Are Ugly Black Streaks Ruining Your Roof?

by on 02/20/2015 - 03:41 pm

Categories: Building & Remodeling, Home Exteriors, Windows & Doors

By Jaime Johnson

Owner, Perfect Home Restoration

The beautiful appearance of your roof can be quickly ruined by black streaks and stains caused by an obnoxious algae called gloeocapsa magma. The persistent growth algae, which began to invade Northeast Ohio about 10 years ago, not only looks terrible, but they also feed on the limestone filler contained in most asphalt shingles. That can result in the premature deterioration of your expensive roof.

When they first attach themselves to your roof, the algae may be difficult to detect. They will have a faint bluish green hue. At that stage, they are relatively harmless. However, as they multiply, they develop a dark, hardened outer coating, which appears black in color. The larger algae population begin to cause damage as they feed on the shingles. Since algae thrive in dark, damp areas, you’ll probably notice considerably more streaking on the north facing, shaded side of your roof than on the dry, sunny south side. All colors of shingles are susceptible to algae growth, but they are most visible on lighter colored roofs. If you have a black roof and don’t see any streaks or blotching, you may want to do a close inspection to see whether algae are hiding up there.

If your neighbor’s roof is infested with gloeocapsa magma, watch out. The algae can jump from roof to roof.

Algae is very difficult to remove from shingles. An equal solution of chlorine bleach and water may help to lighten the stains. It isn’t a good idea to use other chemicals on your roof without first checking with the shingle manufacturer. Doing so just may make a bad situation worse.

And since it’s always dangerous to work on a high roof—especially a wet one—you may want to have the work done by a professional roofing contractor.

High pressure power washing also is not a good idea. Shingles are designed to withstand rain and snow, but not a pressure washer, and damage may occur.

The Asphalt Roofing Manufacturers Association cautions that the effectiveness of cleaning with bleach is temporary, and discoloration will likely reoccur. However, the association says, several types of algae-resistant roofing products have been developed and are now commercially available. You may want to ask your professional roofing contractor about them.

One way to reduce the risk of algae growth is to have zinc or copper strips along the top course of your shingles. When the strips are hit with rain, they release an algaecide that runs down the roof and prevents algae from developing. The strips, however, won’t kill the algae that’s already established on the roof.

To avoid the problem before it starts, next time you re-roof your home, you may want to ask your contractor about algae-resistant shingles.

Perfect Home Restoration is located at 750 East Washington Street, Suite B1, in Medina, Ohio. Call them at 330-591-9641, or visit