As an Amazon Associate I earn from qualifying purchases.
A shed dormer lifts part of a steep roof into a single, shallower slope so the attic under it gets full-height walls. Its roof runs from the top of the dormer front wall up to a tie-in on the main roof, usually a foot or more below the ridge. The main roof rises its pitch ÷ 12 for every foot in from the wall, so the tie-in sits where the main roof reaches the tie-in height, and the dormer pitch is the rise from the front wall to the tie-in over the horizontal distance between them. The Shed Dormer Calculator returns that pitch and whether shingles are allowed — at least 2/12 under 2021 IRC R905.2.2, with two layers of underlayment below 4/12 — plus the dormer rafter and tail, the roof area, the front and cheek wall areas, the floor that gains a 7 ft ceiling (the R305.1 minimum for habitable rooms) and the sheathing and shingle bundles. On a 28 ft wide house with a 12/12 roof, a 16 ft dormer with an 8 ft front wall over the outside wall and a tie-in 1 ft below the ridge pitches 4.62/12, has about 270 sq ft of roof, adds 112 sq ft of 7 ft headroom and needs 9 bundles.
A shed dormer roof runs from the top of the dormer front wall up to a tie-in on the main roof. The main roof is (house width ÷ 2) × pitch ÷ 12 high at the ridge and climbs pitch ÷ 12 per foot in from the wall, so the tie-in sits where the main roof reaches the tie-in height; the dormer pitch is the rise from the front wall to the tie-in over that horizontal distance. Shingles need at least 2/12 (2021 IRC R905.2.2) with two layers of underlayment below 4/12 (R905.1.1); habitable rooms need a 7 ft ceiling (R305.1).
| Main roof | Front wall | Dormer pitch | Shingles |
|---|---|---|---|
| 8/12 | 6 ft | 2.24/12 | Yes, double underlayment |
| 8/12 | 7 ft | 1.28/12 | No — roll roofing or membrane |
| 10/12 | 7 ft | 3.44/12 | Yes, double underlayment |
| 10/12 | 8 ft | 2.50/12 | Yes, double underlayment |
| 12/12 | 7 ft | 5.54/12 | Yes |
| 12/12 | 8 ft | 4.62/12 | Yes |
| 14/12 | 8 ft | 6.70/12 | Yes |
Find where the dormer roof meets the main roof (the tie-in) and how high and how far in from the dormer front wall it is. Pitch = (tie-in height − front wall height) ÷ horizontal distance × 12. On a 28 ft wide house at 12/12 with an 8 ft front wall and a tie-in 1 ft below the ridge, the dormer rises 5 ft over 13 ft — 4.6/12.
For asphalt shingles, 2 in 12 under 2021 IRC R905.2.2, with two layers of underlayment between 2/12 and 4/12 (R905.1.1). Mineral-surfaced roll roofing is allowed down to 1/12 (R905.5.2); anything flatter needs a low-slope membrane.
The floor under the dormer gets a ceiling at least as high as the front wall. On a 12/12 roof the old roof only reached 7 ft at 7 ft in from the wall; a 16 ft wide dormer with an 8 ft front wall over the wall line adds 7 × 16 = 112 sq ft of floor with at least 7 ft of headroom, the IRC minimum for habitable rooms (R305.1).
Far enough to frame and flash the joint into the main roof — commonly a foot or more below the ridge, so the main roof's ridge, ridge vent and upper rafters stay intact. A tie-in at the ridge needs the ridge itself reworked, often with a structural beam.
Dormer roof area ÷ 100 × 1.10 for waste × 3 bundles per square. The 16 ft dormer in the example has about 270 sq ft of roof with 12 in overhangs, so 9 bundles — plus step flashing along both cheek walls.
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.