How to Clean Roof Stains

A great guide to cleaning off roof stains without causing a bunch of damage.

The Family Handyman

Do shingles on your north, west or other shaded roofs have streaky, moldy stains? Here’s how to clean the ugly discoloration and keep it from coming back.

Make your shingles last longer

Black streaks on the north- and west-facing and shaded areas of your asphalt-shingled roof can really wreck the appearance of your home. The streaks look like mold, but they’re actually algae colonies that form in your shingles and feed on moisture and the limestone filler agents in the shingles.

Using shingles that have been treated with algicide keeps the growth at bay for about 10 years (thus the 10-year algae warranty). But once the algicide wears off, your roof hosts an all-you-can-eat buffet for the neighborhood algae spores. As the algae eat away at the limestone, they dig into the asphalt and dislodge the light-reflecting granules. That’s the beginning of the end of your roof. So it pays to clean your shingles as soon as you spot algae growth.

Choose the right chemicals

If you search online, you’ll see hundreds of posts on roof-cleaning methods. In less than 10 minutes, you’ll sign off convinced that all you need is a few gallons of household bleach and a power washer set at its lowest setting.

We don’t recommend that approach. Even at low pressure, a power washer can seriously damage shingles. Plus, chlorine bleach is a corrosive agent that can damage metal roof flashings, gutters and downspouts. It can lighten the color of your roof and “bleach” anything the overspray contacts. And the runoff harms plants. But here’s the kicker. Bleach may kill the top layer of algae and lighten the stains, but it doesn’t kill the underlying algae. So the algae colony gets right back to work.

Sodium hydroxide (lye) products, on the other hand, work better than bleach and are less harmful to vegetation. But they’re also corrosive, and using them requires you to don full protective gear.

So look for a roof-cleaning product that’s noncorrosive and safe for the environment. We chose Defy roof cleaner for this story (, but there are other brands.

Choose the right day and prepare the area

Check the weather forecast and choose a cool or overcast day with little to no wind so the spray hits your shingles, not the neighbors’. Those conditions allow the cleaning solution to soak deep into the algae colonies without evaporating too quickly.

Next, repair any loose shingles or flashings, and clean the gutters and downspouts so they can drain freely.

Then prepare the area by moving lawn furniture and covering vegetation, because you’re going to have overspray. Even though the product we chose isn’t toxic, the runoff can be pretty ugly. So a little prep work will save you cleanup time later.

The cleaning process: Soak the shingles

Mix the product with water for a 1:7 dilution ratio (a gallon covers about 700 to 900 sq. ft). Pour it into a pump sprayer, strap yourself into a full-body harness, tie it down and climb to the roof.

Before applying the cleaner, spray the roof with water to cool it down. That’ll prevent the cleaner from drying out too quickly. Then spray the cleaner onto the shingles. Wait about 20 minutes, then rinse.

The Family Handyman

Blast off the crud

If the staining is fairly light, you can rinse off the cleaning solution with just a garden hose sprayer. But go slowly and use even strokes. If you don’t, you’ll wind up with clean patches that were rinsed properly alongside dirty patches that you skipped over too quickly. For severely stained roofs, a garden nozzle won’t exert enough pressure to dislodge the stains. In that case, you’ll want to invest in a specialized rinsing tool or pressure washer.

Prevent regrowth

Depending on weather conditions, you can expect algae regrowth in as little as one year. There are two ways to slow the regrowth process. One is to install zinc or copper strips along the entire ridge. Theoretically, rainwater picks up algae-killing ions and spreads them over the roof. In reality, the protection falls short because algae can still feed off humidity when it’s not raining. But you don’t have a lot to lose by trying it.

The second method is to spray on a coating of stain-blocking solution (Defy Stain Blocker for Roofs is one product, but there are other brands as well.) A stain-blocking product can buy you up to three years of protection from algae. If you decide to try it, apply it shortly after you’ve cleaned the roof.

Whether you install the metallic strips or apply the stain-blocking solutions, you’re still going to experience algae regrowth sometime down the road. Get back up on your roof and clean it early, so the stains don’t set in permanently.