Hip Roof vs Gable Roof: Calculating the Area
The counter-intuitive part
People expect a hip roof to have more surface than a gable roof. It does not — not meaningfully. Both roofs cover the same footprint at the same pitch, and the surface area of any simple roof is:
roof area = footprint area × slope factor
A gable roof is two rectangles. A hip roof over the same rectangle is two trapezoids plus two triangles. Add the pieces up and, at a constant pitch, you land on the same number. What genuinely differs is everything around the field.
What actually changes
| Gable | Hip | |
|---|---|---|
| Field area | Footprint × slope factor | Same |
| Waste allowance | 10% | 15% — every course meets a hip on an angle |
| Ridge length | Full length of the building | Shorter, by roughly the building width |
| Hip length | None | Four hips, each run × hip factor |
| Ridge and hip cap | Ridge only | Ridge plus all four hips — much more |
| Rake trim and drip edge | Two rakes | None — eave drip edge all round |
| Labour | Lower | Higher: more cutting, more detail |
Hip length is not the slope factor
A common error is using the slope factor to compute hip lengths. A hip runs diagonally in plan, so it needs its own multiplier:
hip factor = √( 2 + (rise ÷ 12)² )
| Pitch | Slope factor (rafters, area) | Hip factor |
|---|---|---|
| 4/12 | 1.0541 | 1.4530 |
| 6/12 | 1.1180 | 1.5000 |
| 8/12 | 1.2019 | 1.5635 |
| 10/12 | 1.3017 | 1.6415 |
| 12/12 | 1.4142 | 1.7321 |
Multiply the hip factor by the horizontal run the hip covers in plan to get its true length. For a common hip where the run is half the building width, a 30 ft wide building at 6/12 gives 15 × 1.5 = 22.5 ft per hip.
Worked comparison
A 40 × 30 ft building at 6/12, slope factor 1.1180:
| Gable | Hip | |
|---|---|---|
| Field area | 1,342 sq ft | 1,342 sq ft |
| Waste | 10% → 1,476 | 15% → 1,543 |
| Squares | 14.8 | 15.4 |
| Bundles at 3/sq | 45 | 47 |
| Ridge | 40 ft | 10 ft |
| Hips | — | 4 × 22.5 = 90 ft |
| Ridge + hip cap to buy | 40 ft | 100 ft |
So the field material is nearly identical, and the cap material is two and a half times more. That is the real cost difference between the two shapes, along with the labour.
Using the calculator for a hip roof
- Enter the pitch and the building footprint as normal — the field area and squares are correct for a hip roof as well as a gable.
- Set the waste allowance to 15% rather than 10%.
- Compute the hip lengths separately with the hip factor above and add them to your ridge cap order.
- Order eave drip edge for the full perimeter, and no rake trim.
Convert between rise-over-run and degrees, then turn a footprint into real roof area — with the slope factor, rafter length, squares and shingle bundles you need to order.
Frequently asked questions
Does a hip roof have more area than a gable roof?
Essentially no. Over the same footprint and at the same pitch both have the same surface area — footprint times slope factor. What differs is the waste allowance, the much greater hip and ridge cap length, and the labour.
How do I calculate hip length?
Use the hip factor, the square root of 2 plus the square of rise over run, and multiply by the horizontal run the hip covers in plan. At 6/12 the hip factor is 1.5, so a hip covering a 15 foot run is 22.5 feet long.
How much waste should I allow on a hip roof?
About 15 percent, against 10 percent for a simple gable, because every shingle course meets a hip on an angle and the offcuts are rarely reusable.
- How to Measure Roof Pitch (Safely, From the Attic)
- How Many Shingles Do I Need? Squares, Bundles and the Slope Factor
- How Much Underlayment and Ice Shield Do You Need?
- What Is a Good Roof Pitch?