How to Clean Roof Stains

Wash your shingles where the sun doesn't shine

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Introduction

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.

Tools Required

  • Full-body safety harness
  • Garden hose
  • Garden spray nozzle
  • Lye or other nontoxic
  • Noncorrosive roof cleaner
  • Supplemental water pump

Make your shingles last longer

Roof stains are a common roof problem. 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 roof 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. So it pays to clean your shingles as soon as you spot algae growth.

Do it yourself and save big bucks

Professional roof cleaners charge hundreds of dollars; the bigger the roof, the more hundreds you’ll spend. And they have to repeat the cleaning every few years. If your roof slope isn’t too steep and you’re comfortable working on it, you can clean it yourself and save the dough.

You’ll need a full-body harness, a garden sprayer, a garden hose and a nontoxic, noncorrosive roof-cleaning chemical. Some manufacturers sell a special tool applicator and rinsing tool, but if the staining isn’t severe, you may not need them.

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 (saversystems.com), 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.

Project step-by-step (3)

Step 1

Tools that make the job easier

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One manufacturer (saversystems.com) has taken roof cleaning to a new level and developed a special rinsing tool to dislodge dug-in algae colonies. The Roof Rinsing Tool is pricey, but far more effective than an ordinary garden spray nozzle. If your household water pressure isn’t enough to generate the proper nozzle pressure at the jets, the manufacturer recommends boosting it with a supplemental 1/2-hp pump. (One choice is the Wayne PC4 transfer pump shown here, available at homedepot.com.)

If you really want to speed up the cleaning process and are willing to spend even more, buy the Defy Roof Cleaner Applicator (find a dealer at saversystems.com). Pour in the concentrated cleaner. The special sprayer dilutes the cleaner as you spray, eliminating the need to continually pump and refill a traditional garden sprayer.

Step 2

The cleaning process

Soak the shingles

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Saturate a large area of shingles with the cleaner. Start at the bottom row and work up to the peak. Spray until you see runoff. Respray any areas that dry out. Fix any roof leaks first.