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We have all seen roofs discolored and unattractive – covered with mold, algae, and bacteria. Like most issues affecting the integrity of a house, the problem goes much deeper than the unattractive appearance.

common roof stains
The gloeocapsa magma survives by eating minerals and excreting oxygen. The bacteria multiply rapidly by feeding on moisture and calcium carbonate (in the form of the limestone crystals used to manufacture asphalt shingles).

The problem has both spread and worsened dramatically during the last two decades. Internet research finds the main reasons cited for this worsening of the problem are:

1. The trend to rising humidity and temperatures (favorable conditions) has lead to increased reproduction (more spores)

2. Fiberglass shingles (the most commonly seen amongst today’s residential homes) are made with limestone as a filler (in the asphalt). These shingles hold moisture and organic “bacteria food” material longer (particularly on the north-side of the roof) than the paper/asphalt/ceramic shingles of 20 years ago. This is their food source.

Once the bacteria have taken hold of a roof surface that is favorable, it takes a significant amount of time before they become noticeable. From that stage, the visible evidence gets more noticeable every year until they take over the surface. This bacteria colony forms a tough outer shell for protection, and spends its days eating away at the infected roof.

It can be argued that there is little scientific evidence that these organisms cause actual harm to the roof. That is primarily because there is no financial support for creating a long term scientific study of the problem. The roofing manufacturing industry has no interest in proving to potential customers that there is no reason to replace their roof, for example. While this kind of study would help property owners make better decisions, it is really only financially valuable to you as a roof cleaner.

As contractors, we see the kind of damage done – up close. Common sense tells us that holding moisture in the shingles while breaking the shingles gown on a molecular level cannot help but contribute to the death of a roof. We see evidence every day of premature aging, rotting, and granule loss.

thick shingles due to moisture

Shingles swell in thickness with this retained moisture. Most of the other visible damage is along the edges, where we see rounded and uneven edges, as well as curling. As the shingle degrades, it becomes a perfect host for further culprits, like algae and lichen. Once lichen appears, a roof may not even be salvageable.

This problem can appear at any time in the life cycle of a roof, even when the roof is relatively new. A shingle that might have lasted 20 years or 30 years if exposed only to weather may now become useless long before that time.


Roof moss is one of the other common reasons for roof cleaning. It has its own chemical solutions. Controlling the growth of moss is a potential service contractors can offer with seemingly no end of new customers.

moss control

Zinc Strips

Zinc strips are commonly used for a long-term solution to controlling mosses (see photo at right: skylights are surrounded by galvanized flashing that has suppressed the growth of mosses below the skylights). The remaining parts of this cedar shake roof are covered mainly with the moss Dicranoweisia. Zinc strips and galvanized flashing are apparently relatively safe and inexpensive. They effectively kill or retard the growth of mosses and fungi and appear to have effect up to 15 feet below the zinc flashing along the length of the flashing. Zinc strips are considered to be effective for up to one year, after which they can be replaced. The effect of galvanized flashing (example above) can persist for decades. Success rates vary with the degree of moss development and weather. Zinc strips or flashing are most effective before mosses are well developed. Physical removal of existing moss followed by installation of zinc strips or flashing is an effective long-term strategy for suppressing moss growth.

Negative Side Effects: Direct runoff from the zinc strips or flashing to surrounding vegetation, fish ponds, or water supplies should be avoided, because some contamination by zinc is likely to occur. Zinc strips should not be used with strong acids or bases (like bleach).

Zinc Sulfate

The active ingredient for most moss killers is zinc sulfate monohydrate – usually at concentrations of 99%. Zinc sulfate will not stain roofs or corrode aluminum and galvanized gutters. To use zinc sulfate one can apply powder directly to moss areas. Manufacturers recommend that powder should be applied at the rate of up to three pounds for every 600 square feet. For spraying combine three pounds to five gallons of water and apply to 600 square feet. This product needs to be applied on a calm day.

Powder application has been known to control mosses for two years and spraying application may need to be applied annually. Some roofing companies will guarantee no mosses for up to five years after using this product while treating roofs. Avoid runoff that can reach aquatic areas. Toxic to fish.

Zinc Chloride

Zinc chloride comes in two different concentrations, 13% and 62%. The 13% concentration can be applied directly to moss without mixing with water. Spray directly from nozzle six to ten inches away from target. Make sure to wet the area thoroughly. The 62% concentration needs to be mixed with water before applying to an affected area. Mix one pint of concentration to three gallons of water. Using a backpack sprayer, one manufacture recommends using one gallon for every 100 sq ft. Zinc chloride should be applied just prior to fall rains or in the early spring.

Zinc chloride is effective in controlling mosses from one year up to five years. Increased application concentrations may be needed in areas of higher moss concentration. Avoid drift and runoff when using this product. Zinc chloride will affect other plants and lawns. Application should only take place when air is still and when no rain is expected within 24 hours. Zinc chloride is corrosive and should not be used when copper fixtures are present. This product is toxic to fishes and aquatic invertebrates. Avoid contaminating water sources with zinc chloride. If zinc chloride comes into contact with a painted area damage is possible.

Zinc – Copper Sulfate mix

Zinc and copper sulfate comes dry, and can be applied as a powder or mixed with water. This product reacts electrolyticallly with water to stimulate a slow release reaction. Zinc and copper sulfate will not stain patios, decks, walls, walkways, buildings or roofs. However, zinc and copper sulfate may react with red bricks. This product is not harmful to lawns, ornamental shrubs, trees, turf or other vegetation such as flowers and vegetation. To apply simply sprinkle areas thoroughly with powder when it is wet, either after a rain or when early morning dew is present. Do not use this product in high wind. For spray application a wet applicator may need to be purchased. It is not clear if this product is still available commercially. According to information from one manufacturer, apply one pound of moss killer to 1000 sq ft. This product may be applied anytime during the year, but should not be applied while it is raining. Since this product specializes in the slow release reaction; allow plenty of time for the chemical to act. This product is corrosive and should not be used if copper fixtures are present.

Applications of zinc and copper sulfate are said to last for up to one year depending on the concentrations of moss. Annual application is generally needed. Though this product is supposedly safe for surrounding plants, it is still toxic to fishes and aquatic invertebrates. Do not apply this product to water or let the product come into contact with water sources. When applying this product or any product to rooftops it is essential to avoid runoff. Collecting the runoff would greatly reduce the negative effects on the environment.


Chlorine bleach (sodium hypochlorite) can be used on a number of surfaces contaminated with mosses including decks, patios, walks and roofs. When used to proper concentrations bleach is non corrosive to metals and will not stain treated areas. One should, however, avoid contact with clothing. Brand names of bleach especially for mosses can be found in the moss control area in garden centers. Bleach is applied just as you do for roof cleaning, making this solution perfect for what you already plan on doing.

Bleach applications remain effective for up to one year, but annual applications are usually necessary. Bleach at these concentrations will be toxic to plants if left on for more than ten minutes. After ten minutes, injury or “burning” of foliage will occur. Since this product is toxic to fishes and aquatic invertebrates contact with water sources should be avoided.