What the calculator measures
The common run is horizontal distance from the outside wall face to the ridge face. When you enter the full span, half the ridge thickness is removed from each half-span. Rise is measured vertically from that wall-bearing reference to the ridge reference. The common line length is the hypotenuse: √(run² + rise²). Overhang is entered horizontally, then converted to its length along the roof slope.
The displayed blank length adds line length, sloped tail, and your stock allowance. It does not silently add a ridge, fascia, backing, or saw-kerf deduction. Mark the reference faces on the worksheet before transferring dimensions to stock.
Birdsmouth: three dimensions that are often confused
The seat is horizontal bearing length. The heel drop is the vertical cut at the inside end of that seat: seat × tan(roof angle). Structural notch depth is measured perpendicular to the rafter edge, so this worksheet separately reports seat × sin(roof angle) and the actual rafter depth remaining. The editable percentage is a warning threshold only; permitted notches and required bearing depend on the applicable design, code, member, loads, and connection.
Regular hip, valley, and jack geometry
For equal-pitch roof planes meeting at 90° in plan, the hip or valley travels diagonally: its plan run is common run × √2. Its 3D line length is common run × √(2 + slope²). A theoretical jack schedule steps across the plate by the entered spacing and applies the common-rafter slope factor to each run.
Those are centerline/top-edge geometry values, not finished cheek-cut lengths. Actual hip, valley, and jack layout can require deductions for member thickness, ridge or hip support, side cuts, backing, unequal pitch, offsets, and connection details.
Planning boundary
This workbench solves geometry; it does not design a roof. It does not select lumber size, species, grade, spacing, fasteners, hangers, connections, bracing, sheathing, uplift resistance, snow/wind/dead/live loads, or allowable spans. Verify the drawing, local requirements, manufacturer instructions, and structural design. Measure the actual building and test one rafter before production cutting.
Frequently asked questions
Does rafter length include the overhang?
The “common line length” stops at the outside wall reference. “Total to tail plumb line” adds the sloped length of the entered horizontal overhang. “Blank length” adds the separate stock allowance as well.
Why is half the ridge thickness deducted?
With a centered ridge board and a full outside-to-outside span, each rafter runs to one ridge face rather than the roof centerline. Direct-run mode is available when you have already measured that horizontal run.
Can I use the hip length for a valley?
On a regular equal-pitch 90° intersection, their theoretical centerline geometry is the same. Their support, cuts, member widths, backing, and connection details are not necessarily the same.
Are the angles circular-saw settings?
No. They are clearly named layout-line angles relative to the member's top edge or level. Saw miter and bevel settings change with machine conventions and how the board is held.
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