I see a lot of houses In my area with roof edges where one side of shingles overlaps the other. For example:
Is anything special needed to prevent water from getting underneath the overlap?
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Yes, there are several things required to make a “valley” installation waterproof.
There are two ways to install valley shingles: 1) “open cut”, and 2) “closed cut”.
Here’s an illustration of each:
You have the “closed cut” system.