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The following roof valley bypasses the gutter and spills too much water over the adjacent roof overhang. I figure a bent piece of aluminum flashing installed appropriately could divert the valley water into that gutter. But how should the flashing be shaped and installed here?

(I know the flashing should be tucked under some of the shingles, but how far, and how should it be secured under them?)

Roof valley bypassing gutter

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  • Is that valley best-practice for shingles ? Round here there would be slope of metal or plastic guttering in the inside corner.
    – Criggie
    Commented Aug 21, 2021 at 0:23
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    @Criggie, lapped and woven valleys are occasionally seen in my area. They have a clean appearance, but they don't shed tree debris well at all. They should not be used where that's expected during the life of the roof. They do have metal or rubber membrane underneath.
    – isherwood
    Commented Jan 31 at 14:27

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The right solution here is a second gutter on the lower fascia. A cobbled-up flashing diverter will look hideous and collect debris, which your lapped valley already has a tendency to do. It also introduces an obstruction in an already sensitive and high volume area.

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