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Thread: Zinc or Copper flashing to prevent roof algae

  1. #1
    Product R&D for a powertool manufacturer dgold's Avatar
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    Default Zinc or Copper flashing to prevent roof algae

    Hey folks,
    I have a colonial in the Baltimore suburbs with a four-sided hip roof with one central ridge along the center 1/3rd, and no valleys. It's covered in "architectural" asphalt shingles. The shady sides (the Northern sides) are beginning to show black streaks -- which from what I understand is a type of algae.

    The best way to permanently prevent this - from what I've read on the web - is to install copper or zinc flashing under the top course or ridges of the roof. Has anyone here done this successfully?

    Because I have a hip a roof, I'd also need to install flashing under my ridge caps as well - since my top course is a relatively short horizontal run in the center of the roof.

    Can I simply slip in a piece of flashing and then get in there with some roofing cement (using a caulk gun) every few feet to hold it in place? Any reason this would be a bad idea?

    Thanks in advance for any help.
    Not a pro, but happy to share my lessons learned whenever I can. This forum has been a fantastic resource along the way.

  2. #2
    DIY Senior Member jch's Avatar
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    We had the cap shingles on our gable roof redone this summer. We installed a 3-1/2" zinc strip (with 3/4" covered) sticking out from the cap shingles (i.e. 2-3/4" of exposed zinc). Too early to tell how well it works *but* I must say that the roof has always been cleaner "downstream" of the other metal flashings on our roof (skylights, chimney, stink stack).

    We're in the Pacific Northwest so zinc strips are pretty common here--seems like they provide about 8 feet of protection (downslope).
    ----------
    - John

  3. #3
    DIY Senior Member jch's Avatar
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    p.s. be careful about using copper flashings. The runoff can destroy aluminum gutters (galvanic corrosion). Zinc is far more compatible with steel roofing nails and aluminum gutters.
    ----------
    - John

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    Engineer Furd's Avatar
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    I had a house about ten miles north of Seattle and when I had it re-roofed I had the zinc strips added at the ridge cap. After several years the south-facing roof had no moss but then it never had any moss with the prior roof. The north facing roof, which did have a serious moss problem on the old roof still had a moss problem although maybe not as bad as before. After a bad windstorm the zinc strip was torn out from under the shingles for about ten to fifteen feet.

    So, from my experience with that house I would not advise using the zinc strip as a cure-all for moss or algae growth.

  5. #5
    DIY Senior Member jch's Avatar
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    I agree that it's not a cure-all.

    But I can say that here it seems that any moss on zinc-stripped roofs appears to be at least 8 feet down from the ridge.

    If you also installed a *second* zinc-strip mid-roof, then you'd probably get full roof coverage.

    Remember though, you need to maximize the amount of zinc exposed to the rainwater. Only put as much under the shingles as you need to cover the nail holes through the zinc (about 3/4").

    Now the question becomes: which is uglier--moss on your roof? or zinc-strips?
    ----------
    - John

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