Roof Overhang Calculator – Eaves Projection and Rafter Extension
Calculate the rafter extension required for a specified horizontal roof overhang, and distinguish projected eaves depth from length measured along the roof slope.
Horizontal overhang is not sloping rafter length
A typical roof overhang is specified as its horizontal projection beyond a reference such as the exterior wall line. A rafter is sloping, so an overhang of 400 mm horizontally does not correspond to 400 mm measured along the timber.
Formula for a simple straight eaves extension
With roof pitch θ measured from horizontal, the theoretical extension along the slope is sloped extension = horizontal projection ÷ cos(θ). The vertical change across the same projection is rise or fall = horizontal projection × tan(θ). These formulae apply to a straight roof plane; they do not determine the final fascia, end-cut or support detail.
Worked example: 400 mm overhang at 35°
Sloping extension = 400 ÷ cos(35°) ≈ 488 mm. Vertical change = 400 × tan(35°) ≈ 280 mm. Therefore measuring 400 mm along the rafter would deliver a shorter horizontal overhang than intended.
Where does the overhang measurement start?
Specify whether your input is taken from the outer face of the wall, centerline of the wall plate, or another project datum. Include the position of the fascia and any end cut in the workshop drawing. A finished roof-edge projection may differ from the bare rafter extension.
Common mistakes
- Mixing horizontal eaves projection with sloping length.
- Adding the projection directly to a cut length without checking its datum.
- Confusing timber projection with the finished roofing overhang.
- Ignoring changes due to the rafter tail or fascia detail.
For roof slope conversion use the roof pitch calculator; for the complete member length see the rafter calculator. The geometry here is indicative, and final structural dimensions must follow the project design.
Continue with related roof calculations
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