How to Flash a Dormer on a Roof

Flashing on a dormer, a structural addition that extends out from a building's sloped roof, sheds rainwater away from the dormer toward the building's main roof covering.
Dormer flashing prevents rainwater from leaking into the dormer.Dormer flashing prevents rainwater from leaking into the dormer.
Installers place the dormer flashing over the seams where the dormer's walls and roof meet the building's main roof prior to installing the dormer's siding and the building's roof covering. This keeps the flashing hidden from view while maintaining its waterproofing qualities. Flashing manufacturers usually make dormer flashing from strips of galvanized metal or aluminum.

Step 1

Measure the width of the dormer's horizontal wall where it meets the building's main roof, using a tape measure. Add 12 inches to the measurement and transfer the total to a piece of angle flashing, using a permanent marker to place the measurement mark.

Step 2

Cut the angle flashing along the measurement mark, using aviation snips.

Step 3

Measure six inches in from one end of the angle flashing with the tape measure and place a measurement mark on the angle flashing's vertical side, using the permanent marker. Repeat this on the angle flashing's other end.

Step 4

Cut the angle flashing's vertical side along the two measurement marks, using the aviation snips.

Step 5

Set the angle flashing on the building's main roof and butt its vertical side against the dormer's horizontal wall, keeping each 6-inch-long cut sections in line with the dormer's corners. Bend the angle flashing's 6-inch-long cut sections toward the main roof's peak until the cut sections match the main roof's slope. The angle flashing must lay flat against the dormer's wall and the building's main roof.

Step 6

Secure the angle flashing to the dormer and the building's main roof with 1-inch roofing nails, using a hammer to drive a roofing nail every 8 inches along each edge of the angle flashing.

Step 7

Measure the length of the dormer's sloped wall where the dormer meets the main roof, using the tape measure. Transfer the total to a piece of angle flashing and place a measurement mark at the appropriate spot on the flashing, using the permanent marker. Cut the angle flashing along the measurement mark, using the aviation snips. Repeat this procedure for the dormer's second sloped wall.

Step 8

Position one piece of the angle flashing against its respective dormer wall and secure it to the dormer wall and the building's main roof with 1-inch roofing nails, using a hammer to drive a nail every 8 inches along the flashing's edge. The bottom end of the sloped angle flashing overlaps the 6-inch-long cut end of the angle flashing secured against the dormer's horizontal wall. Repeat this procedure with the remaining piece of angle flashing.

Step 9

Measure the length of one of the valleys between the dormer's roof and the building's main roof, using the tape measure. Add 12 inches to the measurement and transfer the total to a piece of valley flashing, using the permanent marker to place a measurement mark at the appropriate place. Cut the valley flashing along the measurement mark, using aviation snips. Repeat this for process for the dormer's second valley.

Step 10

Set a piece of valley flashing on its respective dormer roof valley and align the bottom of the valley flashing with the bottom of the dormer roof. Fold the excess valley flashing over the peak of the dormer roof, using your hands to bend the flashing at the appropriate spot. Tack the valley flashing to the dormer roof and the main roof with the 1-inch roofing nails and hammer, spacing a roofing nail every 8 inches along the edge of the valley flashing. Repeat this procedure with the second piece of valley flashing.

Step 11

Measure 24 inches from one end of a piece of flat flashing with the tape measure and place a measurement mark at the appropriate spot, using the permanent marker. Cut the flat flashing along the measurement mark with the aviation snips.

Step 12

Center the 24-inch-long piece of flat flashing over the seam where the two pieces of valley flashing meet on the building's main roof. Drive a 1-inch roofing nail into each corner of the flat flashing, using the hammer.

Things You Will Need

  • Tape measure
  • Angle flashing
  • Permanent marker
  • Aviation snips
  • 1-inch roofing nails
  • Hammer
  • Valley flashing
  • Flat flashing

About the Author

Based out of Central Florida, Robert Sylvus has been writing how-to and outdoor sports articles for various online publications since 2008. Sylvus has been a home improvement contractor since 1992. He is a certified HVAC universal technician.