall the rafter continues as an overhang tail cut at the same slope, with a plumb cut at the ridge, a birdsmouth seat notched over the top plate, and a plumb cut at the tail.

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  <a href="https://remodelcalculators.com/construction-diagrams/rafter-anatomy" target="_blank" rel="noopener">
    <img src="https://remodelcalculators.com/api/figure-embed/rafter-anatomy" alt="A common rafter drawn to scale at a 6:12 pitch. The run is measured horizontally from the top plate to the ridge FACE — half the ridge board thickness is subtracted from half the building width, so the rafter does not reach the building centerline. The rise is the vertical height over that run, and the common rafter length is the sloped distance, equal to the run times the square root of one plus the pitch over twelve squared. Beyond the wall the rafter continues as an overhang tail cut at the same slope, with a plumb cut at the ridge, a birdsmouth seat notched over the top plate, and a plumb cut at the tail." width="800" height="520" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/rafter-anatomy" target="_blank" rel="noopener">Common rafter anatomy — run, rise, length, overhang tail, and the ridge, seat, and tail cuts</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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Rafter Length Calculator

Free rafter length calculator: common, hip, valley & jack rafter lengths, plumb/seat cut angles, birdsmouth depth, and board count. Per IRC R802. No signup.

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