Excuse me if this is a dumb question. When you put a valley gutter on a iron roof what's stopping the water building backing up under the roofing iron? Like if it rains heavily or you get some kind of debris build up to stop the flow.. Are you just relying on water good flow so it doesnt happen or do you use some kind of membrane under the iron for the worse case senario... On a flat roof line that is not a pitch...

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