Enter the building span, roof pitch, overhang and ridge board thickness and get the common rafter line length and the total rafter length including the overhang.
Common rafters only. Lengths are the theoretical slope lengths from the figures entered.
A common rafter spans half the building width. The horizontal run is half the span minus half the ridge board thickness, because the rafter meets the side of the ridge board rather than the centreline. The slope multiplier for a pitch of x per 12 is the square root of 144 plus x squared, divided by 12; multiplying the horizontal run by it gives the rafter line length from ridge to wall plate, and the overhang, also multiplied by the multiplier, is added to give the total length along the slope. The birdsmouth is the notch that seats the rafter on the wall plate: it affects where the rafter sits and its effective bearing, and its layout depends on the plate and framing details, so this calculator covers the length maths only and does not lay out the birdsmouth or any other cut. Results cover common rafters only, not hip or valley rafters.
Why is half the ridge thickness deducted? The rafter length is measured to the face of the ridge board, not the building centreline. Each rafter stops half the ridge board thickness short of the centreline, so that amount comes off the horizontal run before the slope length is calculated.
Is the overhang measured horizontally or along the slope? The overhang entered here is the horizontal projection, the distance the eave sticks out from the wall. The calculator multiplies it by the slope multiplier so the total rafter length is consistently measured along the slope.
Does this calculator lay out the birdsmouth cut? No. It calculates common rafter lengths only. The birdsmouth is the notch that seats the rafter on the wall plate, and its layout depends on the framing details of the specific build, so it is outside what a length calculation can settle.