How to Apply Step Flashing Around Roof Dormers

Measure the sides and ends of the dormer -- a projecting structure often containing a vertical window -- with a tape measure to figure the amount of flashing needed, typically in 8-inch-long sections, bent at 90-degrees into an L shape. Get flashing at least 5 inches wide on both sides. Remove any siding that has been installed on the dormer so the flashing will fasten to wall sheathing and siding will cover it. Make sure roofing paper has been installed properly..

Install corner flashing first. Use tin snips to cut a notch in the bottom of one end of a flashing strip. Cut along the horizontal bend line and at a 45-degree angle from the outside of that strip to the point where the top half is bent around the wall corner. Face the point of the triangle down the roof, with an open triangle inside between the flashing and the wall.

Nail the wall side of the flashing to the wall sheathing with roofing nails and a hammer. Put just one nail at the top corner of each flashing end. Seal the edges of the flashing triangle with roofing cement and run another bead of cement at the corner where the next piece of flashing will begin.

Lay flashing across the bottom of the dormer, alternating a strip of flashing with a shingle, so every shingle side is covered with flashing and every flashing joint is covered by a shingle. Use a utility knife or big shears to trim shingles as needed to fit. Overlap the two dormer corners with bottom flashing.

Start installing step flashing up the dormer walls at a bottom corner. Cut a triangular notch in the top half of the first strip to bend around the corner and overlap the corner flashing. Overlap that piece with a shingle, then make a weave up the wall, laying down flashing, then a shingle so every shingle joint and every flashing joint are covered.

Step up both sides to the top of the dormer. Put step flashing at the top of the dormer depending on the style of dormer roof. Install step flashing horizontally along the seam of a shed or single-slope roof. Run step flashing up both sides of a gable roof to a peak. Overlap step flashing at a peak by cutting along the bend line so the top half goes up the wall and the bottom half bends over the peak. Do this on both sides of the peak and cover the peak with cap shingles.