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The desired horizontal (plan-view) distance the roof overhangs the wall.
What you want in PLAN — the shadow the overhang casts at noon, so to speak — because that is what is usually specified and what the planner drew. The number this returns is longer, because the tail runs down the slope to reach that plan distance. Set it from the wall's outer face and decide before cutting whether the fascia sits on the tail's end or over it: that choice is worth the fascia's thickness on every rafter.
The main roof pitch, expressed as rise per 12 units of run.
The pitch of the plane the tail belongs to. Everything here is one triangle: the horizontal overhang is the run, the pitch gives the rise, and the tail is the hypotenuse — so the steeper the roof, the more rafter it takes to reach the same plan overhang. At 12/12 the tail is 41% longer than the overhang; at 3/12 it is only 3% longer, which is why shallow roofs forgive a guess here and steep ones do not.
Rafter tail length
2.236 ft
They open the calculator with your figures already in it
Roof Overhang Tail Length Calculator: 2.24 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- The rafter tail's along-slope length equals the horizontal overhang × √(1+(pitch/12)²), the same slope factor used for common rafter length
Inputs used
- Horizontal Overhang
- 2 ft
- Roof Pitch (rise per 12)
- 6
What this calculation does not cover
- This is the EAVE overhang, where rafters run down the slope. A gable rake overhang is carried on level lookouts out to a fly rafter, and a lookout's length is the horizontal projection with NO slope factor at all — cut rake framing to this number and every piece is long by the factor: 12% at a 6/12, 41% at a 12/12.
- Gives a length and says nothing about whether the tail can carry it. A rafter tail is a cantilever off the birdsmouth, and what limits how far it reaches is the wood left above the seat cut together with wind uplift on the eave. A wide overhang is a sail, and on an exposed site it is uplift trying to peel the tail off, not the weight of the roof, that sets the reach and the tie-down needed at the wall.
- Everything depends on where the horizontal overhang was measured from. Overhang is usually dimensioned from the outside face of the wall framing, sometimes from the sheathing or the finished cladding, while the carpenter cuts from the heel of the birdsmouth — cross those reference points and the eave lands out by the sheathing and cladding thickness on every rafter in the run.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- The rafter tail's along-slope length equals the horizontal overhang × √(1+(pitch/12)²), the same slope factor used for common rafter length
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