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Community and Q&A

Copper flashing over aluminum and zinc

nj_homeowner | Posted in General Questions on


How bad would it be for us to install 16 oz copper counter-flashing over the aluminum step flashing that roofers installed on our house?

We would like to use copper counter-flashing for the length of all our roof connections, but are concerned about galvanic corrosion of the aluminum (and some zinc nails?) by the copper being so nearby.

The metals would not be in direct contact.  We would separate them by adhering a layer of butyl-based flexible flashing tape over the step flashing.  The tape would cover all but about 1/4″ of the aluminum.  The aluminum is also coated with PVC, but exposed cut aluminum edges would be within 1/4″ of the copper sheet.  Both metals are within 1/4″ of the roof surface that will see plenty of water and weather.

Our cladding will be Hardie-Plank lap siding, but it’s not installed yet.

Is this a bad idea?  I have read that “small areas of aluminum” near “large areas of copper” are at high risk.  I have also read that direct contact is required, but also that water flowing over copper that comes into contact with bare aluminum can cause corrosion.

Is the amount of corrosion that would happen in this scenario enough that we should avoid doing this?  Or is the expected amount of corrosion so small as to be insignificant?

Thank you,


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  1. Expert Member
    BILL WICHERS | | #1

    If you nail through both materials then you’ve made a connection. Galvanic corrosion happens because you have inadvertently built a battery. Electric current from that unintentional battery running through water is what rots away the metal.

    Best would be to replace that aluminum step flashing. If you can’t do that, if you can paint any exposed areas with some aluminum primer (not all paints/primers will adhere to aluminum), you can at least minimize the galvanic corrosion you get.

    I’d use stainless steel fasteners here. Galvanized metal in contact with copper will have galvanic issues too. Stainless steel is commonly used as a transition material to interface between the two to keep corrosion at bay.


    1. nj_homeowner | | #3

      Thank you for the response. We would not be nailing through both materials because the copper is taller than the aluminum, so it would be nailed above the aluminum, with copper nails.

      The metals would not be in contact. They would be about 1/4" from each other at the roof.

      Water will flow and could sit there (eg snow), immersing the two types of metals. Is that enough to "make a battery?"

      1. Expert Member
        BILL WICHERS | | #5

        If you use butyl tape you’ll probably be ok. Make sure to tape any in-painted areas, edges and nail holes included. Even if you get some corrosion of the aluminum over time, the copper flashing above it is still giving you weather protection.

        Is there any reason you can just remove the aluminum step flashing and let the new copper flashing do the job?


        1. nj_homeowner | | #6

          We can replace the aluminum. It's just more work and cost. The copper counter flashing at these locations does not add any water protection at the bottom. If the aluminum below failed, we would have no protection.

  2. gusfhb | | #2

    While on other forums I frequently have issue with panicked cries of galvanic corrosion whenever dissimilar metals are used, this is a bad one. Copper over aluminum will destroy the aluminum. Heating installers tacked an oil line up against the Z flashing between the siding and trim on my last house, chunk of the flashing gone in less than 10 years[could have been sooner, who knows]

    I love copper, but in this case I would use aluminum, use coated if you want a different color.

    1. nj_homeowner | | #4

      Thank you. I wonder if you would have been fine if the materials were isolated by something durable such as butyl flashing tape?

      Ours would not be in direct contact but would be near each other and water will flow between them.

      1. gusfhb | | #7

        I have actually done this where I put copper flashing over exterior fir deck beams and existing aluminum flashing would have required stripping quite a bit of siding to replace for just one corner of contact.

        I only hesitate because my case was opposite yours, aluminum above copper. I think if you put I&W on the aluminum it would last a very long time, but if it fails...

        I guess I am not understanding the whole situation, is the copper already installed?

        In this situation I just cannot see the risk, since copper is usually used for longevity, or in contact with PT lumber. Longevity is at serious risk here and I you have not mentioned PT

        If you are not in contact with PT, the risk is higher than the reward IMHO

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