Where two sections of roof intersect (such as the gable roof of a dormer and the main roof, or two wings of the house), they create a “valley”. This joint collects extra rainwater, and can be a great way for water to get into the house, so it needs extra protection. Here are a couple ways to make sure you’re covered.
There are basically two ways to flash a valley. You can leave the joint open, so the flashing is visible, or you can interweave the shingles over the flashing. The second method can be visually more appealing, but may deteriorate faster and is, in fact, less traditional. It should also be limited to roofs with pitches from 4:12 to 6:12. So we’ll concentrate on the open valley.
Install roofing felt along the joint, and cover this with metal valley flashing. This may be copper, galvanized steel, or rolled aluminum with a colored baked-enamel coating. The flashing is usually pre-creased to make it easier to install. Nail it in position so that the nails will be covered by the shingles.
By making the exposed flashing wider at the bottom, you ensure the water will flow smoothly down and off the roof and eliminate edges which can trap water and debris. Snap a chalkline so that it tapers toward the top at roughly 1/8″ per foot. You may also want to cut off the upper corners of the shingles on the flashing.