I have a customer who called concerning the copper gutters I installed two years ago. It was on a low pitch porch roof so after I installed the gutters the drip edge didn’t spill the water into them the way they should, a fact I didn’t realize untill it was too late, so I put a thin strip of aluminum flashing under the dripedge, and it worked. the question is will the aluminum react with the copper? does it matter if the materials are not touching (I don’t think they are any where). thanks!
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They will eventually react as the have are different. Just like in bolt building, one of them, if touching, will be the sacrifical element. Copper to copper would have eliminated that. That is why they use copper nails with copper flashing.
It does matter if they are touching. If they are not, you should be OK. The other poster was right that dissimilar metals react with each other (just like in a battery). But, the metals either need to be touching or immersed in a solution that conducts electricity (like saltwater as the other poster pointed out). Unless the house is underwater, the second scenario is not going to happen. So, if the metals don't touch (and are not in indirect contact via fasteners or something like that), you should be fine.
If the aluminum is above the copper, there should be little consequence. If the run-off from copper is going across the aluminum, the aluminum will eventually fail, but not fast enough to lose sleep over. The copper is more noble, so the copper gutters will not be harmed. Also, if the aluminum has paint on it (anything but mill finish alum.), it's effectively isolated from the copper. I've installed copper roofs over aluminum gutters numerous times with the admonition that the gutter's lifespan will be reduced, but have yet to replace the aluminum gutters with copper.
What "greencu" said.
I will add that the water wash from copper onto aluminum can eat the aluminum quite fast. Acid rain might be a factor here.
I don't do much carpentry, or roofing, but on service drops, where cu/al connections are common, I have seen the aluminum corrode badly in just a few years when placed down stream of bare copper.
I think you find that there will little effect on the aluminum,I have replaced thousands of feet of stolen copper downspout on churchs with little or no effects years later
The galvonic reaction will happen quite fast when galvinized is used.I have seen them rot out in a year.If you are worried a bitumous paint will insure no problems