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A saltbox roof is a gable with the ridge off-centre — a short slope over the tall front wall and a long slope that sweeps down over a lower back wall. The Saltbox Roof Calculator works each slope on its own: the horizontal run from the wall to the ridge plus the eave overhang, times that slope's pitch multiplier √(144 + rise²) ÷ 12, gives the rafter, and the rafter times the ridge length (the building length plus both rake overhangs) gives the slope's area. Because both slopes meet at one ridge, the back wall plate sits lower by the difference in their rises. The calculator also returns squares, shingle bundles at 3 per square with your waste allowance, the drip edge for eaves and rakes and the area of each gable-end wall. A 36 × 24 ft house with the ridge 8 ft back from the front wall at 8/12 has a 10.82 ft front rafter, a 20.43 ft back rafter, a back plate 5 ft 4 in lower, 1,187 sq ft of roof and needs 40 bundles with 10% waste.
A saltbox is a gable roof with the ridge off-centre: a short front slope over a taller front wall and a long back slope that comes down lower. Each slope is worked out on its own — its horizontal run plus the eave overhang, times its own pitch multiplier √(144 + rise²) ÷ 12 — and the two slopes meet at one ridge, so the back wall plate sits lower by the difference in their rises.
| Ridge from front | Front rafter | Back rafter | Back wall lower by | Roof area | Bundles (10% waste) |
|---|---|---|---|---|---|
| 12 ft (gable) | 15.62 ft | 15.62 ft | 0 ft 0 in | 1,187 sq ft | 40 |
| 10 ft | 13.22 ft | 18.03 ft | 2 ft 8 in | 1,187 sq ft | 40 |
| 8 ft | 10.82 ft | 20.43 ft | 5 ft 4 in | 1,187 sq ft | 40 |
| 6 ft | 8.41 ft | 22.84 ft | 8 ft 0 in | 1,187 sq ft | 40 |
A gable roof with the ridge off-centre: a short, often steeper slope over a two-storey front and a long slope that sweeps down over a one-storey back. The name comes from the lidded wooden boxes colonial New England kitchens kept salt in.
Treat each slope separately. Each rafter is its horizontal run (including the overhang) times its own pitch multiplier, and each slope's area is that rafter times the ridge length. A 36 × 24 ft house with the ridge 8 ft back from the front wall at 8/12 has a 10.82 ft front rafter and a 20.43 ft back rafter; with 12 in rakes the roof is 38 × 31.25 = 1,187 sq ft.
Divide the roof area by 100 for squares, add waste (10% is a common starting point, more with dormers or valleys) and multiply by the bundles per square on the wrapper — 3 for most architectural shingles such as GAF Timberline HDZ. The 1,187 sq ft example needs 13.06 squares and 40 bundles.
The difference between the rise of the two slopes. With the ridge 8 ft from the front and 16 ft from the back at 8/12, the front rises 5 ft 4 in and the back 10 ft 8 in, so the back wall plate sits 5 ft 4 in below the front plate.
Yes. Many saltboxes and lean-to additions use a steeper front and a shallower back. Keep the back slope at or above the minimum for your covering — 2/12 for asphalt shingles under IRC R905.2.2, with two layers of underlayment below 4/12.
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.