Roof stains can be caused by a variety of factors, but observing the stain color, location, and relationship of the stain color and location to roof components such as chimneys and flashings, as well as the stain’s relationship to roof conditions (pitch, shading, nearby trees, debris, roofing materials), will usually lead to a quick and accurate diagnosis of the cause of the staining.
Knowing what causes those ugly black, brown, green, red, rust, or other colored stains on building roofs can help us find a solution. It is important to know how to recognize and explain the most common causes of black, brown, red, gray, green, or white stains on roof shingles and other building surfaces, what they mean, and what they can result in.
Roof staining can come from a variety of causes, each with varying degrees of importance, ranging from cosmetic to roof-damaging.
Black algae, bleed-through, or extractive bleeding of asphalt, dirt, soot, or organic debris are common causes of dark black or brown roof shingle stains. There are, however, various types of roof stains and their causes.
By proximate cause of a roof stain, we imply that it’s important to pay attention to the placement of a stain on a building, as well as the nature and condition of neighboring buildings and site characteristics that could be the source of the stain.
If you notice stains around or below a chimney, for example, investigate the chimney as the source of the stains. Isn’t it pretty simple?
The rusty metal spots on the roof visible at the left are almost definitely from the rusted chimney flashing.
Algae or fungal growth on shingles used to be limited to warm, humid climates, but now they can be found on houses as far north as Canada. The increased use of crushed limestone as a filler material in asphalt shingles, according to some experts, is to blame for the spread.
Limestone is cheap and forms a long-lasting shingle, but the calcium carbonate in it encourages algal growth. Zinc or copper granules are incorporated with the colored stone topping in algae-resistant (AR) tiles. When the zinc or copper in the shingles is exposed to water, it is released, limiting algae development.
Algae resistance warranties are often for fewer than ten years because the protection ceases when the mineral is washed away. Due to a higher concentration of AR granules in some shingles, they provide longer-lasting protection.