What Causes Roof Discoloration And What Can You Do About It?

If you see roof discoloration or dark streaks on the roof shingles of your house, it is quite likely a result from algae growth.

Roof discoloration like this is commonly seen in the Gulf States as well as Northwest and Eastern Seaboards. These issues are not specific to these regions, however.

Algae can grow on all roofing materials except for metal. This is because galvanized sheet metal contains copper and zinc which are toxic to algae. The reason you may see streaks on your roof is due to trace amounts of copper and zinc draining into your flashings after rainfall, however, this inhibits algae from growing on your roof.

Does Discoloration Cause Harm to a Roof?

Discoloration will not harm your roof’s surface, but it can detract from the overall appearance of your home. Before selling your house, it is often advisable to solve the discoloration problem on your roof. A streaked roof may give people the impression that your house needs repair or that your roof is about to need replacement. People may hesitate to make offers on your home or attempt to make a bid on your home if they believe older roofing materials are present.

Can Roofing Discoloration Be Fixed?

Chemical cleaners can be applied to any roof using a low-pressure spray to remove roofing discolorations. But cleaning your roof is usually only a short-term solutionThe discoloration problem may require a pressure washer, but know that asphalt shingles can be damaged by repeated pressure washing (as granule loss) and this will reduce the life expectancy of the roof. It is therefore recommended that you do not use a pressure washer. . 

It is difficult to remove algae discolorations from roofing surfaces, but they can be lightened by spraying a solution of household chlorine bleach (sodium hypochlorite) and water. Manufacturers may specify mixing directions for these ingredients according to the degree of discoloration. 

In general, you should mix equal amounts of chlorine bleach and water together to clean a roof. Bleach and trisodium phosphate (TSP) are common ingredients in cleaning formulas meant to solve this problem. While bleach is less effective than sodium hydroxide (lLye), you need to wear gear to protect yourself during the application. But chemicals and methods other than those approved by the shingle manufacturer should not be used for cleaning.

How to Clean Your Stained Roof (DIY)

To prepare your roof for cleaning, cover any vegetation and remove lawn furniture. Even when you use a non-toxic product, overspray can’t be avoided. Its runoff can make a big mess when it hits the ground.

Plan your shingle cleaning on a day that is (ideally) cool and cloudy with very little wind. This  reduces the chance that the sprays will blow off your roof and into other areas. An overcast or cloudy day helps prevent the cleaner from evaporating before it takes effect.

If you know the manufacturer of your shingles, find cleaning instructions or recommended products on the website of the company that manufactured your roof shingles and follow the instructions carefully. If you don’t know the shingle manufacturer, the following formula is recommended by William Woodring, the Director of Technical Services at GAF. Simply mix a cup of trisodium phosphate with five gallons of water and add a gallon of bleach to make a standard cleaning solution.

Wear protective clothing, shoes that prevent slipping, and rubber gloves along with eye protection. To make sure that you stay safe and to help you access any equipment as needed, it is helpful to complete this task with a friend.

Spray the mixture on your roof using long, even strokes with a garden sprayer (the kind commonly used for spraying fertilizers). Spraying water up at your roof's shingles can loosen them, so start at the top and work your way down.

Clean only a 4-foot wide section of shingles at a time. As you are cleaning your roof, be careful and avoid stepping on the wet part as it will get slippery. Consider wearing a harness to keep you safe from falling while cleaning.

The roof should be left to sit for at least 15 minutes and up to 20 minutes after wetting it with the solution. The combination of the algae and the solution on the roof can make the shingles slippery and potentially hazardous, so avoid walking on the roof until it’s dry.

After about 20 minutes, the solution should be rinsed off with a garden hose. Be sure to use steady, consistent strokes and go slowly. You could end up with spotty patches on your roof if you miss sections of your roof by going too fast.

Resist any temptation to scrub the shingles, as this may loosen granules and encourage granule loss. This will reduce the life expectancy of the roof. Take all necessary safety precautions whenever you are on or near a roof. 

Consider Getting Your Roof Cleaned by a Professional

You should contact a professional roof cleaning service to remove algae roof stains if you would rather avoid the dangers of a slippery roof. Your warranty may also be at risk of being voided if you try to clean your roof yourself.

During algae treatment, our team here at Baltic Roofing uses non-corrosive and biodegradable products. Our eco-friendly method uses a garden sprayer instead of a power washer and leaves no residue behind.

Can You Prevent Future Black Streaks From Appearing on Your Roof?

Is it possible to permanently remove algae? In a word, no. There's no completely effective cure for roof algae. In the same way that cold viruses are airborne, algae spores are in the air and can’t be fully avoided. While the symptoms can be treated, eventually algae will come back (especially if your neighbors also experience algae growth).

However, don't lose hope! With a little routine maintenance, you can get your algae under control. The following tips can help reduce roof algae growth.

Keep Your Roof Well-Maintained

Tree branches should be trimmed so that sunlight can shine on the roof more to help prevent debris from accumulating. Create a routine maintenance plan and clear any debris that accumulates on the roof manually. By doing so, you can create a less inviting environment for roof algae.

Additionally, clear out clogged downspouts and gutters so that rainwater is able to drain properly. A gutter that drains to a lower section of roof area encourages algae growth as well as creating pools of water on your roof, which can damage shingles and underlying wood structures.

Install Zinc or Copper Strips on the Roof's Sides

Copper or zinc strips can be installed underneath the shingle row at the roof's peak (as long as the lowest edge of the metal strip is exposed by 2 to 4 inches). The water resulting from these additions will effectively kill algae on the roof each time it rains, as the metal in the roof will contain molecules that kill algae.

Galvanized metal and copper are available in various thicknesses and widths. Home improvement stores or online retailers should carry them. Zinc strips specifically are designed to rectify the problem of algae on roofs.

It's important to note that the installation of zinc or copper strips will assist in resolving the problem, but it won't guarantee 100% complete protection. Regardless of whether it is raining or not, algae spores feed on moisture and humidity. Using these strips will add some protection.

Should You Get New Shingles?

If you have black stains on your roof that are simply the result of algae, you should not replace the roof shingles just to resolve the problem. Additionally, you should be wary of a roofer who suggests that the replacement of your roof shingles is the only possible solution.

However, if you have shingles that are damaged as opposed to stained, it's important to replace them as soon as possible. Roof leaks are often caused by long-term shingle damage, leading to water damage to the wood sheathing under the shingles. As the sheathing becomes more damaged, it eventually leaks. If the damages are left unrepaired, the water damage will worsen and the repair costs will increase significantly the longer it is left unattended. 

If you're trying to combat the problem of algae on your roof, roofing manufacturers have made shingles that contain copper, a metal that is toxic to algae - copper. It may be worth investing in copper-infused shingles if your roof is aged or in poor condition and you need to replace your roof anyway.

Just make sure to invest in the highest quality of shingles with the longest warranty. The results that consumers experience after installing these shingles can be mixed. There is no regulation regarding the measure of copper content that this type of shingle contains. It is subject to the specifications of each manufacturer.

Keep Your Home Looking Fresh

Black streaks on your roof can make it look worn, but it can be resolved quite easily and you can take steps to reduce the likelihood of their return.

If you would like to speak to one of our specialists about black streaks appearing on your roof, contact Baltic Roofing to schedule a free no-obligation roof inspection!

Louis Hanley

Community Manager

Baltic Roofing

Louis is a community manager for Baltic Roofing where they help the greater Chicago community with everything from high-quality roof replacements to skylight repairs. He is an expert on all things roofing and home improvement. Outside of work, he’s a huge Cubs fan and enjoys spending time with his dog, Roscoe.