Metal valley underneath closed valley

I’ve had a roof replacement quote from an installer who prefers open (metal) valleys. Personally I prefer closed valleys because apparently they’re state of the art. So the installer suggested that they would close the valley, but still install the metal valley underneath “for better protection”.

Is this a sound strategy? Or is it likely to cause problems later on?

Not just a sound strategy, code requires a valley lining. I prefer ice & water barrier to metal but either will work.

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They will do both !

Install ice & water shield underneath new pre-painted metal valleys, then close the valley with shingles.

I am assuming this method doesn’t have any issues?

On valleys with a rough/choppy transition I’ve always used 24" aluminum to true the valley and protect the shingles. I&W over that.

That’s kind of like wearing a belt and suspenders. I don’t see the value and could create some minor installation issues with the additional thickness. If you have someone that knows what they’re doing, shouldn’t be a problem. But then again, if you have someone that knows what they’re doing, no need for IWS and valley metal either. A good roofer doesn’t need any but having one or the other is good in case of debris buildup in the valley.

Are you in Ontario? I see a lot of guys do that for closed valleys here.
If you are I recommend open valleys in our climate