Coping with Rooftop Moss and Algae

A primary concern for many home owners is the condition of the roof. To ensure the maximum life span of a roof, a plan of routine maintenance should be established. Part of that plan should include keeping the roof clean and dry.


In the northwest it is very common to find moss and algae growing on asphalt shingles due to the region’s damp climate combined with an abundance of tall trees. This is a situation that needs to be addressed.


Plant life growing on shingles indicates the presence of moisture and excessive moisture can damage a roof in several ways. Moisture can freeze promoting the degradation of the roofing materials, but even if it doesn’t freeze it can induce rot in the roofing structure if the moisture makes its way under the shingles.


As uncontrolled moisture encourages plant growth, over time the growth can lift the asphalt shingles subjecting them to a condition known as ‘wind lift’ where an elevated shingle is lifted by the wind to the point where it can break off in a heavy gust. Even if the shingle does not break off, a raised shingle makes it easy for windblown rain to get under the shingle allowing water to seep into areas of the roof where it can cause tremendous damage over time.


To ensure your roof stays dry and wards off as much moisture as possible it should have maximum exposure to sunlight. Sunlight is a natural enemy of moisture, moss and algae. If a home is surrounded by tall trees it is recommended to trim back some branches to increase the amount of sunlight it receives.


Another aspect of maintaining a healthy roof is keeping the roof clean. Accumulations of leaves and branches absorb and retain moisture that comes in the form of dew or rain. As we mentioned before, moisture promotes plant growth in addition to promoting wood destroying insects like termites.


Fortunately, cleaning asphalt shingles from moss and algae is something most home owners can do with very few tools. This list can, but does not have to include, an industrial type broom, leaf blower, garden hose, household bleach, liquid dish soap and a hand pump or garden sprayer.


For the sake of brevity we will not address any safety issues in depth except to say that even with proper preparation and experience, climbing on your roof is one of the most dangerous things you can do around your house. If you are unsure about how to climb on your roof, please hire an expert. For those that are willing to take on such a job, this link is an excellent article on rooftop safety from Fall Protection Services. Another worthwhile article is Popular Mechanic’s article on how to safely use a ladder.


The first step in removing moss or algae from your asphalt shingles is to clean the roof of any loose, excess debris. Use a broom with a light sweeping motion to clear the roof of pollen, leaves, branches and other detritus. You can also use a garden hose or a leaf blower to clean the roof but if you opt for either of these methods be sure to use them at their lowest possible setting.


No matter which method you decide to use, it is vitally important that you protect the integrity of the shingle. Asphalt shingles are covered with tiny granules that block the sun’s ultra violet rays and protect the shingle from being baked and dried out. Therefore you want to avoid any abrasive contact with the shingle if possible. It is highly recommended you avoid any power sprayer of any type. In fact ARMA, the Asphalt Roofing Manufacturer’s Association recommends using no pressure at all.


Once the roof is clean the next step is to apply a solution to kill the plant growth. There are many off the shelf products that work very well but mixing up your own homemade solution is certainly possible and probably cheaper than buying store bought products.


Homemade bleach solutions usually contain just bleach and water. There are recipes that recommend a ratio as little as 1 part bleach to 4.5 parts water. Other sources recommend a mixture as strong as 50% bleach to 50% water. Commercial firms use a solution that is 10 – 12% bleach to water but remember they most likely are using commercial grade bleach, not the household bleach purchased in grocery stores.


A final ingredient to be considered when preparing your cleaning solution is the addition of dish soap. The addition of dish soap is controversial because the addition of dish soap that contains ammonia to undiluted bleach causes toxic gases that can be very dangerous to your health. Therefore it is advisable to add dish soap that does NOT contain any ammonia whatsoever.


Once you have your cleaning solution mixed the next step is to apply the solution to the affected areas. Before spraying anything on the roof, it would be wise to water down any hedges, bushes or tress located directly under the area where you will be spraying. Watering down these plants, the plants whose growth you want to promote as opposed to the plants on the roof you’re hoping to eliminate, will dilute any accidental overspray of the cleaning solution you’re applying on the roof.


If it is a hot, sunny day you may also want to consider spraying down the roof with water before applying any cleaning solution to reduce the chance of premature evaporation of the cleaning solution.


The most common way to spray your asphalt shingles is with a pump type hand sprayer found at most hardware stores. When spraying the shingles, start at the bottom edges of the roof and work your way to the top. Allow the solution to sit at least 15 – 20 minutes if you plan on rinsing. Reapply cleaning solution to areas where dried out areas occur due to absorption or premature evaporation.


When it comes to rinsing the rooftop you have two options; to rinse or not to rinse. If you purchased a store bought cleaning solution it would be wise to follow the manufacturer’s directions regarding rinsing.


If you made your own cleaning solution, the first option is to respray the roof with water to give the roof a final cleanse. This may be your strategy if you live in an arid climate or work for a commercial roof cleaning company. Rinsing the roof typically reduces the chances of streaking. Rinsing is usually performed by starting from the very top of the roof and working your way to the bottom. Spray very gently. Just enough to do the job and no more. Do not loosen any grains on the shingles if at all possible.


Rinsing the roof also provides an opportunity to wash down the metal components of a roof (gutters, vent pipes) as bleach solutions and industrial cleaners have been known to leave stains on rooftop metal work.


The second option is to ‘spray it and forget.’ Let the rains or Mother Nature take care of any necessary rinsing. This is feasible and many do it this way but it may not be aesthetically pleasing if streaks are left behind after the roof dries.


Regardless of whether or not you need to rinse your roof, do respray any plants below the area where you were working with water to dilute any of the chemical or home mixed overspray.


Most home owners find cleaning a roof just once a year keeps the roof clean, dry and aesthetically pleasing to the eye. If for various reasons this schedule does not suffice there are physical systems that can be installed to help minimize plant growth.


Strips of lead, zinc or copper can be installed just beneath the very top row of shingles. A few inches of these strips should be exposed to the eye. The basic premise is that when it rains, the rain water runs across the exposed metal strip and picks up a few molecules of whatever the strip is composed of. As the water runs down the roof it deposits these molecules onto the asphalt shingles where the plant life grows, thus damaging and eventually killing the plants.