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Calculate rafter length, birdsmouth cuts, HAP (Height Above Plate), and lumber requirements for your roof framing project.
From wall plate to ridge (horizontal distance)
Enter run to calculate rafter measurements.
Enter dimensions to calculate birdsmouth.
Enter dimensions to calculate cuts.
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A birdsmouth is a notch cut into a rafter that allows it to sit flat on the wall's top plate. It consists of two cuts: a seat cut (horizontal, parallel to the plate) and a plumb cut (vertical). The birdsmouth provides a secure bearing surface and prevents the rafter from sliding.
HAP is the vertical height of the rafter measured above the wall plate after the birdsmouth is cut. It determines how much of the rafter remains above the plate to carry loads. A minimum HAP of 2" is typically required for structural integrity, and the birdsmouth cut should not remove more than 1/3 of the rafter depth.
Rafter size depends on span, spacing, and load requirements. As a general guide: 2×6 for spans up to 10', 2×8 for spans up to 14', 2×10 for spans up to 18', and 2×12 for longer spans or heavy loads. Always verify with local building codes and span tables for your specific situation.
Rafters are individual framing members cut on-site, offering more attic space and flexibility for complex roofs. Trusses are pre-engineered, factory-built assemblies that install faster and typically cost less for simple roofs. Rafters are better for vaulted ceilings or finished attic spaces.
Collar ties are horizontal members that connect opposing rafters in the upper third of the roof. They resist wind uplift and help prevent rafters from separating at the ridge. Most codes require collar ties every 4 feet along the roof length. They're typically made from 1×6 or 2×4 lumber.
Rafter overhang on the slope is longer than the horizontal projection. To calculate: Overhang (on slope) = Horizontal overhang ÷ cos(pitch angle). For example, a 12" horizontal overhang on a 6:12 pitch (26.57°) equals 13.4" on the slope.