Design / Structural
Rafter Length & Birdsmouth Calculator
Compute common rafter length, birdsmouth plumb and seat cuts, and overhang — from rise/run and pitch.
Calculator
What makes this different
Most rafter calculators give you the rafter length and stop. This one outputs the actual birdsmouth geometry (plumb cut, seat cut, depth) so you can lay it out on the lumber. It also checks for tail length vs total rafter.
Who it's for & when to use it
Framing a roof, building a shed, drawing rafters for a timber-frame porch.
How to use
- Enter the dimensions of your material or the parameters of your project.
- Adjust the optional knobs (waste factor, kerf, planer allowance) to match your shop practice.
- Click Calculate and read the output. Use the per-item breakdown for ordering, the totals for budgeting.
- Compare the result to your lumber yard's quote — the effective $/BF column is the apples-to-apples number.
Formula: Length = √(rise² + run²). Birdsmouth seat cut is along the run, plumb cut is along the rise. Tail length = √(rise² + run²) − run + overhang adjusted for slope.
Notes & tips from the shop
- Birdsmouth depth should not exceed 1/3 of the rafter depth — deeper weakens the rafter at the bearing point.
- Lay out the birdsmouth before cutting the plumb cut at the peak. It's easier to adjust in the right order.
- For long spans, use a ridge board the same depth as the rafter, not a structural ridge beam.
Frequently asked questions
What if my roof has two pitches?
Calculate each pitch separately and use a transition valley or break in the rafter line.
Do I need collar ties?
Yes if the ceiling joists aren't tied to the rafters (e.g., cathedral ceiling). Collar ties prevent rafter spread.
What's the difference between a ridge board and a ridge beam?
A ridge board is non-structural — it just provides a nailing surface. A ridge beam carries the load and needs support at the ends.
Limitations
Common rafters only. Hip and valley rafters need additional calculations.
All calculations happen in your browser — nothing you type is sent to a server.