By
eHow Home & Garden Editor
Difficulty: Moderately Challenging
Step1
Measure the length of the roof valley and calculate the amount of valley flashing you'll need. Include an extra 4 inches for each overlapping piece that you need to install.
Step2
Install valley battens on either side of where the valley flashing is installed. Attach the valley to the top flange and extend 3/4 of an inch over the edge of the flange. If the roof has a fascia, extend the valley past it by 3/4 of an inch as well. Scribe the valley to the fascia if needed.
Step3
Set roof valley flashing in a bead of caulking and use metal roofing nails to attach the flashing to the flange. Overlap each piece the required amount for the slope of roof you are installing. The minimum is 4 inches.
Step4
Account for the pitch of the roof when laying out the lap on the valley flashing. The lower the pitch the greater the lap needed to protect the roof from ice dams.
Step5
Spray granules on any cuts made to eliminate premature rusting of the cut material. Caulk in any gaps created by the installation process to avoid water infiltration underneath the installed flashing.