Roof overhang calculator

How the roof overhang calculator works

The overhang is measured level, from the outside face of the wall framing to the end of the rafter, and the rafter tail is longer because it runs along the slope. Multiply the level overhang by the pitch factor, the same factor the rafter calculator uses for the main run:

• Rafter tail = overhang × √(1 + (pitch ÷ 12)²). For 16 in at 6/12: 16 × 1.118 = 17-7/8″.
• Drop = overhang × pitch ÷ 12: how far the top of the rafter falls between the wall line and the tail. 16 × 6 ÷ 12 = 8″.
• Plumb cut = rafter depth × the pitch factor: the vertical face the fascia covers. A 2x8 is 7-1/4 in deep^[3], so 7.25 × 1.118 = 8-1/8″.
• Soffit width = overhang − fascia thickness, for a level soffit that tucks behind a 3/4 in (1x) fascia.

Pitch is the rise per 12 in of run; if you do not know it, measure it with the roof pitch methods or convert an angle on the roof pitch calculator.

Rafter tail length chart

For a given overhang, steeper roofs need longer rafter tails and drop further below the wall line. The chart lists the tail length along the top edge of the rafter for common pitches:

Add the tail to the line length from the wall to the ridge to get the full rafter; the rafter calculator does both at once, and the roof calculator adds the overhang to the roof area for shingles and sheathing.

Examples: 12, 16 and 24 inch overhangs

Three common eaves, worked through with the formulas above:

• 12 in at 4/12 with 2x6 rafters: tail 12-5/8″, drop 4″, plumb cut 5-13/16″, soffit 11-1/4″.
• 16 in at 6/12 with 2x8 rafters: tail 17-7/8″, drop 8″, plumb cut 8-1/8″, soffit 15-1/4″.
• 24 in at 8/12 with 2x10 rafters: tail 28-7/8″, drop 16″, plumb cut 11-1/8″, soffit 23-1/4″.

The plumb cut is taller than the rafter is deep, so a 2x8 tail cut plumb needs fascia wider than 7-1/4 in to cover it; many builders square-cut the bottom of the tail or rip it to suit a stock fascia width.

Fascia, soffit and the eave height

Fascia covers the plumb cuts and runs the full length of each eave; soffit closes the underside. The drop matters when the eave must clear a window head, a door or a porch roof below:

• Fascia length is the eave length times the number of eaves: two on a gable roof, one on a shed roof, all four sides on a hip roof.
• Level soffit boxes run from the back of the fascia to the wall, so their width is the overhang less the fascia thickness.
• Eave height above a window is the wall plate height less the drop and the depth of the plumb cut.
• Drip edge and underlayment at the eave are set by the roofing rules in IRC Chapter 9^[2].

Overhang and the rafter design

The overhang adds load to the rafter tail and lengthens every rafter, so it belongs in the rafter design. The IRC rafter span tables measure the span level between supports, not along the rafter, so the overhang past the wall is not part of that span^[1]:

• Check the rafter size for the span on the rafter span chart; the overhang does not count toward that span.
• Keep the birdsmouth seat cut to no more than a quarter of the rafter depth (IRC R802.7.1), so the tail keeps its strength^[1].
• Large overhangs in high-wind areas need uplift connections designed for the site; ask your building department.

Common roof overhang mistakes

• Measuring the overhang along the rafter. The level overhang is the design number; the tail along the slope is longer.
• Measuring from the siding. Measure from the outside of the wall framing, then subtract the siding and sheathing if you want the visible soffit depth.
• Forgetting the plumb cut is taller than the rafter. At 12/12 a 2x8 plumb cut is about 10-1/4 in.
• Leaving the overhang out of the roof area. A 16 in overhang on both sides of a 40 ft gable adds about 119 sq ft of roof at 6/12.