Does rafter length include the overhang?

The line length the calculator shows is measured to the ridge and does not include the overhang by itself. This tool reports both: the line length to the ridge and the overhang added along the slope, so the total cut length already covers the tail. A 12 in overhang on a 6:12 roof adds about 13.4 in of board, not 12 in, because the rafter runs at an angle.

What is the difference between line length and actual board length?

Line length (the theoretical length) is the straight measurement along the top of the rafter from the ridge to the outside wall. The actual board you cut is longer because it includes the overhang tail, and slightly shorter at the peak after you subtract half the ridge thickness. Order lumber by the actual cut length, never the line length, or every rafter comes up short.

How much do I subtract for the ridge board?

Roof run is measured to the center of the ridge, but each rafter stops at the face of the ridge board, so you remove half the ridge thickness from the run-based length. A standard 2x ridge board is 1.5 in thick (its actual size), so you subtract 0.75 in. For a thicker LVL or doubled ridge, measure the real thickness and halve it.

Do I subtract the ridge thickness twice for a hip rafter?

No. The hip rafter is built off the same common run that already accounts for the ridge, so you do not deduct the ridge a second time. A hip runs the 45-degree diagonal between two equal slopes, which makes it longer than a common rafter of the same run — for a 10 ft run at 6:12 the common line length is about 11.2 ft while the hip is 15 ft.

How deep can a birdsmouth seat cut be?

A common rule, in line with IRC notching limits, is to keep the seat cut to no more than one third of the rafter depth. On a 2x10 rafter (9.25 in actual depth) the seat cut should not exceed about 3.08 in. Cutting deeper removes too much bearing material and can fail inspection. If a wide wall plate forces a deeper seat, use a deeper rafter instead.

How do you calculate rafter length from span and pitch?

Halve the span to get the run, then multiply the run by the pitch multiplier. The multiplier is the square root of 1 plus the square of pitch divided by 12. For a 24 ft span at 6:12: run = 12 ft, multiplier = 1.118, so the line length is 13.42 ft. Add the overhang along the slope and subtract half the ridge thickness for the final cut length. No trigonometry needed — the calculator does it for you.

Rafter Length Calculator

Calculate common and hip rafter length from your building span and roof pitch. Includes overhang, ridge deduction, and a birdsmouth seat cut check — no trigonometry required.

Building Width (Span) ft

Roof Pitch rise per 12

: 12

Eave Overhang in

Ridge Board Thickness in

Quick Presets

Common Rafter Cut Length14.47 ft

Line Length to Ridge13.42 ft

Hip Rafter Length18.00 ft

Overhang (Along Slope)1.12 ft

Ridge Deduction−0.06 ft

Pitch Multiplier1.118

Birdsmouth Seat Cut Check

Check that the seat cut stays within the safe depth for your rafter.

Rafter Board Depth in

Seat Cut Depth in

Seat cut is within the limit

A 3.00 in seat cut stays under the 3.08 in limit (one third of the board depth). The rafter keeps enough material to bear safely.

Estimates for planning only. Always confirm cut lengths against your framing plan and local building code before ordering lumber.

Common Rafter Length by Span and Pitch

Line length to the ridge (before overhang and ridge deduction) for the building widths and pitches people ask about most.

Span	4:12	6:12	8:12	12:12
20 ft	10.54 ft	11.18 ft	12.02 ft	14.14 ft
24 ft	12.65 ft	13.42 ft	14.42 ft	16.97 ft
28 ft	14.76 ft	15.65 ft	16.83 ft	19.80 ft
32 ft	16.87 ft	17.89 ft	19.23 ft	22.63 ft

Line length = (span ÷ 2) × √(1 + (pitch/12)²), measured to the ridge centerline. Add your overhang and subtract half the ridge thickness for the final cut length.

How Rafter Length Is Calculated

Every rafter length comes from the run and the pitch. Here is each step the calculator runs.

The Formulas

run = span ÷ 2

Half the building width is the horizontal run for one rafter.

line length = run × √(1 + (pitch/12)²)

The square-root part is the pitch multiplier — it stretches the flat run into the sloped length.

cut length = line length + overhang − (ridge ÷ 2)

Add the overhang along the slope, then remove half the ridge thickness at the peak.

Hip Rafters Run the Diagonal

hip = √(2 × run² + rise²)

On an equal-pitch roof, the hip rafter runs the 45-degree diagonal between two slopes. Its horizontal run is the common run times √2, so a hip is always longer than the common rafter on the same building.

Because the hip is built off the same run as the commons, you subtract the ridge only once — never twice.

Worked Example: 24 ft Span at 6:12

Run = 24 ÷ 2 = 12 ft. Pitch multiplier = √(1 + (6/12)²) = 1.118.

Line length to the ridge = 12 × 1.118 = 13.42 ft. A 12 in (1 ft) overhang adds 1 × 1.118 = 1.12 ft along the slope.

A 1.5 in ridge board removes half its thickness: 0.75 in ≈ 0.06 ft.

Cut length ≈ 13.42 + 1.12 − 0.06 = 14.47 ft per common rafter.

Related Tools

Keep planning your roof framing with these related calculators and references.

Rafter Length FAQ

Common questions about measuring and cutting rafters.