How to clean black algae streaking from your asphalt shingle roof to improve curb appeal and prolong the life of your roof.
If you have noticed black streaks or stains on your roof which are caused by a type of algae called gloeocapsa magma. This type of algae feeds on the crushed limestone that is used in many asphalt shingles. Periodic rains spread the algae down the roof causing the black streaks.
Many people consider the streaking to be just an aesthetic problem, but continuous contact with the algae can cause the shingles to fail prematurely by loosening the protective granules. The algae also absorbs moisture and may prevent the shingles and roof sheathing from drying completely which can shorten the life of the roof system.
Fortunately the black streaks can be cleaned either by the homeowner or professionally.
Facts about Roof Algae
There are a few misconceptions about the black streaking caused by this algae. Some people assume that the streaking is a sign that the roof needs to be replaced. This is incorrect as even new roof shingles can show signs of streaking within a year. All that is required is an algae spore, moisture, and the limestone contained in the shingles.
Some homeowners feel that the streaks are only cosmetic and the cleaning can be put off. The issue is that the streaking is not caused by dirt, but by a living organism, much like mold. The longer the algae is on the roof, more of the protective granules will be destroyed which will cause UV damage to the substrate of the shingle and may increase your summer cooling costs.
While your roof may only have a few streaks on it, that doesn’t mean you should just clean off the darkened areas. Cleaning the entire roof is necessary to kill all of the algae spores and results in a uniform appearance. Dark roofs may also need to be cleaned even though the streaks are difficult to see. If you notice streaking on roofs in your neighborhood, you may want to consider cleaning your roof as well. The dark streaks do not appear until the algae has matured and some of it has begun to die off.
Tools and Materials
Powdered oxygen bleach, TSP, or a deck cleaner with bleach
Garden or pump sprayer
Soft broom or brush with extension handle
Long garden hose with spray nozzle
Safety harness; if applicable
Roof and Deck Cleaner
Always work safely as you will be walking on a roof and working with a hose, sprayer, and broom. If you do not feel comfortable working on your roof or your roof is very steep, hire a professional. You can expect to pay anywhere from $300 to $750 for cleaning your entire roof. Make sure your contractor has liability insurance and get several references.
Some professional roof cleaners use specialized sprayers to clean the roof
It is best to cleaner you roof when the temperatures are above 50 degrees and there is little or no wind. It is also better to work on cooler, overcast days to prevent the cleaners from evaporating. Wear goggles and rubber gloves when working with chemicals. If you feel uncomfortable rent a safety harness at your local home center or rental center. It is also best to work with a partner to make the job go quicker and safer.
1. Mix the roof cleaning solution with water in a bucket according to the package directions.
2. Pour the solution into a garden sprayer. You should use a sprayer that is small enough to handle while walking on your roof. For oxygenated bleach: Place 1 gallon of hot water and 4 oz. of oxygen bleach powder in a bucket. Wait about five minutes for the powder to completely dissolve and then fill your sprayer with the mixture.
3. Spray the roof cleaning solution over the black streaks on the shingles until it covers the area thoroughly. Do not let the solution dry. If the weather is windy or sunny, stay on the roof and spray another light coat every few minutes to allow the solution to work. If using a powdered bleach mixture, allow it to sit on the area for 20 to 30 minutes. If you notice that the solution is drying too quickly, you should hose down your roof first to cool off the shingles.
4. For very dark areas with heavy build up gently scrub the areas with a soft broom only using downward strokes. You can use an extension handle where you can stay on a ladder and reach to the ridge of the roof or straddle the ridge of the roof and work from the top down.
5. Rinse the area thoroughly with clean water by pointing the hose downward. Spraying underneath the shingles can damage them or cause leaks in your home. Never use a pressure washer to clean your roof.
- It is a good idea to remove any debris from the roof before cleaning.
- If possible, prune any overhanging branches from trees that may shade the roof and keep the shingles moist.
- Cover landscaping below with plastic sheeting to protect it from the cleaning solutions.
Asphalt Roofing Manufacturers Association
Algae Discoloration of Roofs