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The Gable Roof Calculator turns a building's length, width, pitch and overhangs into everything a gable roof needs: the rafter length, the ridge height above the wall plate, the ridge length, the area of both slopes, roofing squares, shingle bundles with waste, the area of each gable-end triangle and the drip edge for the eaves and rakes. The rafter is the horizontal run — half the width plus the eave overhang — times the pitch factor √(rise² + 144) ÷ 12; the roof area is two rafters' worth of rectangle as long as the ridge. A 40 × 30 ft house at 6/12 with 12 in overhangs has 17.89 ft rafters, a 7.5 ft rise to the ridge, 1,503 sq ft of roof (15.03 squares) and needs 50 bundles with 10% waste.
A gable roof is two identical rectangles leaning against each other. Each is as long as the ridge — the building length plus both rake overhangs — and as wide as the rafter, which is the horizontal run (half the width plus the eave overhang) times the pitch factor.
| Building | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| 30 × 24 ft | 877 sq ft | 930 sq ft | 1,000 sq ft | 1,083 sq ft | 1,177 sq ft |
| 40 × 30 ft | 1,417 sq ft | 1,503 sq ft | 1,615 sq ft | 1,749 sq ft | 1,901 sq ft |
| 50 × 30 ft | 1,754 sq ft | 1,860 sq ft | 2,000 sq ft | 2,166 sq ft | 2,353 sq ft |
| 60 × 40 ft | 2,745 sq ft | 2,911 sq ft | 3,130 sq ft | 3,390 sq ft | 3,683 sq ft |
Divide by 100 for roofing squares, add 10% waste for a plain gable, and multiply by three for bundles of standard shingles.
| Pitch | Angle | Rafter length | Ridge height |
|---|---|---|---|
| 4/12 | 18.4° | 16.87 ft | 5.00 ft |
| 5/12 | 22.6° | 17.33 ft | 6.25 ft |
| 6/12 | 26.6° | 17.89 ft | 7.50 ft |
| 7/12 | 30.3° | 18.52 ft | 8.75 ft |
| 8/12 | 33.7° | 19.23 ft | 10.00 ft |
| 9/12 | 36.9° | 20.00 ft | 11.25 ft |
| 10/12 | 39.8° | 20.83 ft | 12.50 ft |
| 12/12 | 45° | 22.63 ft | 15.00 ft |
Add the eave overhang to half the building width to get the run, multiply by the pitch factor to get the rafter length, then multiply by the ridge length (building length plus both rake overhangs) and by two for the two slopes. A 40 × 30 ft house at 6/12 with 12 in overhangs has 17.89 ft rafters and 2 × 42 × 17.89 = 1,503 sq ft of roof.
Rafter length = (half the span + the eave overhang) × the pitch factor. At 6/12 the factor is 1.118, so a 30 ft wide house with a 12 in overhang needs rafters about 16 × 1.118 = 17.89 ft long, measured along the top edge before the ridge and tail cuts.
Ridge height above the wall plate = half the span × rise ÷ 12. A 30 ft span at 6/12 rises 15 × 0.5 = 7.5 ft; at 12/12 it rises 15 ft. Add the rafter's height above the plate for the finished ridge height.
Divide the roof area by 100 for squares, add 10% waste for a simple gable, and multiply by three bundles per square. The 1,503 sq ft example is 15.03 squares, 16.53 with waste, which rounds up to 50 bundles.
About 10%. A plain gable has only straight rake cuts, so it wastes less than a hip roof (about 15%) or a cut-up roof with valleys and dormers (15–20%).
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.