How to Prevent and Treat Roof Mildew and Staining


Did you know that mold-like growth and stains that appear on your roof are actually algae that grow quickly under humid conditions? Although it does no damage to your roof, it lowers the appeal of your home and invites moss to invade your shingles. This post will explain how to treat this pesky growth and prevent it from coming back. 


Working on a roof can be dangerous, so taking adequate safety precautions is important:

  • Wearing slip resistant shoes, using a safety rope, and avoiding working on high-pitched roofs are great ways to help your cleaning experience go smoothly. 
  • Choosing the right day is important too: look for cloudy, non-windy days to do roofing work to avoid spreading chemicals in the air and keep them from evaporating too quickly. 
  • Wet plants and shrubbery around areas where runoff may occur, or cover them with plastic wrap. 
  • Avoid powerwashing, which tends to damage roof shingles. 


Before treating roof algae, make sure that your roof is damage-free and your gutters/downspouts are clear to direct runoff away from your belongings.

Equal parts bleach and water in a pump garden sprayer work magic when fighting off roof algae. Before applying cleaner, rinse the entire roof with a hose to reduce evaporation. Starting at the bottom row, spray generously while working your way to the roof peak until you see runoff. Allow the solution to remain on your roof for about 15 minutes before rinsing it off with a hose. The cleaning will not be immediately visible, but after a few rains the algae will wash away.

Oxygen bleach is an alternative to normal bleach that is more eco-friendly and easier on your shingles. It’s also safe for plants and other foliage around the runoff zone. Take a scrub brush and lightly scrub the affected shingles while applying more solution for a deeper clean.


A common way to prevent algae stains is to install a 6 inch wide zinc or copper strip under a row of shingles nearest to the peak of the roof, leaving an inch or two exposed. When it rains, water washes the metal ions over the roof, killing algae and leaving your roof stain free. Alternatively, chemical stain-blocking solutions are available and tend to last 2-3 years after application. In extreme cases, installing dark colored shingles can hide excessive algae staining.

The good news is that with Bundl Home’s handyperson offerings, roof cleaning is covered! See our subscription options to learn more.