How do you flash a dormer?

A dormer is a small piece of roof that projects from or is not in plane with the main roof plane. It has a junction with the main roof and this junction is where the flashing is required.
Dormers can vary in shape, so the exact form of flashing required will vary. The type of flashing also depends on the roofing material, and whether the same material is on the main roof and the dormer roof.
If the main roof plane and the dormer are of the same material, the flashing can usually be the same as used on a similar junction of the main roof.

Important parts of a basic flashing are:

• it laps from under the upper or higher roof cladding so that water will run down onto it from the higher cladding, and it laps over the lower roof cladding so that water running down it will run onto the lower cladding, rather than underneath the lower cladding. Flashings that run parallel to the fall of the roof are a little different but follow the same principle: look at the path the water will follow.

• It is lapped a sufficient distance under and over the cladding to prevent water seeping back. This distance will depend on the roofing material, the material's profile, the strength of wind in the area and the rainfall of the area.
• It may also need a return or kickback on the flashing to prevent capillary action or wind driven moisture.

Flashing materials must be compatible with the roofing material. Water running over some materials will collect ions that may make materials that this water then runs onto be stained or corrode quickly.