Roof Framing Calculator
14.53 ft common rafter
Slope factor 1.12; hip/valley members: 19.5 ft (factor 1.5, the larger of the two). 19 rafters along your wall at 16″ OC.
- Common rafter length = (run + overhang) × √(1+(rise/12)²) = × 1.12. Pure geometry (measured along the slope, ridge-to-tail).
- Hip/valley members use the larger factor √(2+(rise/12)²) = 1.5 per foot of common run; applying the field-area factor to hips instead understates them (JLC field-guide convention).
- Lengths are geometry; rafter sizing (depth, species, grade, spacing for your span and snow load) is structural, so use IRC R802-class span tables or an engineer. This calculator refuses to size members.
- Birdsmouth seat cuts and ridge deductions change the cut length slightly; this is the theoretical line length.
Why This One Is Different
Before you cut a single rafter, you need one number: how long it has to be from ridge to tail. This calculator gets you there without a geometry lesson, using the horizontal run plus any overhang and the pitch you give it (the underlying math is a slope factor derived from pitch, if you're curious, but you don't have to work it by hand). It does that one job well and stops there: it won't tell you what depth or grade of lumber to cut, because that's a structural question answered by your jurisdiction's adopted code edition, not by geometry. What you get here is the ridge-to-tail length for common rafters at your pitch, plus a rafter count if you supply a wall length, with the birdsmouth seat cut and ridge deduction called out as small adjustments you'll make to the theoretical number at layout.
Where the Numbers Come From
- Your rafter's length along the slope comes from one clean piece of geometry, not a guess: common rafter length equals (run + overhang) × √(1 + (rise/12)²), the slope factor derived directly from the pitch, measured along the rafter's top edge from ridge to tail. Source: rafter-length slope-factor geometry, construction-estimating canon, labeled
- Rafter framing sits under three documents: the IRC (binding law once a jurisdiction adopts an edition), the AWC NDS (the engineering design values), and AWC's own span-table publications. This calculator sizes nothing, by design. Source: AWC / ICC code-adoption documentation
- Worked example: a 12-ft run with a 1-ft overhang at a 6/12 pitch gives a slope factor of 1.118 and a common rafter length of 13 × 1.118 ≈ 14.54 ft, the theoretical line length before birdsmouth and ridge-board deductions. Source: worked example, closed-form arithmetic
- A birdsmouth seat cut and the ridge board's thickness both shave a small amount off the theoretical line length calculated here: real but minor adjustments a framer makes at layout, not something this calculator estimates. Source: engine convention, labeled
More Questions
Why is my actual cut length a little different from the calculated length?
The birdsmouth seat cut and the ridge board's thickness both take a small amount off the theoretical line length. This calculator gives you that theoretical number as the starting point for layout.
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Formulas, waste factors, and cost models on this page follow the sourced conventions on our methodology page. Estimates only; verify with your supplier and local code.