Reference chart
Rafter Length Chart & Calculator
Common rafter length from the outside of the wall plate to the ridge, for building spans of 8 to 40 feet at the usual pitches, with a 1½-inch ridge deducted. Add your overhang in the calculator, which also gives the stock length to buy.
Common rafter length by building span and pitch
| Span | 3:12× 1.031 | 4:12× 1.054 | 5:12× 1.083 | 6:12× 1.118 | 7:12× 1.158 | 8:12× 1.202 | 9:12× 1.25 | 10:12× 1.302 | 12:12× 1.414 |
|---|---|---|---|---|---|---|---|---|---|
| 8 ft | 48.7″ | 49.8″ | 51.2″ | 52.8″ | 54.7″ | 56.8″ | 59.1″ | 61.5″ | 66.8″ |
| 10 ft | 61.1″ | 62.5″ | 64.2″ | 66.2″ | 68.6″ | 71.2″ | 74.1″ | 77.1″ | 83.8″ |
| 12 ft | 73.4″ | 75.1″ | 77.2″ | 79.7″ | 82.5″ | 85.6″ | 89.1″ | 92.7″ | 100.8″ |
| 14 ft | 85.8″ | 87.8″ | 90.2″ | 93.1″ | 96.4″ | 100.1″ | 104.1″ | 108.4″ | 117.7″ |
| 16 ft | 98.2″ | 100.4″ | 103.2″ | 106.5″ | 110.3″ | 114.5″ | 119.1″ | 124″ | 134.7″ |
| 18 ft | 110.6″ | 113.1″ | 116.2″ | 119.9″ | 124.2″ | 128.9″ | 134.1″ | 139.6″ | 151.7″ |
| 20 ft | 122.9″ | 125.7″ | 129.2″ | 133.3″ | 138.1″ | 143.3″ | 149.1″ | 155.2″ | 168.6″ |
| 22 ft | 135.3″ | 138.3″ | 142.2″ | 146.7″ | 151.9″ | 157.7″ | 164.1″ | 170.8″ | 185.6″ |
| 24 ft | 147.7″ | 151″ | 155.2″ | 160.2″ | 165.8″ | 172.2″ | 179.1″ | 186.5″ | 202.6″ |
| 26 ft | 160″ | 163.6″ | 168.2″ | 173.6″ | 179.7″ | 186.6″ | 194.1″ | 202.1″ | 219.6″ |
| 28 ft | 172.4″ | 176.3″ | 181.2″ | 187″ | 193.6″ | 201″ | 209.1″ | 217.7″ | 236.5″ |
| 30 ft | 184.8″ | 188.9″ | 194.2″ | 200.4″ | 207.5″ | 215.4″ | 224.1″ | 233.3″ | 253.5″ |
| 32 ft | 197.1″ | 201.6″ | 207.2″ | 213.8″ | 221.4″ | 229.9″ | 239.1″ | 249″ | 270.5″ |
| 36 ft | 221.9″ | 226.9″ | 233.2″ | 240.7″ | 249.2″ | 258.7″ | 269.1″ | 280.2″ | 304.4″ |
| 40 ft | 246.6″ | 252.2″ | 259.2″ | 267.5″ | 277″ | 287.5″ | 299.1″ | 311.4″ | 338.4″ |
Your numbers
Rafter, plate to ridge
13′ 4.16″
160.16″ line length
With overhang
14′ 5.57″
+ 13.42″ along the slope
Buy
16 ft stock
next even length
Run / rise
143.25″ / 71.63″
26.6° · factor 1.118
run = span ÷ 2 − ridge ÷ 2 = 143.25″; length = run × 1.118; overhang along slope = 12 × 1.118. Bird's mouth seat cut sits at the line length; the tail is the overhang.
Questions
How do I calculate rafter length?
Run × slope factor, where run is half the building span less half the ridge thickness, and the slope factor is √(rise² + 144) ÷ 12 for the pitch. A 24-foot span with a 1½-inch ridge at 6:12 has a run of 143.25 inches; times 1.118 is 160.2 inches from the plate to the ridge. Add the overhang along the slope: a 12-inch horizontal overhang adds 13.4 inches.
Is the length measured along the top or the bottom of the rafter?
Along the rafter's measuring line, which runs parallel to the edges from the outside of the wall plate (where the bird's mouth seat cut lands) to the face of the ridge. The chart figure is that line length; the total column adds the tail. The plumb cut at the ridge and the tail cut change the board's extreme length slightly, which the next-even-length stock allowance covers.
What size lumber and how far can it span?
That is a span table question, not a length one: it depends on species, grade, spacing, snow load and pitch. Common answers are 2×6 to about 10 feet of horizontal run, 2×8 to 13, 2×10 to 16, 2×12 to 19 at 16 inches on centre and moderate loads, but read the IRC R802 table for your case or ask the yard.
How much overhang?
12 to 24 inches horizontal is typical, more in hot climates for shade, less in snow country. The chart's total column uses 12 inches; the calculator takes any figure. Overhang is measured horizontally and converted to rafter length by the same slope factor.
Does the ridge board carry weight?
A ridge board in a conventionally framed roof does not; it is a nailing surface, and opposing rafters brace each other with ceiling joists or collar ties taking the thrust. A ridge beam in a cathedral ceiling does carry load and must be sized and supported. Either way its thickness comes off the run, which the calculator does.
Charts are computed from the formulas and code figures stated on the page (International Residential Code limits where cited). They are planning and take-off numbers, not engineering: species, grade, span tables and local amendments govern anything structural. Confirm with your inspector before you cut.