When roofing your home, you have to install roof flashing anywhere that the shingles come into contact with a fixture such as a chimney or vent, but also in what is called a roof valley. Installing flashing on a roof is necessary to prevent moisture from seeping in and rotting the underlaying materials and eventually causing leaking. A roof valley is a particularly sensitive area of a roof because it is a point where two roof faces meet. Valleys create a channel for rainwater to drain, so ensuring a proper waterproofed seal is vital to the life of your roof. Start by installing the roof underlayment over the wood sheathing. On top of this will go the flashing followed by the shingles or other roofing material.
Step 1: Lay Down Flashing
At the point where the roof faces come together at an angle and create a valley, measure the distance from the eaves to the ridge. Take a piece of aluminum flashing 18 inches wide and long enough to span this distance with a single piece, and fit it into the valley spaced evenly over the seam in the roof. Take the shears or tinsnips and cut the flashing to match the shape of the eaves and the ridge. Making sure it is flat against the roof on both faces, apply a band of roofing cement on the bottom outside edges of the flashing from bottom to top and set in the valley. Nail down the edges of the flashing over the cement up and down on both sides.
Step 2: Cover with Roofing Paper
Cut a strip of roofing paper 36 inches wide and, like the flashing, long enough to span the distance from the eaves to the ridge. Roll out this piece of roofing paper over the flashing and trim the edges at the bottom and the top in the same manner as the flashing. Staple or nail down this roofing material at its edges from top to bottom on both sides.
Step 3: Snap Chalk Lines for Guides
Position the hook at the end of the chalk line at the ridge 3 inches from the center seam on one side. String the chalk line down to the eaves and, held 3 inches from the seam on the same side at the bottom, snap the line. Repeat this process on the other side of the seam. When complete, the two lines should be 6 inches apart with the valley seam directly between them.
Step 4: Install Shingles
With the chalk lines set and the flashing and roofing material in place, you can now add shingles. Follow the appropriate application directions for the type of roof you have. What is important is when you get to the valley, do not shingle past the chalk line on either side. The valley seam should be visible after the shingles are installed.
By some accounts, having a base layer or roofing paper, flashing and an overlaying piece of waterproof roofing paper atop that is a bit much. Some simply place flashing on top of the roofing paper and shingle over that, while others use the heavy duty roofing paper over the base layer of paper. However you do it, the idea is the same: to prevent moisture from creeping in and underneath the shingles.