Does this calculate a finished rafter cut length?

No. It calculates a geometric reference line. Birdsmouth, fascia and ridge cuts, lumber depth and the chosen reference edge must come from the framing layout.

Why subtract half the ridge thickness?

Each side of a symmetric gable ends at its ridge-board face, half the ridge thickness away from the centerline. The subtraction belongs in horizontal run before applying slope.

What does the reported rise mean?

It is the vertical change along the selected sloping reference line from the outside wall to the ridge face. It is not building height or a birdsmouth heel measurement.

Can I use the result to choose a 2×6 or 2×8?

No. Member selection needs loading, span definitions, spacing, species, grade, bracing and the applicable design requirements. This tool does not evaluate them.

Common rafter geometry

Rafter Length Calculator

Calculate common rafter reference length from gable span, roof pitch, ridge-board thickness and horizontal overhang. See body length, tail length and rise.

Ridge thickness adjustment
Separate body and tail
Saved geometry and CSV

Geometry and layout planning

Find the common rafter line

Save to this browser

Use a symmetric gable profile. Results describe reference lines and preliminary positions, not lumber sizes, joint details, load capacity or safe installation spacing.

Gable geometry

Outside-wall spanftAcross a symmetric gable, perpendicular to the ridge.Pitch rise per 12 in run/12Same pitch on both sides. Zero is allowed for a geometry check, not roof suitability.Actual ridge-board thicknessinDeducts half the actual board thickness from the horizontal half-span.Horizontal eave overhangftBeyond the outside wall, measured horizontally—not along the rafter.

Rafter reference line including tail

14.4646 ft

Geometric line length only—not a stock length or finished cut list.

Scaled reference triangle only. Heel, joints and overhang are not drawn.

Geometry and layout details

Adjusted horizontal run: 11.9375 ft; Half the span less half the ridge thickness.
Rise to ridge face: 5.9688 ft; Relative to the same sloping reference line at the outside wall.
Body reference length: 13.3465 ft; Outside wall to ridge face along the slope.
Tail reference length: 1.118 ft; Horizontal overhang × slope factor.
Roof angle from horizontal: 26.5651 °; Not a saw-bevel or compound-cut setting.

Before construction, use an approved framing design or manufacturer’s truss drawings. These results do not calculate member sizes, webs, plates, bracing, connections, birdsmouths or building-code compliance.

Transparent

Formula, intermediate values and rounding rules are visible.

Private by default

Saved project measurements stay in this browser unless you copy a share link.

Purchase-aware

Inputs, units and limitations stay visible so you can verify each estimate.

How a common rafter reference length is calculated

For an equal-sided gable, the horizontal run starts as half the outside-wall span. If rafters meet the faces of a ridge board, subtract half its actual thickness from that run. The calculation applies roof slope to this adjusted distance; it does not subtract ridge thickness directly from the sloping length.

A pitch of 6/12 means a rise of 6 units for every 12 horizontal units. The slope multiplier is the square root of 1 plus the squared rise/run ratio. Multiplying the adjusted run by that factor gives the body reference line. The horizontal overhang is converted independently to a sloped tail line and added afterward.

These are geometric reference dimensions. Lumber depth, the chosen measurement line, birdsmouth geometry, ridge and fascia cuts, and construction tolerances affect a real cut list. The result is not the length of stock to purchase, a permissible span or a structural member size. Use the framing design to establish those details.

Adjusted run

(span ft − ridge in ÷ 12) ÷ 2

Outside-wall span to ridge face for an equal-sided gable.

Slope multiplier

√(1 + (pitch ÷ 12)²)

Uses pitch as rise per 12 units of horizontal run.

Total reference line

(adjusted run + overhang) × multiplier

Includes tail geometry, but no allowances for cuts or stock selection.

How to use this calculator

1Confirm a symmetric gable and measure outside-wall span across the ridge.
2Enter actual ridge-board thickness in inches; use zero for an ideal centerline calculation.
3Enter pitch and the horizontal eave overhang.
4Review body and tail separately, save or export, then reconcile with the framing details.

Common mistakes

• Using the entire building span as one rafter’s horizontal run.
• Mixing nominal lumber thickness with actual ridge thickness.
• Entering sloped overhang when the input requests horizontal overhang.
• Treating roof angle as a compound saw setting or reference length as a finished cut length.

Use this calculator when

You need a common-rafter geometry check for a symmetric gable.
You know span, pitch and ridge thickness and want the assumptions visible.

Use another method when

You need hip, valley, jack, curved or unequal-pitch rafter geometry.
You need lumber sizing, safe spans, connections or a fabrication cut list.

Worked example

A 3–4–5 slope triangle with a tail

Use an 8-foot span, 9/12 pitch and zero ridge thickness. Each side has a 4-foot run and 3-foot rise, so the body line is 5 feet. A 2-foot horizontal overhang adds a 2.5-foot tail line.

5 ft body + 2.5 ft tail = 7.5 ft reference length