Gable Siding: Area and Rake Length from Pitch
A gable is a triangle: its height comes from the roof pitch, its area is half the base times that height, and its two sloped edges take J-channel.
To calculate siding for a gable you need two numbers: the width of the wall under it and either the gable's height or the roof pitch.
The pitch is easier to get, from the roof plans or by measuring the rise over 12 in of level run. BYU-Idaho's estimating text: Slope
is the ratio of Rise/Run. In construction, it is measured in inches per foot
(BYU-Idaho). With it, the gable height is half
the width times the rise ÷ 12.
One gable worked through
A gable 24 ft wide under a roof of 6 in 12:
- Run = half the width = 12 ft
- Height = 12 × 6 ÷ 12 = 6 ft
- Area = ½ × 24 × 6 = 72.0 sq ft
- Rake (each sloped edge) = √(12² + 6²) = 13.42 ft; both sides 26.8 ft
The triangle rule is LP's: For triangle areas, measure from the base of the triangle to the top and multiply that number by half of
the base length.
(LP). The rake is the rafter length along the roof's underside line; Swanson Tool Co.'s table
for a 6/12 roof prints 13.42
for a 12 ft run (Swanson), and the same function gives 13.42 ft.
Gable area by width and pitch
Square feet of one gable, before openings and waste. Each cell also gives the gable's height; the rake per side is in the table after it.
| Width | 4 in 12 | 5 in 12 | 6 in 12 | 7 in 12 | 8 in 12 | 9 in 12 | 10 in 12 | 12 in 12 |
|---|---|---|---|---|---|---|---|---|
| 20 ft | 33.33.33 ft | 41.74.17 ft | 50.05 ft | 58.35.83 ft | 66.76.67 ft | 75.07.5 ft | 83.38.33 ft | 100.010 ft |
| 24 ft | 48.04 ft | 60.05 ft | 72.06 ft | 84.07 ft | 96.08 ft | 108.09 ft | 120.010 ft | 144.012 ft |
| 26 ft | 56.34.33 ft | 70.45.42 ft | 84.56.5 ft | 98.67.58 ft | 112.78.67 ft | 126.89.75 ft | 140.810.83 ft | 169.013 ft |
| 28 ft | 65.34.67 ft | 81.75.83 ft | 98.07 ft | 114.38.17 ft | 130.79.33 ft | 147.010.5 ft | 163.311.67 ft | 196.014 ft |
| 30 ft | 75.05 ft | 93.86.25 ft | 112.57.5 ft | 131.38.75 ft | 150.010 ft | 168.811.25 ft | 187.512.5 ft | 225.015 ft |
| 32 ft | 85.35.33 ft | 106.76.67 ft | 128.08 ft | 149.39.33 ft | 170.710.67 ft | 192.012 ft | 213.313.33 ft | 256.016 ft |
| 36 ft | 108.06 ft | 135.07.5 ft | 162.09 ft | 189.010.5 ft | 216.012 ft | 243.013.5 ft | 270.015 ft | 324.018 ft |
| 40 ft | 133.36.67 ft | 166.78.33 ft | 200.010 ft | 233.311.67 ft | 266.713.33 ft | 300.015 ft | 333.316.67 ft | 400.020 ft |
| Width | 4 in 12 | 5 in 12 | 6 in 12 | 7 in 12 | 8 in 12 | 9 in 12 | 10 in 12 | 12 in 12 |
|---|---|---|---|---|---|---|---|---|
| 20 ft | 10.54 | 10.83 | 11.18 | 11.58 | 12.02 | 12.5 | 13.02 | 14.14 |
| 24 ft | 12.65 | 13 | 13.42 | 13.89 | 14.42 | 15 | 15.62 | 16.97 |
| 26 ft | 13.7 | 14.08 | 14.53 | 15.05 | 15.62 | 16.25 | 16.92 | 18.38 |
| 28 ft | 14.76 | 15.17 | 15.65 | 16.21 | 16.83 | 17.5 | 18.22 | 19.8 |
| 30 ft | 15.81 | 16.25 | 16.77 | 17.37 | 18.03 | 18.75 | 19.53 | 21.21 |
| 32 ft | 16.87 | 17.33 | 17.89 | 18.52 | 19.23 | 20 | 20.83 | 22.63 |
| 36 ft | 18.97 | 19.5 | 20.12 | 20.84 | 21.63 | 22.5 | 23.43 | 25.46 |
| 40 ft | 21.08 | 21.67 | 22.36 | 23.15 | 24.04 | 25 | 26.03 | 28.28 |
Rakes and J-channel
Siding meets the roof along both rakes, and the cut ends are covered with J-channel. CertainTeed lists at rake edges of gables
among the places to measure for J-channel (CertainTeed). A pair of gable ends 30 ft wide at 6 in
12 adds 67.1 ft of J-channel before waste; Siding trim turns it
into pieces with the rest of the house.
Two things that change the answer
- CertainTeed measures a gable at its center and adds a foot,
Measure height at center (add 1' to allow for waste).
(CertainTeed). CladCount does not add that foot; its waste percentage covers the whole house instead. - Only the part of the gable you will cover counts. A gable over a porch roof or with a vent in it covers less: enter its height above the lower roof, or take the vent off as an opening.
Open in calculator to add your own gables, by height or by pitch, next to the walls of the house.