Flashing, along with underlayment and shingles, is a vital part of any roofing system.

How to Fasten a Roof Flashing After a Reroof

by Matt Brown

After a building has been reroofed, it may require repairs at some point. One potential failure point is roof flashing, which is metal sheeting that is installed beneath the shingles and above the roof’s underlayment. Specifically, flashing boots are installed around each of your roof’s vents; step flashing is placed anywhere the roof meets a vertical surface such as a wall, dormer or chimney; and valley flashing lines the valleys of your roof where two roof surfaces meet. In many cases, once the source of your leak has been pinpointed to a piece of flashing, a durable repair can be made.

Roof Vents

Measure the inside diameter of the roof vent and purchase a flashing boot that size.

Pry up the shingles around the roof vent flashing with a putty knife to expose the nails that hold the flashing in place. Typically, two nails at the bottom of the boot are exposed, two nails at the sides are covered by shingles, and two at the top secure both the boot and a row of shingles.

Remove the nails with a pry bar and slide the flashing boot up and over the vent pipe. This will cause the shingles above the flashing boot to come loose, but take care to leave them in place.

Turn the flashing boot 90 degrees and slide it up and over the vent pipe, taking care not to damage the shingles.

Cut a hole in a piece of roofing membrane equal to the diameter of the vent pipe. Apply a bead of silicone around the roof opening through which the vent pipe protrudes. Peel the sticky back off of the roofing membrane and slide it over the pipe. Apply another silicone bead around the opening.

Slide the new flashing over the vent pipe at the same angle you removed the old piece. Gently lift the top shingles and rotate the new boot 90 degrees so its top portion slips under the top shingles.

Hammer in two nails through the bottom portion of the boot and two through the sides. Nail the top shingles back into place through the flashing.

Apply a bead of silicone to the underside of the shingles you lifted and press them into place.

Step Flashing

Measure and cut a piece of step flashing with tin snips equal to the size of the damaged piece.

Attempt to slide the cut piece of flashing underneath the roof shingles and the wall siding, covering the damaged piece. If successful, slide the nose of your caulk gun underneath the shingle and apply roofing cement to hold the new piece in place.

Pry up the shingles and siding adjacent to the damaged flashing if necessary. Remove the damaged flashing, and nail the new piece to the wall -- not the roof. Replace the siding and shingles over the flashing.

Roof Valleys

Remove the shingles covering the valley beginning at the top. Each shingle is secured with eight nails – four through its own nailing hem and four that penetrate both the course above and that shingle.

Attach peel-and-stick roofing membrane to the valley. Lay the flashing into place. Bend the bottom of the flashing around the edge of the roof. Nail the flashing into place approximately every 10 inches, 1 inch from the edge of the metal. If more than one piece of flashing is required to cover the area, start at the bottom, use a chalk line to align the pieces, and overlap pieces per the manufacturer’s recommendation.

Use the roof’s existing shingles as layout guides for reinstalling shingles on either side of the valley. Avoid nailing through the new flashing, except for within an inch of the metal’s edge.

Items you will need

  • 4-inch putty knife
  • Pry bar
  • Roofing membrane
  • Silicone
  • Hammer
  • Roofing nails
  • Tape measure
  • Tin snips
  • Caulk gun
  • Roof cement
  • Chalk line


  • Working on a roof can be dangerous. Only work in dry conditions, wear proper footwear and, on steeper roofs, always wear a harness.

About the Author

Matt Brown has been writing professionally for more than 15 years. He shares his experience in home remodeling and do-it-yourself projects with his readers. Brown earned his bachelor of arts in communications from Marist College in Poughkeepsie, N.Y.

Photo Credits

  • Comstock/Comstock/Getty Images