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Moss on Roof

How To Handle Moss

Green, velvety masses of moss often grow on north-facing roof surfaces and on tree-shaded roofs. Unlike algae, moss left on roof surfaces can develop beyond an aesthetic problem. It can infiltrate the roof structure underneath the shingles and make their edges lift and curl, which can lead to cracking and blow-off during high winds and storms. Heavy moss growth can actually form dams that can cause water to back up under the shingles and damage the roof deck. It's best to clean moss off a roof as soon as you notice it's growing there.

This picture was taken at a home in Carmichael.  This is a North facing roof with a overhanging branch providing shade. As you can see here,  the moss has started to lift up the shingle.  Fortunately, removing moss is a fairly simple task that you can perform on a seasonal or as-needed basis to keep Moss off your roof. Keeping your roof weather tight and great-looking for years to come.

Moss Close-Up

Safety First

Moss can be easily removed from asphalt shingles with a 50/50 solution of chlorine bleach and water. Laundry-strength bleach is sufficient, or you can opt for any of a number of proprietary roof cleaners available at hardware or big box stores, some of which don't contain bleach, lye, or other potentially harmful chemicals.

Moss can be easily removed from asphalt shingles with a 50/50 solution of chlorine bleach and water. Laundry-strength bleach is sufficient, or you can opt for any of a number of proprietary roof cleaners available at hardware or "big box" stores, some of which don't contain bleach, lye, or other potentially harmful chemicals.

Since bleach and some cleaners can be harmful to plants and humans, it's a good idea to take some precautions when working with them, including the following:

  • Wait for a calm, windless day to clean your roof.
  • Spray landscape plants near the house with water and cover them with tarps to protect them from chemical over-spray and runoff.

Wear protective clothing, including long sleeves, pants and gloves, as well as goggles to protect your eyes, and shoes with high-traction soles. A roof with moss can be very slippery to walk on.

How to Clean Algae and Moss from a Roof

After Wash

Apply the bleach solution with a garden sprayer. Let it stand on the surface for about 20 minutes, then rinse it off with spray from a garden hose. Don't let the bleach solution stand on the roof for more than 30 minutes or so without rinsing. And don't use a pressure washer, which can damage the shingles by removing their protective layer of asphalt granules.

If accumulations of moss are heavy, at least some of it should wash off the roof surface right away with the stream from the hose. You can try brushing off algae and moss with a brush or broom with medium-stiff bristles, but don't scrub too hard. You don't want to separate the mineral granules from the shingles.

If chunks of moss remain on the surface after rinsing, let the roof dry, then spray on the bleach solution again. Wait 30 minutes and rinse.

How to Prevent Moss from Recurring

Moss tends to grow on roof surfaces that are shaded and retain moisture. So, it’s a good idea to cut away tree branches that overhang the roof and block sunlight. Keep the roof surface clean by blowing off leaves and fallen branches during seasonal maintenance.

Following these maintenance tips can help you enjoy an attractive roof. They can also help extend the roof’s service life, which is important whether you plan to stay in your home or sell it in the future.