Algae Discoloration
on Roofs


Technical Bulletin #13

Current as of March 2012 



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Occasionally light coloured asphalt shingle roofs may discolour with a brown to black stained appearance. Although this staining often is mistaken as dirt, moss, or even granule loss, it is caused by a type of algae known as Gloeocapsa. Natural pigments produced by the algae cells result in the dark discoloration, usually visible after the roofs are about five to ten years old. 

Historically this phenomenon has been more common in warm, humid climates but is becoming more common throughout North America. Although most noticeable on light shingle colors, the algae stain affects all asphalt shingle roofs, including darker colors.

The staining caused by the algae is in no way indicative of a shingle defect. This is an aesthetic concern caused by naturally occurring environmental factors and has never been shown to seriously harm the shingle or significantly shorten the life of the roof. If the homeowner wants to restore the natural colour and appearance of the roof, there are various options available.

1.  High pressure washing is sometimes recommended by roof cleaning service companies. This is NOT recommended as it can quite often result in damage to the asphalt shingles, primarily due to the loosening and removal of the mineral granular surfacing. It is very important that the granules be left intact on the shingles as they provide protection for the asphalt from ultraviolet radiation. 

2.  For anyone with a new roof construction, it is possible to install a zinc metal or other galvanized type metal strip near the ridge of the roof. As the metal ions oxidize and erode off of the metal strip, they wash down the roof inhibiting cellular algae growth. (This is why the staining on algae discolored roofs is usually less or non-existent below metal roof attachments such as flashings, aluminum-sided dormers, antennae anchor wires, etc.) Please consult your roofing material wholesaler or dealer for information on these types of metal strips. 

3.  The algae discoloration is difficult to completely remove from shingle surfaces, but it may be lightened with a cleaning solution made of diluted chlorine bleach.  A typical solution would be one part chlorine bleach to one part water. The ratio of the products in the solution may vary and depends on the amount of discoloration to be removed.  Gently spray the solution on the shingles. Use normal precautions when handling bleach including eye protection. Apply the solution carefully to avoid damaging other parts of the building or the landscaping below.  Mild scrubbing with a soft-bristled brush in severely stained areas may be required, but avoid harsh scrubbing as this may loosen and remove the granules.  Thoroughly rinse the solution off the roof with a gentle spray from a hose. Historically this cleaning option has been shown to be only a temporary relief from the algae discoloration. Repeated applications may be required every few years. 

CAUTION:  The bleach solution may make the roof slippery so it is advisable to work from a ladder or walkboard to avoid walking directly on the shingles.

4.  Commercially available roof cleaning solutions may be available (via internet searches) CASMA makes no comments or recommendations regarding these cleaning solutions. 

5.  Due to the increased occurrence of algae discoloured roofs, shingles are now available with small quantities of zinc or copper granules embedded in the mineral surfaced granule coating. These particles inhibit the algae growth through some of the life of the asphalt roof.

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The information contained in this bulletin is for general education and is not intended to replace advice from a qualified contractor or direction on usage/installation from the manufacturer. Consumers should be aware of the safety hazards associated with work on roofs and, before doing so themselves, should consider following CASMA s advice of using qualified contractors. This bulletin may be reproduced with permission on condition that it be reproduced in whole, unedited, with attribution of copyright to CASMA.

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