How to Install Roof Valley Flashing Where Two Valleys Meet

When working on a roof, keep in mind that water always goes downhill. Overlay flashing so that the water sheds over the flashing rather than running underneath it. When flashing roof valleys, start at the bottom of the roof and work up, laying the top course over the bottom and this will work to keep the roof from leaking. Use caulk, asphalt tar or code-required methods to seal any areas that might present opportunities for leaks, such as nail holes or along joints.

Felt or tar paper
Asphalt tar
Caulk
Roofing nails

Instructions

- 1
  Measure the roof valleys to determine how much flashing is needed. Include a 4-inch overlap of one flashing over the one below it as a minimum measurement.
- 2
  Install metal flashing after installing felt paper, but before installing shingles. Work from the bottom of the roof up in the single roof valley, nailing at the edge of the flashing. Flashing nails get covered after shingles are installed, but you can seal the nail holes with caulk or asphalt tar for extra coverage. Building codes generally require flashing to extend 12 inches beyond the center of the valley on either side.
- 3
  Note the point where the roof valley splits into two valleys and stop flashing installation at exactly that point. This may require cutting the flashing to fit. If desired, make a cut in the flashing and bend so a piece of flashing extends into each valley.
- 4
  Place a piece of flashing, first in one valley overlapping the single valley flashing below. Nail at far outside edges. Make cuts in the flashing to accommodate bends.
- 5
  Go to the next valley and install flashing, ensuring it overlaps any flashing below and the flashing coming in from the adjoining valley.
- 6
  Seal flashing seams with asphalt tar or caulk.
- 7
  Paint flashing, if desired to match roof shingle colors and to make the flashing blend in with the roof color.

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