Thread: Gambrel Roof Geometry

A few people have asked about a Gambrel Roof Calculator so I've had a go at a test one.
Not many Gambrel roofs (or roof builders) around here, so I'd appreciate comments and suggestions from anyone who knows about this type of roof, on how the test calculator is set up, what's needed etc.

Gambrel Roof Geometry Calculator

It only works out the geometry, with the Gambrel fitting a semicircle.
It starts off as a half octagon with equal upper and lower length rafters.
Visit with Chrome, latest Firefox or IE10 and there are slider controls to edit the sweep angles and animate the diagram.

You can add a ridge thickness, and (what I've called a) purlin thickness to adjust dimensions if needed. (Is purlin the correct term here?)
If you check the 'Overhang' box, it overhangs the rafters the width of the end 'plumb cut', and shows a slider control to edit the seat width, adjusting dimensions to suit.
You can just extend the length of the lower rafter for extra overhang?

Also, to directly measure any horizontal or vertical dimension at scale in the upper diagram, left click, release and drag your mouse.

Thanks for any suggestions.

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