Algae on High

[Lakal]

I'm not sure this is the correct thread to post this but anyway...........

We have black stains on our roof ( sorry it's not a copper one Lone Protester )
and after a bit of a Google it seems the problem is algae - the stains are only on the north side of the roof.

Can the roof be cleaned without using hazardous chemicals? I don't want to hurt my trees and shrubs etc. Also, what product will clean and prevent a recurrence? What sort of qualifications should I look for from a roof cleaning contractor?

[swampfox]

If it's algae I would think that an ordinary pressure washer would do it, but I don't know how to tell if that would be a good idea for a particular roof. It would probably force water underneath the shingles if it's a shingle roof.

Another idea, and only an idea at this point, is that the stuff that kills mildew in the shower would certainly kill algae, and the bleach therein might actually destroy their color too. I'm not physically up to roof work anymore, but if I were I'd be tempted to try one of those cleaners (I think Tilex is one brand).

Don't take this advice as expert roofer advice. My science teacher instinct loves to solve problems, if only theoretically.

[cuebald]

Algae is cleaned from brick with a weak solution of muriatic acid. I don't know what it would do to a roof.

The pressure washer sounds like a good idea, but you'd have to clean from the top down.

[Lakal]

No one in this house is going to attempt this. I definitely need to employ a contractor. Pressure washing with Tilex or Muriatic Acid sounds a tad perilous.