Let’s talk about those ugly roof streaks

By Angie Hicks

www.angieslist.com

YOU MAY HAVE the most attractive landscaping, beautiful windows and charming mailbox, but black streaks running down the roof can ruin your home's curb appeal.

What causes those dark marks, and what can be done to eradicate them?

The source is an algae called Gloeocapsa magma. Depending on the age and condition of your roof, cleaning may be the most cost- effective solution since it's about 5 to 10 percent of the price of a roof replacement, which can be $10,000 or more.

Algae-caused marking isn't preventable, but it can be removed, though not always permanently. The algae survive through photosynthesis and by feeding on limestone filler used in asphalt shingles.

Black marks became a problem about 20 years ago, when manufacturers began adding limestone granules to add weight to material used to coat shingles.

Roof experts say that what while other components are being added to shingles to hinder algae growth, they still get calls to deal with black marks on relatively new roofs. Shingle manufacturers offer products treated with copper or zinc to inhibit algae growth, but their effectiveness wears down over time.

Most black streaks form on the northern slopes of roofs, where it's darker and wetter - ideal for algae growth. Areas of the country with low humidity have fewer instances of roof streaks, while the problem is relatively common in the Southeast, where humid and warm.

The algae appear blue-green when the organisms form an outer coating to protect themselves from ultraviolet rays. Algae turns black with decay.

The dark streaks are unsightly, but the greatest danger to the roof is from moisture retention or root damage that algae and other life forms can cause.

Also, algae and fungus can grow together to form lichen, the roots of which can wrap around and feed on the granules covering the shingles. Once established, lichen is not easily removed. Even if it dries out, it can come back to life with the next rain.

Scrubbing or power washing lichen will only cause more damage.

Before determining whether having your roof cleaned is the right option, know your roof's age and condition, and compare costs accordingly. A cleaning can cost around $200 to $1,500, depending on the size of the roof, its pitch and height.

When hiring a company to clean your roof, consider this advice:

* Make sure the cleaners don't use high-pressure washing systems, which can remove granules coating shingles, and lower their life expectancy.

* Ask what kind of cleaning solution the company uses. (One roofing expert we interviewed said he uses a chlorine-based chemical wash with a soaping agent.) Also, make sure the company has a plan for preventing possible damage to your home or landscaping from runoff.

* Ask how long the cleaning method should keep the roof algae- free. A range of two to five years is normal.

As with hiring any contractor, ask family and friends for references or check online consumer reviews from a trusted source, get several bids, seek and contact references, and confirm liability and workers' compensation insurance.

And then, prepare to once again enjoy how your home looks without those dark ugly roof streaks.

Angie Hicks is the founder of Angie's List, a resource for local consumer reviews.