Free Rafter Length calculator with clear step-by-step results.
The rafter is the hypotenuse of a triangle whose base is half the span and whose height is the rise. The eaves overhang is measured horizontally but adds material along the slope, so it is multiplied by the slope factor rather than added directly.
Common rafter length
Length = square root of (run^2 + rise^2), where run = span / 2
Overhang along the slope
Sloped overhang = horizontal overhang x (rafter length / run)
Roof structure design, including member sizing for span, spacing, snow and wind loading, must comply with the applicable building code or be specified by a structural engineer.
Order longer. This is the finished length before the plumb cut, birdsmouth and any fascia detail, so add at least 150 mm per rafter for cutting.
It depends on the covering: slate and plain tile generally need 30 degrees or more, interlocking concrete tiles work from about 17.5, and membranes can go far lower. Check the manufacturer's minimum.