Gambrel Roof Calculator

Gambrel Roof Dormer Calculator: Width, Height, and Placement

Headroom gain, dormer framing height, header size, and rough opening for a shed or gable dormer on a gambrel roof.

Headroom Gain
2.00 ft
Dormer Height
3.20 ft
Rough Opening
8.25 ft
Recommended Header
2× 2×10

Shed vs Gable Dormer

A shed dormer is a single-pitch box that lifts a horizontal section of the roof. It gives the most floor area per dollar and is the dormer of choice when the goal is usable second-floor space. A gable dormer is a smaller triangular roof popping out of the main roof. It looks classic, sheds snow well, and works best as a window dormer rather than a headroom-adding dormer.

Upper vs Lower Placement

Almost every gambrel dormer goes on the upper slope. The upper pitch is shallow (15–30°), which makes the cheek walls reasonable and the flashing standard. The lower slope at 60°+ is too steep: the dormer rafters would meet the lower roof at a near-90° intersection, requiring custom flashing and structural framing.

If the goal is more loft floor area, plan a shed dormer on the upper slope that spans most of the building length, set back at least 4 ft from each gable end for structural reasons. Snow drift accumulates on the upslope side of any dormer, so add an ice-and-water shield strip 36 in wide.

Which Placement Should You Pick?
Put the dormer on the upper slope unless you have a specific reason not to. The shallow pitch there takes standard flashing details and a normal cheek wall, while a dormer on the steep lower slope forces a near-vertical intersection that most framing crews have not built before. Save the lower slope for a small gable dormer meant purely for a window, not for headroom.

Frequently Asked Questions

›Shed dormer or gable dormer on a gambrel?
Shed dormers give more headroom and more floor area for the same opening size. Gable dormers shed snow and water better and look better from the street. On a gambrel the shed dormer is almost always on the upper slope; the gable dormer can go on either.
›Where do dormers go on a gambrel?
Most dormers go on the upper slope because the shallow pitch makes flashing simpler. A dormer on the steep lower slope is structurally tricky: the rafter sits almost vertical and standard dormer framing details do not apply.
›How much headroom does a dormer add?
A shed dormer 8 ft wide on the upper slope of a 24 ft gambrel typically adds 18–24 in of headroom at the dormer face. The exact gain depends on the upper pitch and how far the dormer cheek wall extends.

Related Calculators

Full Gambrel Calculator

Roof geometry.

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Attic Space

Calculate the headroom you start with.

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Framing

Headers and openings.

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Snow Load

Drift loading at dormers.

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