How To Remove And Prevent Black Stains On Your Roof

If you've noticed black streaks along your roof, you'd probably like to know what it is and how to get rid of it. Keeping your roof in good condition is important, not just in the prevention of unnecessary repairs, but to also improve the aesthetic value of your home. Here's what's causing those unsightly stains on your roof and what you can do to treat and prevent it.

What Causes Staining and Streaking

Common in areas of high humidity, the substance that leaves black streaks down your roof is actually a roof algae, otherwise known as roof mold or shingle mold. It can be caused by dirt and multiple strains of mold and mildew, but the most likely culprit is Gloeocapsa magma. This particular type of algae loves moisture and tends to thrive on asphalt shingles because it feeds on the limestone.

Why it Needs to Be Removed

The limestone in your shingles is there to act as a protective barrier against the elements. If left untreated, algae (as well as other mosses and mildew) will consume the limestone, leading to shingle decay. The algae also absorbs heat, which will effectively make your roof hotter, placing an additional load on your air conditioning and adding to your fuel costs.

There isn't anything you are doing to cause the growth of algae. The spores tend to spread from house to house with the help of winds, as well as animals like birds and squirrels. But there are a few measures you can take to remove it and prevent it from coming back.


The safest way to remove algae from your roof is to hire a professional roofing contractor and let them do the cleaning. That way you don't have to worry about getting injured or exposing yourself to chemicals that are bad for you, your plants, and the environment.

But if you insist on doing it yourself, here are a few recommendations. You can purchase a commercial roof cleaner, or you can make your own oxygen bleach solution and use that. It's best to stay away from chlorine bleach as it can harm surrounding plants and break down your roof shingles much faster than oxygen bleach.

Whatever you do, do not pressure wash your roof, as this can literally blast away the grout on your shingles. It's perfectly fine to use a regular hose with higher than average water pressure, but when it comes to anything beyond that, let the professionals handle it.

Shade Control and Gutters

Tall trees can be a wonderful source of shade, particularly in the summer. But because algae loves darkness and moisture, any branches that hang over your roof tend to provide the perfect breeding ground for this substance. That's why a good preventative is to trim the trees.

Also, your gutters are another spot that can harbor moisture if not properly maintained. When your gutters are clean and clear of debris, it's much easier for the roof to drain properly and stay dry.

New Roof

If your roof is really old, cleaning may not be as effective, and it might be time to look into a replacement.

Nowadays, you can purchase roofing shingles that are infused with copper granules, a natural algae preventative. The great thing is that it will also help prevent the growth of other molds and mildew, as well as lichens. Be prepared to add about 10-15% over the average $0.80-$1.20 cost per square foot for algae resistant shingles. Or you can look into installing a 100% copper roof, which runs around $8-$9 per square foot. You definitely won't have to worry about algae with one of those on board.

Sheet Metal For Prevention

If you look, you may notice that those black stains are not visible below or near a metal chimney. This is because of the copper content in the flashing. As it rains, a small amount of the metal gets washed down the roof, preventing the growth of molds and algae.

If you're able to get your roof fairly clean and don't feel the need to replace your roof, you can actually install sheet metal along the ridge of your roof. All you need is one row of six-inch wide metal that contains copper or zinc, leaving at least one inch exposed to rain water beneath the shingle.

For more information, contact a local roofing company, like Acoma Roofing.