Here are some pictures of a typical roof leak and it's repair in a dead-end valley. (Click on the thumbnails to enlarge)
Not much to see, but under this neat appearing job is a large leak. So many roofers think that the more metal they hide, the better. Truth is, when the metal is buried, so is the exit for the water. The 'secret' to a good working roof system is in it's ability to shed water.
Three other roofers had worked on this before me. I found a mess when I opened it up. Peel and Seal wrapped the corner to no avail. There are three layers of metal on this corner! Sad thing is, it's not unusual to find these problems. Though I didn't bother to take pictures of everything, I removed shingles and step flashing from about 5' of that wall, and about 6' more around the corner. Then, I started step flashing my way back up to the corner.
Since this is in a high wind area, I put a dab of caulk near the top of the step flashing. This is all that's necessary to stop windblown water. As you can see, the next flashing hides the caulk.
The tops of opposing valleys are supposed to meet in the center, at the apex of the ridge. This one was 3" off for some reason.
All the shingles were caulked in place for some unknown reason. The caulk and the unclipped tops caused lots of water to get under the roof, as can be seen by the lighter streaks and the debris trails.
I used a scribe and valley tongs to get the angle and bend the end of the valley to fit the wall. Though it's not visible, the last step flashing did not go in until I reached this step. It and the valley are mated for fit before being secured. I use cleats to hold valleys in position. You are not supposed to put nails in the metal! The screw was installed by the owner because of all the prior leaks. This is how the first step flashing and shingle are placed around the corner. As added protection, the I&W go over the lip of the valley. See how the shingle is squared off. ( The exposed nail above will be hidden when the cap are finished)
Looks kinda odd, eh? This is called a 'Tinner's Wing'. It does serve a purpose. Remember the downspout? Without a downspout there, the wing would have been shorter. It's raining. See how the water freely exits the area?
Albert's Roofing Repairs | Slate and Window Flashing Co., Rich., Va.