Gable / Triangle Area Calculator
The area of a gable is a triangle: half the base times the height. Add every gable, dormer and triangular section to the rectangular walls.
A gable is a triangle — half the base times the height. Each 30 ft × 8 ft gable is 120 sq ft, so 2 of them add 240 sq ft. Measure the width at the eaves and the rise to the peak, and add every gable, dormer and triangular section.
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A gable is the triangular wall under the two slopes of a roof. It is easy to overlook because it sits above the eaves line where the tape measure does not naturally reach — and forgetting it is the single most common reason a siding order comes up short. Dormers, porch gables and any triangular wall section follow the same rule.
The area of a triangle is half the base times the height. For a gable, the base is the width at the eaves and the height is the vertical rise to the peak (not the sloped rafter length). This tool multiplies one gable’s area by the number of matching gables so you can add the total to your wall rectangles.
Base and height, measured right
The base is horizontal — the span across the top of the wall. The height is vertical — straight up from the middle of the base to the ridge. Do not use the sloped edge of the roof as the height; it overstates the triangle. If you can only reach the slope, use the roof pitch to get the vertical rise, or measure the rise from inside the attic.
Formula
gable_area = count × (½ × base × height)
Height is the vertical rise from the eaves to the peak, not the rafter length. Gables of different sizes should be run separately and summed.
Worked example
Two identical gables, each 30 ft wide at the eaves and 8 ft to the peak:
- One gable: ½ × 30 × 8 = 120 sq ft
- Two gables: 2 × 120 = 240 sq ft
Add that 240 sq ft to the 1,400 sq ft of walls for a 1,640 sq ft gross area on the sample house.
Common gable mistakes
- Height is vertical, not sloped. Using the rafter length overstates the area — measure the rise straight up to the peak.
- Count every triangle. Main gables, cross-gables, dormers and porch peaks all get sided.
- Different sizes, separate runs. Only group gables that are truly identical; otherwise sum them individually.
- Gables raise waste. Angled cuts along the rake produce more offcuts, so a gable-heavy house leans toward the higher end of the waste range.
Reference table
Labeled planning snapshot — measure your actual layout. Waste covers cuts, corners, gables and mistakes.
| House layout | Waste factor |
|---|---|
| Simple rectangle, few openings | ~10% |
| Average house, some gables & corners | 10–12% |
| Complex — many corners, gables, dormers | 12–15% |
| Cedar shakes / shingles / diagonal pattern | 15%+ |