How To Clean Moss From a Roof

Moss and algae form on roofs when moisture is present or lingering and the humidity is high. These organisms are difficult to remove and can damage roofing materials over time. The moss root system grows under the tabs of asphalt shingles and raises them allowing water to enter and damage the roof substrate causing dry rot.
Algae do not harm the roof, in general, but cause unsightly black stains. There are three main methods of removing moss and algae. These are mechanical, chemical, and wash down. Which method you use depends on how rapidly you want to dispose of the moss, the type of roofing material you have, and whether or not you have access to the roof of your home.

Mechanical

1. You can sweep the moss off the roof with a stiff bristle broom or brush if you have access to the roof. Sweep downward, not across the shingles or upward. Protect the shrubbery alongside the house with a tarp or plastic sheeting
2. Pressure washing is also a good cleaning alternative but keep the water pressure extremely low or you will damage your roofing material. You should be able to place your hand in the pressure stream without discomfort.

Moss and algae can damage a roof

Chemical

1. There are many different chemicals for moss removal. Some are harmful to the environment and some are biodegradable. Check the label and follow the manufacturer’s instructions.
2. In general, apply the chemical to the moss and wet it thoroughly. Allow the chemical to sit on the moss for the prescribed amount of time, then brush the dead moss off carefully with a stiff bristle broom or brush in a downward motion.
3. You can also flush the dead moss away with a stream of water or simply allow rain to do it for you.
4. Protect the shrubbery at the base of your house and keep in mind the effect the chemicals may have on your gutters, asphalt shingles, ground water, and pets.

Washdown

1. Use a stiff bristle brush to loosen the moss and then a garden hose or pressure washer under very low pressure to wash it away.
2. Once again, sweep the moss, and direct the water only in a downward direction to avoid lifting the shingle tabs or damaging the shingles.
3. You can install strips of copper or zinc sheeting under the top row of shingles near the peak of the roof. Rain will wash the metal particles down the roof and kill the moss and algae over time.

Tips

No matter which method you use, observe all safety precautions: wear a safety harness or tie yourself off with a strong rope. Wear good shoes or boots with a strong high traction sole. Stay away from the edge of the roof.

Unless your roof is very flat or you are a professional roofer, it may be best to hire a roofer or roof cleaning professional to avoid injury to yourself or the roof.

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