Dormer and obstruction loss calculator

Why a dormer costs more than its footprint

Dormers cast a moving shadow, not just a static footprint

Unlike a flat obstruction sitting flush with the roof, a dormer projects upward and throws a real shadow across the surrounding roof plane. That shadow moves throughout the day and grows longer in winter when the sun sits lower in the sky, a seasonal swing the seasonal shading variation calculator tracks month by month. A proper obstruction estimate needs both the dormer's own footprint and its shadow reach.

How shadow length depends on height, sun angle, and roof tilt

Over flat ground, shadow length is the obstruction's height divided by the tangent of the sun's altitude. A roof changes that, because a shadow falling up a slope that faces the sun lands on a surface rising to meet it, which shortens it: length equals height divided by the sum of the sun-altitude and roof-tilt tangents, divided again by the cosine of the tilt, with height measured vertically. A 4-foot dormer under a 25 degree winter sun reaches about 8.6 feet over flat ground but only about 4.9 feet up a 22.5 degree roof, which is why this calculator asks for roof tilt. To pick the sun altitude, use 90 degrees minus your latitude minus 23.4 degrees for noon on December 21, or run a specific morning or afternoon time through NLR's Solar Position Algorithm calculator. At the example values on this page, the dormer excludes about 66 square feet and costs 4 panel positions.

Why hard shading from a dormer hits harder than diffuse shade

A dormer creates hard shading, a solid, sharp-edged shadow rather than the soft dimming from clouds or haze. NLR's test of a partially shaded grid-tied string found that in one configuration a shadow cut power by more than 30 times its own size. That mismatch is why panels sitting inside a dormer's shadow zone often need their own string or MPPT input, or module-level electronics, separate from panels in full sun.

Dormers often face the wrong direction too

Beyond the shadow they cast, dormers frequently face a different compass direction than the main roof plane, which can make the dormer's own face a weaker candidate for panels regardless of shading. The dormer addition impact on solar calculator covers this from the roofing side, for anyone still deciding whether to add a dormer at all. Most installers prioritize the main roof surfaces first and treat dormer faces as secondary space at best, much as the skylight obstruction calculator treats another feature that competes with panels for the same roof space.

Where the displaced panels can go

Lost panel positions don't have to mean a smaller system. If another roof face has room, the multi-roof plane array splitter shows how to move the displaced panels there, and the usable roof area calculator shows how much room each face really has once setbacks are removed.