Free Tool
Roof Pitch and Rafter Length Calculator
Enter the roof pitch and the building span. Get the rafter length along the slope, the roof angle and the plumb cut, with the overhang and the ridge board allowed for.
Roof Pitch and Rafter Length Calculator
Result
Preliminary planning only. This tool does not replace the drawings. The designer or engineer of record and the building permit set govern every size, span and detail.
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How the calculator works
- Slope factor. The square root of one plus the square of the pitch ratio, where the ratio is the rise divided by 12. A rafter on a 6/12 roof is 1.118 times its horizontal run.
- Run. Half the building span, less half the ridge board thickness, because the rafter bears on the face of the ridge board.
- Rafter length. The run times the slope factor, plus the overhang times the slope factor. The result is measured along the top edge of the rafter from the ridge cut to the tail cut. A bird's mouth, fascia thickness and cut details are not deducted.
- Roof angle and plumb cut. The angle is the inverse tangent of the pitch ratio. The plumb cut is the angle from vertical, which is 90 degrees less the roof angle.
- Ridge height. The rise from the top of the plate to the top of the rafter at the ridge is the run times the pitch ratio, before the height of the plumb cut below the rafter top. This is a geometry figure and not a measurement of the finished roof.
This is geometry only, with no code tables involved. The rafter size and the span limit come from the Ontario Building Code span tables and the drawings. This tool is for preliminary planning. The designer or engineer of record and the building permit set govern.
Frequently asked questions
What does a 6/12 pitch mean?
Six inches of rise for every twelve inches of horizontal run. The angle is the inverse tangent of 6 divided by 12, which is 26.57 degrees.
How is the rafter length worked out?
The rafter length along the slope is the horizontal run times the square root of one plus the pitch ratio squared. For a 6/12 pitch the factor is 1.118, so a 12 foot run gives a rafter length of 13.42 feet before the overhang and cuts.
Why is the run less than half the span?
A ridge board has thickness. Each rafter stops at the face of the ridge board, so the run is half the span less half the ridge board thickness. A ridge beam or a structural ridge is treated by the designer.
Does this give the longest rafter I may use?
No. The Ontario Building Code limits the horizontal span of a rafter by size, spacing and snow load. Use the roof rafter span calculator to check that, because the span is measured horizontally between supports.
What are the plumb cut and the level cut?
The plumb cut is the vertical cut at the ridge or the tail. The level cut is the horizontal cut where the rafter seats on the wall plate. On a framing square the plumb cut is marked at the rise per 12 on one leg and 12 on the other.
Is this a quote?
No. It is a planning tool. Contact us with your plans and we will arrange a free site visit.
For the choice between rafters and trusses, read Stick-Built vs Truss Roofs.
Related: Roof and truss framing, Roof rafter span calculator, Sheathing sheet calculator.
More free tools are on the framing tools page.
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