How to Remove Black Stains from Your Asphalt Composition Roof

Posted on: 9 May 2016


Black, streaky stains on your asphalt roof are not only unsightly, they occur due to a process that can actually cause permanent damage to the shingles and shorten the lifespan of the roof. Fortunately, replacing your roof is usually not necessary, as the black stains can be removed in most instances. Below is more information on the mysterious cause of black roof staining and what you can do to remove it yourself.

What causes black roof stains?

Contrary to what you may have heard, roof staining is not caused by the asphalt "seeping" from the shingles. Instead, it is due to a biological process resulting from a proliferation of Gloeocaspa magma, a common species of bacteria. While the origin of the bacteria is not usually known, it is clear why G. magma finds asphalt roofs appealing: they contain lime, which is a major food source for the colonies of hungry bacteria. Lime is added to asphalt shingles to provide rigidity and strength, but its high nutritional content allows bacteria to flourish. The black stains you see are caused by an accumulation of bacterial byproducts and are not removable by rainfall or ordinary washing. However, with the right materials, tools, and technique, you can easily eliminate most G. magma stains in an afternoon of work:

What you will need to remove roof stains

  • 100 feet of garden hose

  • spray nozzle

  • hose end mixer/sprayer

  • laundry bleach

  • extension ladder

  • rubber boots

  • safety glasses

  • old clothing

  • gloves

  • plastic drop cloths or garbage bags

Step-by-step procedure for removing roof stains

1. Protect plants and other bleach-sensitive items. Before you begin cleaning, it is important to take a few minutes to cover landscaping and patio plants to protect them from the bleach that will be used. In addition, be sure to cover bare metal objects, unprotected wood, and other items that could be damaged by the bleach. Use plastic drop cloths suitable for covering items during painting or cut apart large garbage bags and lash them to objects with some string or light rope. After covering all sensitive items, use a garden hose to spray the vicinity of the home along its drip zone with plain tap water, including grass and any other places where bleach is likely to drip during cleaning. This will help further dilute any bleach that finds its way to to the ground.

2. Work with safety first in mind. Climbing onto your roof can have deadly consequences if you fail to keep safety as the first priority. Here are a few important guidelines that will help keep you safe from harm:

  • Use ladders in a safe manner and only in stable locations. Obtain help when placing and climbing a ladder, and utilize ladders with roof bracing to increase their stability.

  • Wear eye protection, appropriate footwear, and gloves when working. When working with bleach, the eye protection and gloves will protect your eyes and hands from chemical irritation. In addition, be sure to wear footwear with slip-resistant soles. Rubber boots provide good traction and will also protect your feet and ankles from the bleach spray.

  • Maintain vigilance at all times. It is critical to concentrate on your work at all times when on a roof or ladder, and don't allow yourself to daydream or lose focus. A fall could prove to be fatal from even a single-story roof.

3. Give the roof a preliminary washing. To begin the job of cleaning your roof, place your extension ladder where you will be able to safely and easily access the rooftop. Climb the ladder and use a garden hose to spray the shingles to knock off loose debris and soak the shingles in preparation for subsequent cleaning. If there are any large objects, such as tree limbs, leaf accumulation, or other bigger items, you may need to remove them by hand at this time.

4. Soak the roof with a bleach-water mixture. After you have sprayed the shingles, add laundry bleach to the hose-end mixer/sprayer chemical reservoir. Adjust the dial settings so the mixer/sprayer will add 1 part bleach for every 9 parts of water that flow through the unit. In addition, also set the sprayer to a stream nozzle to provide the most pressure for projecting the bleach-water mixture.

Once you have made your preparations, climb on to the roof and spray the shingles with the bleach-water mixture. Be sure not to miss any areas of the roof, and avoid overspray whenever possible. Once you have covered the entire roof, or section of the roof if you decide to work in segments, allow the bleach-water mixture to soak for 15 minutes so it can kill the G. magma bacteria.

5. Rinse the roof with clean water. After soaking the roof with bleach and water, use a garden hose and spray nozzle to remove the bleach residue from the roof. Only spray with normal tap pressure; never use a power washer to spray shingles, or you may cause permanent damage to the roof.

For further information about getting rid of stains on your roof, contact a company like Drey Roofing.