What Causes Black Streaks on Asphalt Roofs?

Extractive Bleeding

• Extractive bleeding refers to a manufacturer's defect in the shingles that leads to the discoloration of the roof. Asphalt shingles contain a black pigment which can darken the roof, if it escapes the shingle. Black stains from extractive bleeding typically show up as dark discolorations that run vertically down the roof in streaks. The streaks will appear to originate from a single point on the roof.

Algae

• Asphalt shingle roofs have a susceptibility to the growth of algae and moss, depending on the site conditions. Shady areas of a roof often produce algae, as overhanging trees block the sunlight that inhibits their growth. Although various types of algae and moss can appear on a roof, the most prominent culprit, an algae called Gloeocapsa magma, usually causes no damage to the shingles, but merely distracts from the home's appearance. The algae shows up most frequently in regions that have hot and wet summer climates.

Trim

• Trimming your trees so that branches do not overhang your roof helps prevent the buildup of algae and mosses. Decreasing the amount of shady areas on the roof gives these organisms less favorable growing conditions. Pruning your trees also helps reduce the amount of debris falling onto your roof, another cause of discoloration. Consult with an arborist to determine the best strategy for trimming any tall trees near your roof.

Solutions and Prevention

• For dark streaks caused by extractive bleeding, contact the the installer, since the problem might be covered by the warranty. For streaking caused by algae or moss, wash the roof, using a mixture of 1 part bleach and 1 part water, with a garden sprayer. Avoid using a pressure washer, which can harm the roof. To prevent algae buildup, put copper or zinc metal strips under the course of shingles closest to the roof ridge, advises the website of This Old House.