Rafter Length Chart: Common Rafter Lengths by Pitch and Span (2026)

A chart is faster than a calculation when the roof is a standard size, and most roofs are. The table below lists the common rafter length, from the ridge center to the wall plate, for the pitches and runs that cover nearly every house in Waukegan and Lake County. Find your pitch down the left side, find your run across the top, and read the length where they meet. The run is half the building span on a gable roof, so a 28 ft span is a 14 ft run and a 24 ft span is a 12 ft run. If your roof is not on the chart, the rafter calculator works out any span and pitch to the sixteenth of an inch.

The Rafter Length Chart

All lengths are ridge center to wall plate, in feet and inches, rounded to the nearest sixteenth of an inch. They do not include the ridge board allowance or the tail: subtract the ridge allowance for the cut length and add the tail for the board length, as explained below the chart.

Pitch	10 ft run	12 ft run	14 ft run	16 ft run
4/12	10 ft 6 1/2 in	12 ft 7 13/16 in	14 ft 9 1/16 in	16 ft 10 3/8 in
5/12	10 ft 10 in	13 ft	15 ft 2 in	17 ft 4 in
6/12	11 ft 2 3/16 in	13 ft 5 in	15 ft 7 13/16 in	17 ft 10 11/16 in
7/12	11 ft 6 15/16 in	13 ft 10 11/16 in	16 ft 2 1/2 in	18 ft 6 1/4 in
8/12	12 ft 0 1/4 in	14 ft 5 1/16 in	16 ft 9 15/16 in	19 ft 2 3/4 in
10/12	13 ft 0 3/16 in	15 ft 7 7/16 in	18 ft 2 11/16 in	20 ft 9 15/16 in
12/12	14 ft 1 11/16 in	16 ft 11 5/8 in	19 ft 9 9/16 in	22 ft 7 1/2 in

Three patterns in the chart are worth noticing. First, the steeper the pitch, the longer the rafter for the same run: at a 14 ft run, 4/12 pitch gives 14 ft 9 1/16 in and 12/12 pitch gives 19 ft 9 9/16 in, a difference of more than 5 ft on the same building. Second, the length grows faster than the run: doubling the run from 10 ft to 20 ft more than doubles the length at a steep pitch. Third, the 5/12 row is unusually round: a 12 ft run at 5/12 pitch is exactly 13 ft, because 5, 12, and 13 are a Pythagorean triple. Carpenters remember that one, because a 5/12 roof on a 24 ft span has a rafter length you can check without a calculator at all.

From Chart Length to Cut Length

The chart length is the starting point, not the board you cut. Two adjustments apply. The ridge board allowance shortens it: for a 1.5 in ridge board, subtract 0.75 in times the slope factor, which is 0.84 in at 6/12 pitch and 1.06 in at 12/12 pitch. The tail lengthens it: multiply the horizontal overhang by the slope factor, so a 12 in overhang at 6/12 pitch adds 1 ft 1 7/16 in. Put together for the most common roof in the chart, a 28 ft span at 6/12 pitch with a 12 in overhang: the chart length is 15 ft 7 13/16 in, the ridge-to-birdsmouth cut is 15 ft 7 in, and the total board is 16 ft 8 7/16 in. For a 24 ft span at 6/12 pitch with the same overhang: the chart length is 13 ft 5 in, the cut is 13 ft 4 3/16 in, and the total board is 14 ft 5 9/16 in. A 16 ft board is too short for the 28 ft span roof, while the 24 ft span roof fits a 16 ft board with room to spare, and a 14 ft board is too short for it, which is the kind of mistake the chart prevents.

Small Spans and Large Spans

For a small span, the chart still applies, but the numbers get short enough to check by hand. A 12 ft span has a 6 ft run: at 4/12 pitch the rafter is 6 ft 3 7/8 in, at 6/12 it is 6 ft 8 1/2 in, at 8/12 it is 7 ft 2 9/16 in, and at 12/12 it is 8 ft 5 13/16 in. A 16 ft span has an 8 ft run: at 4/12 pitch the rafter is 8 ft 5 3/16 in, at 6/12 it is 8 ft 11 5/16 in, and at 8/12 it is 9 ft 7 3/8 in. These are the spans of a garage, a porch, or a small addition, and the rafters are short enough that a single 10 ft or 12 ft board covers most of them with the tail included.

For a large span, the chart stops at a 16 ft run, which is a 32 ft span, and the lengths get long. A 36 ft span at 8/12 pitch has an 18 ft run and a 21 ft 7 5/8 in rafter, and with a 24 in overhang the total board is 23 ft 11 9/16 in. Boards over 20 ft long are a special order at most yards, so a wide roof needs the lumber ordered early and checked for straightness on delivery. If your span is wider than 36 ft, the roof usually has an intermediate support or a ridge beam instead of a plain ridge board, and the rafter calculation changes: the run is measured to the beam, not to the center of the building, and a structural engineer should size the beam.

When the Chart Is Not Enough

The chart covers common rafters on a gable roof. It does not cover hip rafters, valley rafters, or jack rafters, which run diagonally and are longer than the common rafter on the same roof. On a 28 ft span at 6/12 pitch the hip rafter is exactly 21 ft, against 15 ft 7 13/16 in for the common rafter, and the jack rafters step down from there. It does not cover shed roofs either, where the run is the full span instead of half of it: a 12 ft shed at 3/12 pitch has a 12 ft 4 7/16 in rafter, which is not on this chart at all. And it does not cover a roof where the two sides have different pitches, which happens on additions and saltbox roofs. For those cases, use the rafter calculator for the common lengths, the hip rafter post for the diagonal members, and the shed roof post for single-slope roofs.

One last check before you order lumber: measure your actual span. The chart assumes the span on the plans, but the building as built is often an inch or two different, and an inch of span is half an inch of run and more than half an inch of rafter length on a steep pitch. Measure wall plate to wall plate, read the chart, run the calculator as a second check, and cut one pattern rafter before you cut the rest. The chart gets you close in seconds; the tape measure and the pattern rafter get you exact.

Related reading: rafter calculator, plus our guides on how to calculate rafter length and shed rafter length calculator, and the roofing calculators hub.

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