To understand how to deal with the recurring problem of algae and moss growth on rooftops, it is important to learn what they are and how they grow and spread. Algae and moss have some similarities, but they are quite different and require different methods to be prevented or removed.

Many roofs in the United States and Canada have streaks and black discoloration that are mistaken for mold or mildew. Algae can grow on roof surfaces where moisture is most prevalent. Algae growth is most likely to occur in humid or coastal climates. Gloeocapsa magma, also known as blue-green algae is the most common. It produces a dark pigmented sheath to protect itself against harmful ultraviolet rays. The algae discoloration usually begins in small spots that then rapidly transform into streaks on a roof plane. It is likely that the algae have been present for several weeks or more before it becomes visible.

Algae spores can be carried by wind and animals, and quickly spread from rooftop to roof. It is not uncommon to see algae growth on roofs in affected neighborhoods. The same applies to apartment and condominium complexes, rowhouses, townhouses, and townhouses. Although there is no evidence to support the claim that algae can cause damage to asphalt shingles roofs, it does affect their aesthetics. On highly reflective or “cool” roofs, algae will undoubtedly have a negative impact on the long-term effectiveness of these roof systems. It is crucial to deal with an algae problem immediately.

The non-vascular Moss plant gets water through its leaves. This is in contrast to most other plants which get water through their roots. To survive, moss needs a moist environment. North American moss prefers north-facing roofs. These planes receive less sunlight and remain damper longer than south-facing ones. The roof is further protected by overhanging branches of trees, which can also drop debris and provide shade. This acts as a food source and a way to retain moisture.

Moss spores can also be carried by animals or wind and spread easily through neighborhoods. Moss, unlike algae, can cause asphalt shingle failure. Moss can cause the shingles’ leading edges to curl or lift, increasing the likelihood of them blowing off during wind events. In severe cases, moss build-up can cause lateral water movement resulting in moisture damage to the roof deck or may even cause leaks.

A 50/50 mixture of laundry strength chlorine bleach and water is the best way to remove algae and moss from roofs. Spray the solution with a sprayer. Let the solution sit on the roof for about 15 to 20 minutes. Rinse thoroughly with low-pressure water. You may need to leave the solution on the roof for longer dwell times. However, this could prevent you from rinsing completely. You should take precautions to protect your landscaping and the surrounding areas from the bleach solution. When working with chlorine bleach, ensure that you have the appropriate personal protection equipment. With each subsequent rainfall, algae will dissolve and wash away. The moss will eventually fall off and can be removed using a leaf blower. Sometimes, it might take several bleach treatments to get rid of all the moss. A pressure washer is not recommended for cleaning asphalt shingle roofs. This will result in granule loss, and possibly premature roof failure.

There are steps you can take to stop moss and alga growth.

To make it more sunny and less likely to accumulate debris, trim tree branches.

* Any debris that accumulates on the roof should always be removed regularly using a leaf blower, or another non-abrasive method, as part of a regular maintenance plan.

* Avoid driving debris under the edges and shingles by directing airflow down the slope.

To promote water drainage, keep gutters clear. Don’t allow gutters from the upper roof to drain on the lower roof. Instead, extend the downspout of the upper roof into your lower gutter.

When it’s time to replace your roof, you can also use preventative measures such as copper or zinc strips to kill algae and moss or asphalt shingles with algae-resistant copper granules. There are currently asphalt shingles that have algae resistance technology.

It is not recommended to add copper or zinc strips to an existing roof. This will either require applying the strips with exposed nails (which could cause leaks over time) or breaking sealant bonds (which may eventually lead to wind damage).

Algae or moss growth on roofs is a nuisance at best. If left unchecked, moss can cause roof damage and shingle blow-off. Regular roof maintenance should include trimming branches and removing debris. To prevent moss and algae from spreading to other roofs, clean the roof using bleach and water.

This post was written by Ted Williams! Ted is the owner of, one of the best roofing companies in Saint Petersburg, FL! Ted is a Master Elite Weather Stopper GAF Roofing Contractor, a double award winner of Best Steep-Slope Contractor from GAF and achiever of Master Elite Consumer Protection Excellence from GAF. He has been serving the Pinellas County area since 1978.  Old Time Roofing has a tradition of quality workmanship, servicing residential and commercial properties.

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