The short answer
A gable is a triangle. Its area is the width at the base times the height to the peak, divided by two. If you know the roof pitch, you do not need to climb up to measure the height.
gable area = width × height ÷ 2
gable height = (width ÷ 2) × pitch ÷ 12
Step by step
- Measure the width of the gable wall along the ground, or at the eave line if the walls below are a different width.
- Find the pitch. Check the plans, measure it from the attic with a level and a tape (rise in 12 in of run), or use a pitch gauge app from the ground.
- Work out the height from the eave line to the peak with the formula above.
- Multiply and halve to get the area. Add the wall below the eave line separately as a rectangle.
Gable area by width and pitch
| Gable width | 4/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|
| 24 ft | 48 sq ft | 72 sq ft | 96 sq ft | 144 sq ft |
| 28 ft | 65 sq ft | 98 sq ft | 131 sq ft | 196 sq ft |
| 32 ft | 85 sq ft | 128 sq ft | 171 sq ft | 256 sq ft |
Areas are rounded to the nearest square foot and measured to the eave line, not including any overhang.
Worked example
A gable 28 ft wide with a 6/12 roof. Half the width is 14 ft, and 14 × 6 ÷ 12 = 7 ft tall at the peak. The area is 28 × 7 ÷ 2 = 98 sq ft. A house with two such gables adds 196 sq ft to the wall area. Enter the width, the height and the number of gables in the siding calculator together with your walls, and it returns the total in squares.
Other shapes you will meet
Shed walls and lean-tos
A wall under a single-slope roof is a trapezoid: average the two heights and multiply by the length. A 12 ft wall that is 8 ft high at one end and 10 ft at the other covers (8 + 10) ÷ 2 × 12 = 108 sq ft.
Dormers
Measure each dormer face as a rectangle plus a small gable triangle, and each cheek wall as a triangle: its length along the roof times its height at the front, divided by two. Dormers produce a lot of short cuts, so add extra waste.
Gambrel and hip roofs
A gambrel end wall is a rectangle with a shallow triangle on top plus two sloped sides; split it into rectangles and triangles and add them. Hip roofs have no gables, so all walls stop at the eave line.
Bay windows and bump-outs
Measure each face separately and add them. They have many corners, which means extra corner posts and trim.
From area to material
Add the gable areas to the wall areas, subtract openings and add waste. Gables waste more than plain walls: two 98 sq ft gables with 15 percent extra come to 225.4 sq ft, or 2.25 squares, before the walls below are counted.
Trim along the gable follows the roof slope, not the width. Each rake is half the gable width multiplied by the pitch factor. For the 28 ft, 6/12 gable, each rake is 14 × 1.118 = 15.65 ft, so you need about 31.3 ft of J-channel or rake trim per gable before waste.
Checking your numbers
If you have the house plans, the elevation drawings usually show the eave height and peak height, so you can compare them with your calculation. A quick check from the ground: a 12/12 roof has a gable exactly half as tall as it is wide, and a 6/12 roof one quarter.
Tips for gables
- Siding on a gable is cut at the roof angle on both ends of every course, so waste is higher than on plain walls. Plan 15 percent or more on houses with several gables.
- Many homeowners use a different material or color in the gable, such as shakes or vertical board-and-batten. Measure the gable area separately if you do.
- Gable vents reduce siding area, but trim around them uses extra material. Most people ignore small vents.
- Working at gable height means ladders or scaffolding. Set them on firm, level ground and keep three points of contact.
Frequently asked questions
How do I find the height of a gable without a ladder?
Use the roof pitch: half the gable width times the pitch divided by 12. A 28 ft gable with a 6/12 roof is 7 ft tall.
What is the area of a 24 ft gable with a 6/12 pitch?
The height is 12 × 6 ÷ 12 = 6 ft, so the area is 24 × 6 ÷ 2 = 72 sq ft.
Should I include the roof overhang?
No. Siding stops at the soffit and rake trim, so measure the wall itself. Soffit is ordered separately.
Do the math for your own project.
Open the siding calculator