Can copper flashing be used over existing galvonized flashing?

Hi- For the past week or so, and as I speak, we have been reroofing our 25-year old Colonial. Much of the house is brick, however, some parts do have cedar siding. In these areas there is a lot of roof-meeting-the-siding going on, and they had been originally flashed with galvanized steel. However, it is impossible to remove the flashing without severely damaging the cedar siding. We had choosen to use copper flashing on all parts of the house, and the roofer thought he probably could just put new copper flashing, for the new fiberglass roof, over the existing galvanized steel flashing, which was for the existing shake roof and is incompatable with the new roof.
So my question is, is it safe to flash a different metal on top of an existing one? We are worried about creating current or some unforseen problem with what should be a fairly mundane thing.
I'd appreciate a really quick reply from anyone who has an experience at all in this, as the roofing is being done as I write this.
Thank you.
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Jay Spenchian wrote:

Don't know the answer but your question strikes as being very apt and to add my two cents; With the prevalence of acid rain and/or a salty maritime climate the possibility of corrosion between the zinc galvanizing and copper; I believe the two metals are somewhat apart on the 'table of elements' the possibility of electrolytic corrosion may be high? However it may come down to practical experience and whether you can find unbiased advice that exactly fits your situation. Maybe we are talking 50 years before any corrosion would, in practical terms (i.e. your grandson will have to fix it?) have any effect. Whereas, if 5 years from now you have discoloured water leaking down the cedar siding due to holes in the 'new' flashing you will be most disappointed? Terry.
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Is it safe from the stand point of the new flashing being prematurely corroded?
Nothing will happen until the galvanized coating is gone, which probably will be pretty quick once it is in contact with the copper. After that the underlying steel will be next. Since the copper acts as a cathode it will not be consumed during this galvanic process (copper only supplies electrons the zine and iron supply the chemical energy during the oxidation process which causes the loss of material, also known as rusting).
Painting both the new and old flashing surfaces that will be in contact will help slow the corrosion process. Any nails which pierce both materials should be copper nails--otherwise the nails will quickly be destroyed by the concentrated galvanic currents present at the point of contact between the two materials created by the nails.
Regards,
John
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I agree totally and would have said the same. A layer or two of felt paper (tar paper) in between wont hurt either.
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Thanks guys for your help. I've yet to relay this to the roofer, but the question remains "what to do?". He claims that he has combined the two before and has not had an incident in twenty years since he originally did it. That case was a galvanized drip edge meeting copper gutters. It's somewhat of a conundrum, because we'd really love to have the more aesthically pleasing look of copper, but don't want to risk roof discolration or having to replace the flashing again. Option A might be to just get rid of the galvanized all together, but that would open the door to a lot of carpentry and painting work for new siding (not to mention cost!).
I don't want to put anyone on the spot, but would you recommend to just flash new galvanized steel over the old galvanized product?
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His experience is probably valid for that particular arrangement. If yours is similar and he looks like he will still be around in twenty years, then go for it.
Galvanized over galvanized would be a no brainer. But, if you have your heart set on the look of copper then he ought to be willing to humor you and do it up with all the bells and whistles the way you want.
Regards,
John
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snipped-for-privacy@aol.com (Jay Spenchian) wrote in message

Why not cover all the bases? Roofing felt is a more significant barrier than painting. While it's true that the copper will not be eroded by the galvanic action with the galvanize flashing; I would recomend placing a layer of 15# roofing felt between the galvanized flashing and the new copper. This will keep them separated and retard any galvanic action.
Use copper nails or 304 stainless steel, galvanic action with the new copper flashing will eat galvanized nails in a very short time.
Good Luck!
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