Roof geometry
Hip versus other
The form distinguishes a hip roof from “other” roof geometry. A hip roof slopes down on all sides and has no vertical gable end. The common rule is that a roof qualifies as hip when at least 90% of its total perimeter is hip. Otherwise it is “other,” which includes gable, flat, mansard, and combinations. Measure the roof perimeter at the eaves, including all roof levels.
Calculation
Measure the total horizontal perimeter and the hip perimeter. Divide hip perimeter by total perimeter. If the result is 0.90 or greater, classify as hip. Keep field notes and photographs of each elevation because geometry is easy to misjudge from the ground.
Worked example
A roof has a total perimeter of 240 ft. The hip portions total 200 ft. 200 ÷ 240 = 0.83, or 83%. That is below 90%, so the roof is “other,” even though most of it looks hip. The reasoning: the form uses a threshold, not a visual impression.
Traps
- Eyeballing a roof and calling it hip.
- Counting only the front elevation.
- Forgetting a lower roof section or porch.
- Treating a flat roof as hip because it is “all perimeter.”