Estimate theoretical common or shed rafter geometry and full-length stock quantities from roof span or run, pitch, overhang, spacing, waste, and optional ridge-board material.
For a centered gable, roof run is half the building span. The pitch ratio determines vertical rise, roof angle, and the slope factor used to convert horizontal run and overhang into sloped rafter length.
Shed roofs use the entered run directly and have one rafter at each spacing position. Gable roofs have a pair at every position along the building length.
span ÷ 2 − optional half ridge thicknesseffective run × pitch rise ÷ pitch run√(effective run² + rise²)horizontal overhang × slope factor(ceiling(building length ÷ spacing) + 1) × roof-plane multiplierceiling(base rafters × (1 + waste rate))A 24-ft span at 6:12 pitch has a 12-ft run, 6-ft rise, 1.118034 slope factor, and about 13.42 ft to the wall. A 1-ft horizontal overhang adds about 1.12 sloped ft, for about 14.53 ft total.
A 20-ft gable at 24 in OC has 10 spaces, 11 rafter positions, and 22 common rafters. Ten percent extra rounds 24.2 up to 25 full rafters.
A 12-ft shed run at 4:12 uses a slope factor of about 1.0541. A 20-ft building at 24 in OC has 11 shed rafters before waste.
An 8 m gable span at a 1:3 ratio has a 4 m run, 1.333 m rise, and approximately 4.216 m rafter length to the wall.
Span crosses the full gable width; run is the horizontal distance for one roof plane.
The calculator converts plan overhang into its longer sloped extension.
Each rafter normally needs stock at least as long as its theoretical line length. Short boards are not pooled into rafters.
Use project plans and qualified guidance for rafter size, span, loads, connections, and code requirements.
Pitch is entered as rise per run, such as 6:12. A zero rise is mathematically supported and produces a slope factor of 1 and angle of 0 degrees.
When selected, the calculator subtracts half the entered ridge thickness from each gable run. It also shows raw half-span, adjustment, and effective run because field layout conventions may instead reference ridge centerline.
Maximum-spacing layout includes both ends. If building length is not divisible by spacing, only the final bay is shortened. Gables use two rafters per position; sheds use one.
Waste applies once to piece count. Purchased linear footage is stock count times stock length, and aggregate offcut is informational only; reuse is not assumed.
Calculated length is theoretical line length before detailed seat or birdsmouth layout. Saw-cut dimensions are not provided.
This calculator does not determine allowable span, rafter size, species, grade, snow or wind loads, birdsmouth limits, ridge-beam needs, bearing, connections, structural capacity, or code compliance.
Find horizontal run and rise from pitch, then use the hypotenuse. Convert any horizontal overhang with the same slope factor.
Run is the horizontal distance covered by one roof plane. On a centered gable it is half the span before an optional ridge adjustment.
Steeper pitch increases rise, slope factor, and rafter length for the same horizontal run.
Use a rise/run ratio of 6/12. The resulting slope factor is about 1.118034.
Enter horizontal overhang. It is multiplied by slope factor before being added to rafter line length.
The optional adjustment subtracts half the ridge thickness from each side's run, not the full thickness.
Spacing positions equal ceiling(building length divided by maximum spacing) plus one. Gables use a pair at each position; sheds use one.
Those values are treated as maximum spacing, with a shorter final bay when necessary.
No. It estimates geometry and material quantity only.
No. Verify the design, loads, member sizing, connections, and code compliance separately.