Roof rafter calculator
Lay out a purlin roof, read ridge position and rafter lengths, and review the main cut geometry before detailing.
Horizontal distance between the left and right eave plumb lines.
Length of the roof along the ridge.
Largest permitted center-to-center spacing between adjacent rafters.
Pitch of the left roof plane.
Pitch of the right roof plane.
Distance from the left foot purlin face to the left eave point.
Distance from the right foot purlin face to the right eave point.
What this tool solves
This tool is for roof geometry, not a structural proof. You define span, left and right pitch, overhangs, rafters, and purlins, and the tool resolves the main layout points from that input.
Use it when you want a fast picture of ridge position, rafter lengths, purlin positions, and checkpoint distances along each rafter before you move into detailing or structural checks.
- Ridge position can shift when left and right pitches differ.
- Overhang is measured from the foot purlin face to the eave point.
- Birdsmouth depth can be entered square to the rafter or plumb.
If you still need the basic trigonometry, use the triangle calculator. If you want a first timber sizing check afterwards, continue with the single-span beam calculator.