Frequently Asked Questions about Roof Cleaning

What are the black streaks on my roof?

For many years, roof discoloration caused by algae has been observed throughout the United States and Canada. The discoloration usually has a brown to black appearance, and may be mistaken for fungus growth, soot, dirt, moss, or tree droppings.

Gloeocapsa magma is probably the most prevalent of several algae species that contribute to discoloration. All species are transported through the air, and tend to collect and grow upon roofing structures. Natural pigments produced by these algae may cause a white or light colored roof to gradually turn dark brown or black. Discoloration may appear as uniform discolorations or streaks.


Why is algae growing on my roof?

Algae feeds on the limestone filler in asphalt shingles.  Because it is grasping onto the shingles it will not wash away over time.  If fact, it will continue to spread and darken until removed.

Growth of algae on roofs has been most widespread in the Gulf States and along the Northwest and Eastern Seaboards. It is not, however, confined exclusively to these regions. Algae growth occurs to varying degrees in all regions of the country, especially those with warm, humid conditions.

Almost all types of roofing systems are susceptible to algae discoloration. It is, of course, most readily visible upon light colored roofs.


Can the algae on roof shingles damage my roof?

Algae feed on the limestone filler used to give the shingles weight.  Over time, the ceramic granules protecting the surface of the shingle begin to loosen and wash off.  This diminishes the roof’s ability to reflect heat rays which can overheat a roof increasing air conditioning bills and shortening shingle life.  Algae on roof shingles also encourages other damaging biological growth, such as lichens and roof moss.

Can the moss and lichens on roof shingles damage my roof?

Roof moss and lichens anchor into the top layer of the shingles and remove granules.  Moss and lichens grow up into the gullets of the shingles and along edges causing them to lift and make the roof more prone to wind damage and leaks.  Acting like sponges, moss and lichens keep the roof damp causing even more growth.

Why shouldn’t I pressure wash my roof?

Using a pressure washer to clean a roof can easily cause irreparable damage, especially on asphalt roofs.  The strong pressure dislodges granules from shingles causing them to fail prematurely. For this reason many roofing manufacturers will void the warranty if a pressure washer is used on a roof. 

Why shouldn’t I power wash my siding?

Power washing can cause serious damage, such as dislodging siding or forcing water under siding or soffits. Power washing is often not recommended by siding manufacturers and may void the terms of their warranty.

How do I prepare for a roof cleaning?

These steps will help ensure your roof cleaning goes smoothly:

  • Please do not schedule window cleaners, lawn care providers, or other contractors on the day of your roof cleaning.
  • Keep pets indoors or well away from the house.
  • Quench your plants thirst ahead of time by thoroughly watering your lawn & plants around the perimeter of your house the night before your scheduled cleaning. If a heavy rain occurred the day or night before your scheduled cleaning, consider this step done!
  • Move all lawn furniture, plants and other movable objects away from the close perimeter of your home.
  • Park cars out of the driveway, or at least 10 feet from the perimeter of your home.
  • Make sure all windows are closed the day of your cleaning.
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