Dormer Calculator
Last reviewed How we calculate
A dormer is a small roof with walls of its own, standing out from the main roof to bring light and headroom into the space beneath. Estimating one means working out its roof, its front wall and the two triangular side walls, called cheek walls. This does all three for a gable dormer.
Gable dormer roof and walls
Roof area and wall areas for a gable dormer, including the triangular side walls.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Dormer roof area
- Front wall, including the gable
- Both side walls The triangular cheek walls.
- All dormer walls
- Length of the side walls Measured level, back from the front face.
The side walls run further back than the depth you entered, so these measurements do not quite fit together. Check the front wall height, the depth and the main roof pitch.
For a gable dormer whose eaves are level with the top of its front wall. The valleys and step flashing where the dormer meets the main roof are extra, and they are where dormers most often leak.
How the calculation works
The dormer roof is its width times the pitch multiplier, run back by its depth. The front wall is the width times the wall height, plus the gable triangle above it. Each cheek wall is a triangle: as tall as the front wall at the face, shrinking to nothing where the main roof has climbed to the dormer eave. How far back that is depends on the main roof pitch.
A worked example
A dormer 6 ft wide and 8 ft deep with an 8 in 12 roof and a 4 ft front wall, on a 12 in 12 main roof, has about 57.7 sq ft of roof. The front wall is 24 sq ft plus a 6 sq ft gable, 30 sq ft in all. Each cheek wall runs back 4 ft and covers 8 sq ft, so the dormer has 46 sq ft of wall to clad.
Where dormers leak
The valleys where the dormer roof meets the main roof, the step flashing along the cheek walls, and the apron flashing below the front wall are the three places dormers most often let water in. Adding a new dormer is structural work that cuts into the roof framing, so it normally needs a building permit and a design that shows how the loads are carried.
Frequently asked questions
What is a cheek wall on a dormer?
One of the two triangular side walls of a dormer, running from the front wall back to where the dormer meets the main roof.
Should I add the dormer to the main roof area?
Yes. Most estimates work out the main roof as usual and add the dormer roof on top, which also leaves a margin for the extra cutting at the valleys. The multi-section roof area calculator can add them together.
Does adding a dormer need a permit?
Usually, because it changes the roof structure. Your building department can tell you what drawings and approvals it needs.