How to remove mold & algae on your roof
A lot of the dark streaky stuff found on roofs in wet climates like ours in the Lower Mainland and the South coast of B.C. is in fact, algae and not mold. There is difference between mold and algae and how they affect your roof. Algae commonly feeds on the limestone powder in asphalt shingles, while mold often thrives on plant and other organic matter that's fallen onto your roof.
You can't do anything to prevent algae and mold spores from flying through the air and landing on your roof, but you can help prevent mold growth by keeping the shingles free of organic matter.
Getting Rid of Mold and Algae
The old standard method for cleaning algae and mold stains from asphalt shingles is to spray on a solution of chlorine bleach and water. You let the solution sit for about 5 minutes, or so, then you rinse it off with a hose. However, bleach is not an ideal solution for a few reasons: it is toxic to plants (and most other living things), and it can damage shingles if it's left on too long or isn't completely rinsed off.
You can protect your plants from bleach spray, but then you have the rinse water pouring down your gutters into your yard, etc.
A better solution to bleach and water for algae
A better solution is to contact a professional roofing company or buy a commercial roof-cleaning product. One drawback of these cleaning solutions is that, unlike bleach, they take a little time to get rid of the staining, sometimes several weeks or months.
Preventing Future Problems
There are a few simple ways to keep mold and algae from returning to your newly washed roof. The simplest is to apply a cleaning or treatment product formulated for long-term protection against algae and mold growth.
The natural way to protecting your roof
There's also the longstanding preventative measure of installing zinc or copper strips near the ridge line of the affected roof planes. Both metals contain minerals that naturally inhibit algae and mold. When it rains, the runoff water carries minerals off of the metal and washes them down the roof.
General maintenance tips for your roof
General maintenance and regular cleaning to remove leaves, twigs and other organic material helps to keep a roof mold-free.
The long term solution to mold and algae on your roof
Finally, for a really long-term, low-maintenance solution, you can specify algae-fighting shingles when it's time to replace your old roof. These are made with copper-clad granules (blended in with the regular mineral granules) on asphalt shingles. The special granules naturally inhibit algae growth by the same process as the zinc and copper strips.
If replacing your roof is the avenue you need to take in order to deal with mold or algae, make sure that you contact a professional roofing service for advice, a quote and the best solution for your home.