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Roof Mildew

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  • Transcript

    LESLIE: Coming up, we have a call from Joe who has mildew on his roof.

    Joe, we want to help you clean it. Tell us about it.

    JOE: Well I went up and cleaned it with bleach and a scrub brush, believe it or not.

    LESLIE: Good start.

    JOE: And it looked great. And when I finished, I put a siliconizer on it, three coats, which is like a rubber when it dries. And the roof looked beautiful. It was white and shiny …

    TOM: Right.

    JOE: … and actually my utility bill went down for the air conditioning. And then I put up there also zinc strips …

    TOM: OK, so far so good.

    JOE:  … to prevent the future mildew.

    TOM: Now let’s talk a little bit about the zinc strips because I know why you did it and Leslie might know why you did it but I’m sure most of the audience does not know why you did it. So why did you put the zinc strips up?

    JOE: It supposedly prevents mildew from building up.

    TOM: Yeah, and do you know why? What happens is the rain hits the metal strips and some of the zinc washes off and becomes a mildicide and cleans the roof on the way down. So it does work; as does copper, by the way. Copper works well, too. And if you ever wonder – if you see like a brick chimney coming up through a roof and you see clean streaks of very clean shingle under the chimney, that’s because of the copper flashing.

    So where are you at now then, Joe?

    JOE: Well I did the strips and this was like a year ago and now I’m getting mildew buildup again and I don’t understand why.

    TOM: The only thing that bothers me is this siliconize step that you talked about because it’s been my experience that sometimes when you put anything on top of an asphalt shingle, it basically creates what I call the greenhouse effect. It allows – the moisture still gets underneath it but it doesn’t have anywhere to evaporate.

    LESLIE: It traps it in there.

    TOM: Yeah. You can get the mildew that’s growing kind of between the shingle and the bottom of that coating and then it’s even harder to get rid of. So …

    LESLIE: Because you’ve protected the mildew.

    TOM: Yeah, and the mildew is very appreciative of that too, Joe, by the way. (Leslie chuckles) So I don’t know about that step. That one is new to me and it concerns me for that particular reason.

    However, I will say that you are tackling this in the right way. It is just true that in some parts of the country, especially in the south, that you’re going to get a fair amount of mildew buildup on a roof on a regular basis. One of the things that you did, though, that also concerns me is this idea of scrubbing the roof. You want to be very gentle about scrubbing a roof because when you’re dealing with asphalt shingles, they have a very thin mineral surface that actually protects them from the sunlight and if you scrub too hard, you can actually scrub that mineral surface away and that can make the shingle last not as long as it might have if you’d left it alone.

    So the best thing to do is to apply a mildicide, and a lot of it; let it sit there for a while and then rinse it off. But don’t scrub it because if you scrub it or if you use an aggressive pressure washer, the chances are you’re going to damage those shingles and cut back on their life.

    Now if my theory holds true that you’ve sealed the mildew in, it’s not going to come off as easily the next time. And if that’s the case, you’re just going to have to live with it. The good news is it’s not causing damage to your roof; it’s just cosmetic. So don’t worry about it shortening – the mildew itself will not shorten the life of a shingle.

    JOE: Well I’ll tell you, a good thing about that siliconizer, we were right in the path of Charlie and we were basically one of the houses on the block that had no roof damage at all.

    TOM: Well the water probably beaded right up and fell off, didn’t it? (chuckles)

    JOE: Well it’s like one, big, rubber roof now …

    TOM: Yeah.

    JOE: … and the wind couldn’t blow under the shingles to lift them up like they did all up and down our street.

    TOM: Well what you should do, Joe, is you should go up there with a little bit of bleach and rub it over a dirty area, the mildewed area, and see if the mildew wipes off. If it does, the mildew’s on top; if it doesn’t, it’s underneath. OK?

    JOE: OK. Thank you.

    TOM: You’re very welcome. Thanks so much for calling us at 1-888-MONEY-PIT, 888-666-3974.

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