Roof Stains of All Colors and Causes

Have you ever noticed black streaks across a rooftop and wondered how and why the shingles would be stained in almost perfectly vertical columns? Have you ever seen a building topped with a layer of green moss and questioned if that growth harmed the integrity of the roof?

Roof stains come in many varieties, sizes and colors. Fairly common on Community Association buildings, these stains can be caused by chimney soot, rust, extractive bleed-through, plant debris and different types of algae and fungi. These discolorations are not only unsightly but can be an indication of damaged roof material. Community Association managers and board members should have all roof stains investigated to verify that the functionality of their roofs has not been compromised.

Chimney Soot and Rust Stains
Soot and rust often cause black and brown roof stains below and around chimneys and other roof penetrations. When there is little or no evidence of staining on other areas of the roof, soot and rust are the first suspects. Soot is the fine, black powder byproduct of combustion. Whether it comes from a fireplace or an oil furnace flue, soot will wash down the slope of the roof and leave a trail of discoloration across the shingles.

Likewise, rust from chimneys, metal flashings, antennas, metal mounts on satellite dishes and other iron penetrations will run down and mark the roof. Rust is the reddish/brown product of iron oxidation and although most rust-stains are cosmetic, they should be investigated. In some cases, rust-stains are an indication that metal flashings are leaking or about to leak, which can cause major roof damage if not corrected.

Extractive Bleed-Through Stains
Extractive bleed-through stains are black streaks that run vertically down an asphalt shingle roof. These stains vary in width and length, but are a clear indication of a defective roofing product. Extractive bleeding occurs when an excessive amount of bitumen or black pigment is released from the asphalt shingles and runs down the surface of the roof. The defect is simply that the bitumen was not well bound in the asphalt mix during production. Since bleed-through stains are a characteristic of the asphalt shingles themselves and not from an exterior source, the discoloration will probably manifest over large areas of roof.

The good news about extractive bleed-through is that it is a product defect described by manufacturers as cosmetic and should not affect the integrity of the roof. The unfortunate news is that there is not much to do to remedy bleed-through stains because power washing and chemical treatments can reduce the remaining useful life of the roof. When it is time to replace the shingles on community buildings, selection of a higher-grade product is likely to reduce the chance of extractive bleed-through and provide a roof with a longer useful life.

Proximate and Debris Stains
Proximate stains are caused by a nearby factor such as a tree, chimney or adjacent building. Black, brown or gray proximate stains are blemishes that develop only in limited areas of a single roof slope and are caused by a specific source. Anything shading a rooftop may prevent it from drying and aid in the growth of algae, fungi and/or moss.

Debris stains are caused by the piling and standing of rubbish on a roof, often from a proximate cause. For instance, a nearby tree may drop leaves, branches and/or acorns onto a rooftop creating debris spots over time. The accumulation of leaves, twigs and other organic debris on a roof creates stains when their decomposition encourages the growth of fungi, algae and moss, and releases organic acids that can cause discoloration. Piles of debris will also hold water on the roof surface, which can cause water infiltration as well as additional wear and tear.

Carefully clearing away debris by gently sweeping or washing it off of the roof's surface can help reduce the discoloration of shingles. Likewise, cutting unnecessary overhanging tree branches may help reduce the amount of shade and organic debris that can cause proximate and debris staining. This type of community maintenance is often overlooked by boards and should be considered when budgeting for future expenses.

Algae, Fungi and Moss
Algae and fungi typically appear as black spots or streaks, varying from what appear to be faint shadows to dark stains. Moss and lichens are greenish and appear to be more of a growth than a stain. All of these building detriments are common on roofs that are subject to little sunlight and high humidity.

Mold is a type of fungus supported by the carbon released during the decomposition of other organisms such as leaves or sticks left on a roof. Algae are plants that use photosynthesis and are supported by moisture in inorganic material such as the calcium carbonate found in the limestone particles of asphalt shingles. Mosses are small soft plants and lichens are composite organisms consisting of fungus and algae. While inherently different, mold, algae, moss and lichens thrive in warm humid environments and normally appear on the northern slope of roofs where shade and moisture are often found. These organisms occur naturally and are hard to prevent. In fact, algae and fungal growth used to be limited to warm and humid climates, but can now be seen on rooftops as far north as Canada.

Gently cleaning the roof is the first step to removing algae and mold stains. Remove all sticks, leaves and other organic material and then scrub the shingles lightly enough to prevent removing their granular surface. To prevent future growths, install copper or zinc strips at the peak of the roof. As rain falls, these metals react with the water to produce a mild acid that kills moss and lichens as it washes over the shingles. Using copper and zinc as a preventive measure works so well that many shingle manufacturers now offer shingles impregnated with these metals.

Whether roof stains are caused by chimney soot, skylight rust, faulty shingles, twigs or too much shade, they are unsightly. Ugly roof stains can detract from a Community Association's curb appeal and may affect resale values. Worse yet, roof stains can be an indicator of a damaged roof or act as a detriment to it. Though primarily a cosmetic concern, all roof stains should be investigated to check on the roof's structural condition.

If you are interested in a roof analysis and/or specification, please contact Kipcon for a proposal.