galvalume copper corrosion

E M
3/15/2011
I have an old house that I recently had re-roofed. I noticed that they left the existing copper valleys in place and put the galvalume valleys right over top of them. To me this sounds like a problem waiting to develop. What is frustrating is that I got this expensive new roof so I just don't have to worry about my roof. Is this something to worry about. Should I insist that they remove the roof and put some kind of barrier between the copper and the valley? If it does corrode through, any idea how long this will take. The roofer's initial response is that it shouldn't be much of a problem because of the paint on the galvalume and the lack of moisture between the two metals.
Info @windowsonwashington.net
An informed customer is our best customer.
3/16/2011
Tough to say without seeing the installation and really observing the amount of contact between the two metal. We have pulled apart roofs that left the old copper flashing intact and right up against unpainted galavalume metal and observed only minor corrosion. Obviously, the point of a 50 year roof is a 50 year roof so I get your frustration. Had he put some ice/water or underlayment between the two the point would be moot. I think you are probably okay if the metal is a painted galvanized and painted with a good kynar finish strength.
Ken Buchinger
NCI Building Systems, Inc.
3/17/2011
I would be concerned about this for two reasons. One, the Galvalume valley will only have a "wash coat" on the back side and two, there are proabably fasteners penetrating both valley layers, which will put "raw" metals directly in contact with each other. I would expect to see some corrosion at these fastener locations within a relatively short time.
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