Scientists may think mold is great, but the rest of us have a hard time not resenting it. The stuff grows everywhere, from the cheese in the fridge to the bathroom ceiling to the very skeleton of the house. And yes, it can even grow on your roof. The good news is that moldy shingles, unlike old cheese, don't need to be thrown out. They probably just need a good cleaning, followed by some simple maintenance step to keep mold (and other life forms) from coming back.
Mold by Any Other Name
A lot of the dark streaky stuff found on roofs in wet climates like Seattle is actually algae. You probably don't care to know the difference, and neither does the roofing industry, as most cleaning solutions and other products for roofs are targeted at mold, mildew, algae, moss, slime…it all works the same. But for the record, 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. Some people add TSP (trisodium phosphate) for extra cleaning power. You let the solution sit for about 5 minutes, or so, then you rinse it off with a hose. However, I'm not recommending bleach for a few reasons: It's 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. Not worth the trouble.
A better solution is to buy a commercial roof-cleaning product, such as Spray and Forget. You still have to spray any surrounding foliage with water before and after applying the cleaner, but there's no rinsing required, and many of these products offer extended protection against future algae and mold growth. The best (and cheapest) way to apply it is with a garden sprayer from the roof, but there also are concentrated versions you can hook up to a garden hose to apply from the ground. 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. 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. You simply slip the strips under the shingles, leaving the metal partially exposed, and nail the strips in place. When it rains, the runoff water carries minerals off of the metal and washes them down the roof. (If your roof has copper flashing around the chimney, you might notice that there's little or no algae growth on the shingles directly below the flashing.)
As mentioned earlier, regular cleaning to remove leaves, twigs and other organic material helps to keep a roof mold-free. 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.