The calculation, without hidden assumptions
A common rafter is right-triangle geometry, but practical planning also needs to distinguish building span from horizontal run, account for the sloped overhang and keep any ridge-board deduction visible. This calculator supports a symmetric gable or a single-slope shed, returns the slope factor and roof angle, and can estimate rafter positions from building length and on-center spacing without pretending to size the structural member.
How to use this calculator
Choose a simple gable or single-slope shed and enter the horizontal building span.
Enter roof pitch as rise per 12, optional horizontal overhang and ridge-board thickness. Ridge deduction applies to the gable run only.
If you want a quantity check, enter building length and rafter spacing to estimate positions per side and the total number of common rafters.
Where people use it
- •Shed, porch and simple gable roof geometry
- •Comparing roof pitches and overhang lengths
- •Rough common-rafter lumber quantity planning
Example: 24 ft gable at 6/12 pitch
A simple 24 ft gable has a 12 ft horizontal run before ridge deduction. A 6/12 pitch has a slope factor of about 1.118, so the common-rafter line to the wall is about 13.42 ft before adding overhang.
What the result does not assume
- •The result is geometric line length, not a finished cut list. Birdsmouth, plumb cuts, tail cuts, ridge connection details and field tolerances still need the actual framing layout.
- •This page does not select rafter size, species, grade, spacing or structural capacity. Use the applicable design documents, span tables and professional requirements.
Frequently asked questions
Is the roof run the same as the full building width?+
For a simple symmetric gable, the common-rafter run is approximately half the building span. A shed roof can use the full horizontal span.
Does the overhang add the same number of feet to the rafter?+
No. A horizontal overhang is multiplied by the roof slope factor to get its sloped line length.
Does this calculator size the rafter lumber?+
No. It calculates geometry and an optional count only; structural sizing requires load, span, species/grade and governing requirements.
Semantic next steps
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These links move to a different input, formula or project stage rather than a keyword variation of this page.
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