Roof Discoloration and Algae Stains

Roof stains are fairly common and can be caused by a variety of factors. You can also view the photos at below to get an idea of what to look for.

Some people mistakenly think the presence of roof stains means they need a total roof replacement. That may or may not be the case --- some of these problems can be fixed with a simple cleaning instead of a total replacement. The flip side is that a cleaning is a waste of money if roof damage has already occurred because of the staining. Check with a reputable contractor to determine your best option.

Typical causes are:

• Algae, Mold or Fungus Stains - Algae stains usually appear as dark black spots or streaks. Sometimes referred to as mold, mildew or fungal stains, they're common in shaded areas and in parts of the country with moderate to higher humidity, and less common in very dry, low humidity areas such as the Southwest. These substances occur naturally in the air and are hard to prevent, particularly on asphalt shingles becuse the limestone particles used in many shingles act as food. Removing algae stains is a simple process, but be careful not to scrub too hard (and avoid power-washing) because that can remove the granules from the shingles.


• Stains caused by product defects (also known as extractive bleeding) - If black or dark staining is present across the entire roof surface, it could be a product manufacturing defect known as extractive bleeding. It's caused by dark pigment from the asphalt shingles leaching to the surface and running down the roof. It's best to get this looked at immediately because in many cases it is covered by your manufacturer's warranty.


• Dark stains caused by deterioration of leaves, twigs and branches that have remained on the roof for too long. - Accumulation of leaves, twigs and organic debris on the roof creates stains because the rotting, decaying debris causes growth of fungi and algae. If you see debris piling up on the roof (especially in valleys where it's protected from wind blowoff), it's worthwhile to use a leaf blower or broom to get rid of it before it starts decaying and causing stains. If you use a broom, make gentle sweeping motions to avoid removing granules from the shingles.


• Chimney- and soot-related stains - usually visible as dark black stains alongside and directly below the chimney with little or no evidence on other areas of the roof.


• Rust stains - usually brown or red stains caused by rust runoff from metal flashings, antennas, metal mounts on satellite dishes, chimney caps, etc. Most rust-related stains are cosmetic, but are worth investigating to make sure the rusted areas (particularly flashings) are not leaking or about to leak, which can cause major roof damage if not corrected.


• Moss and lichens - Moss and lichens are easily identified by a greenish color and typically appear in roof areas that receive little or no sunlight or stay in the shade for most of the day. The main problem with moss growth is that it tends to hold moisture, which accelerates shingle wear and decay, especially during cold months when the moisture freezes and can damage the granules on asphalt shingles. Lichens can grow roots which penetrate the surface of shingles, causing tiny holes and allowing moisture to seep in, causing deterioration to the undersides of shingles and ultimately damaging the roof.


• White or light-colored stains - usually caused by paint residue runoff from surfaces above the roof level, such as gables and dormers.

(click photos for larger view)