What prevents water from getting underneath these roof shingles?

Question

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?

Protip: steel valley flashing is much better if you have trees depositing their goodies on your roof. Lapped valleys have enough friction that debris likes to accumulate. — isherwood, Oct 7 '19 at 17:45

isherwood · Accepted Answer · 2019-10-07 20:33:01Z

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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.

isherwood

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answered Oct 7 '19 at 0:28

Lee Sam

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JRaef · Accepted Answer · 2019-10-07 17:38:23Z

1

Is anything special needed to prevent water from getting underneath the overlap?

Gravity... water doesn't flow up hill...

answered Oct 7 '19 at 17:38

JRaef

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That's all there is to it, just like with conventional metal valley flashing (and literally every shingle on the roof). – isherwood Oct 7 '19 at 17:42
@isherwood We know shingles leak, that’s why underlayment paper is required. For valleys, there’s a double layer required...the manufacturer’s do not rely just on gravity. – Lee Sam Oct 7 '19 at 19:50
Yes, but that underlayment exists under any valley style, and only as a safety measure in the case of ice dams, etc. It's irrelevant here in a discussion of shingling technique. – isherwood Oct 7 '19 at 20:35
@JRaef Actually Water does flow up hill on buildings due to driving wind and wind pressure. – Lee Sam Oct 7 '19 at 22:45

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