Cleaning algae from roof

Q.  This past year, we noticed black markings coming down the roof. Would you have any idea what is causing that, and will a power-washer clean it up? I’ve noticed several homes with shingles on their roofs with the same black markings. Do you have a solution?

Algae on Roof
Algae on Roof

A   Do not use a power-washer to clean roof shingles; it could dislodge the mineral granules that protect the shingles from UV damage. The black streaks are caused by a type of algae that grows on the mineral granules of the asphalt shingles when they do not dry fast enough after rain or after snow has melted off the roof.

There are several ways to remove algae. You can buy special solutions that are sprayed on the roof; you can have the cleaning done by firms that specialize in cleaning roofs, or you can do it yourself. If that is your choice, spray the shingles with a solution of equal parts fresh Clorox bleach and water. Use a plastic garden sprayer and apply the solution on a windless day at the rate of 1 gallon per 50 square feet of roof. The purpose is not to overspray and have the solution run off the roof for a long time.

Zinc Roof Strips
Zinc strips to control algae on roof

Wear rubber gloves and old clothing, as well as eye protection. Spray the solution from a ladder, as it is dangerous to walk on a roof, which may void any warranty left on the shingles.

If you have metal gutters, run water in them while applying the solution because it is corrosive. Soak all vegetation below the roof before spraying, cover it with plastic and rinse it thoroughly when you are done. Make sure there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

Installing special zinc or copper strips at the ridge (or replacing an existing ridge vent with one specially made for this purpose) should prevent recurrence.

Aluminum flashing won’t protect against algae on roof

Q.    I have the black algae problem on my roof that you have mentioned on several occasions. I wonder if using strips of aluminum flashing under the ridge cap would be as effective as the other metals you have mentioned. I am 83 years old and can’t do very much but I have a grandson who helps me a lot.

A.    Sorry, but it does not work. The strips need to be copper or zinc-plated. One good brand of zinc strips is Shingle Shield. Their Web site is www.shingleshield.com. The strips are installed under the first course of shingles below the ridge cap shingles.

Green spots on concrete patio

Q.    I live in a townhouse with a 10-foot by 10-foot concrete patio. The concrete has greenish-colored spots that appear to be moss or mold growing into its porous surface.

I’ve used a 2-to-1 mixture of hot water and bleach, and scrubbed the patio. This removed all the dirt but the greenish spots remained. What is this? Is there a simple and easy way to remove these spots? The patio faces west and does receive full afternoon sun.

A.    Try a mixture of equal parts water and fresh Clorox bleach (it contains chemicals other brands may not have). Pour it on and let it stand for a few minutes to see if it gets rid of the greenish spots. If it does not, increase the ratio of bleach to water and try it in one spot. Once you have succeeded, you’ll probably have to bleach the entire patio so it will have an even color.

Insurance company says to remove moss from shed roof

Q.    The insurance company said we have to get the moss off our shed roof by this coming October. She said to use a power washer. I remember you said not to use a power wash on shingles. We removed a lot of trees from around the shed. Could you please tell me an easy do-it-yourself method? It looks black and light green to me.

A.    I wonder why the insurance representative told you to remove the moss from your shed roof. I can’t see any reason for doing so. Power-washing roof shingles is not advisable; the pressure is very likely to damage the shingles. It will certainly take off the mineral granules, which protect the shingles from UV rays.

When you say black and green, are you seeing a discoloration of the shingles such as is caused by algae? That’s a different story. Moss shows as a soft growth that can easily be picked off by hand or brushed off with a floor-sweeping broom, so it would seem that if its removal is required, it should be simple enough to do so without having to walk on the roof (unless the shed is very large).

Algae takes time to form, and to kill

Q.    I enjoy reading your informative column. The problem I’m having is with fungus on the shingles on the northern side of my roof. The CertainTeed shingles were put on in 1994 and until about 5 years ago, they were OK color-wise with no black streaks at all.

Three years ago I took off 2 shingles and sent them to CertainTeed for testing because I believed there was a defect in the shingles. They naturally wrote back stating that there was no manufacturing defect and the stains were caused by fungus, and the type shingles I had purchased were not fungus resistant.

Now here is the question I have: Why did it take so long for the fungus to show up as black streak marks? A few other factors: there are no trees near the house to block the sun, and the roof does get a fair amount of morning sun as it doesn’t face directly north but northeast. And lastly, I put one of those zinc strips along the ridge of the roof but that has done little if anything to ease the problem.

A.    The black streaks are an algae growth with a mile-long name. These algae develop under moist conditions and it is possible that the morning sun is not strong enough, or stays long enough on the shingles, to dry them completely from morning dew or rain.

It may have taken several years for these black streaks to develop because moisture (including melting snow) may have penetrated the asphalt binder, and it does not dry as quickly as it did before. This is not a manufacturing defect — simply a sign of aging. How long ago did you install the zinc strip? Once the algae have taken hold, it takes a long time for the ions in zinc or copper to kill them.

Metal strips to control algae on roof

Q.    Somewhere on TV, probably in “This Old House”, was a gadget to inhibit the growth of lichens on roofs. I have a lot and would like to get rid of them without the problems with Clorox all over. This gadget was a strip of metal with one of the metals, possibly one of the rare earths, an inhibitor for these plants. It goes in on the ridge pole and the needed metal leaches out, runs down over the lichen colonies and does them in. Very neat, tidy and easy.

I am reminded: if it is too good to be true, it probably is. Have you heard anything about this?

A.    The control of algae, moss and lichen on roofs is well-known in the northwest where the roofers use copper strips set just under the cap shingles on each side of the roof. In the northeast, we can see how these problems are controlled by looking at old barn roofs. Wherever there are galvanized metal roof vents, we see that the shingles below vents are as clear as the day they were installed. This is not the case on the other areas of the same roofs where algae grows unchallenged. The same thing is visible on any roof below copper or galvanized vents and flashings.

You have several choices. You can have a building or roofing contractor install a copper or galvanized metal strip snug against the bottom of the cap shingles or you can buy Shingle Shield zinc strips from The RainHandler and have them installed underneath the first course of shingles below the cap shingles, as per instructions. Home Depot and some hardware stores sell rolls of copper for the same purpose.

Copper is quite expensive; galvanized metal a lot less. You can order Shingle Shield on-line at www.shingleshield.com. Although Shingle Shield strongly recommends cleaning the roof of all growth before installing their strips, this is not necessary unless you are impatient and can’t wait for nature to take its course over several seasons. Having the roof cleaned with their solution is quite expensive.

If you have a ridge vent, you can have it replaced with Air-Vent, Inc’s zinc- coated ridge vent that, when it rains, leaches ions that poison algae and other growth. Any building supply store can special-order this ridge vent. But that’s more drastic than installing metal strips.

Be aware of the fact that these metal strips are most effective on algae — the unattractive black discoloration found on asphalt shingles throughout the country; they are not always as effective in the control of lichen and moss.

Why asphalt roofs turn black

Q.    I have noticed over the years that asphalt shingle roofs turn black — sometimes almost across the entire roof and at other times just in areas that appear streaked. This is more noticeable on lighter colored roofs, especially white ones. This usually takes place sometime after 10 years from the installation. Is this a moisture problem under the roof? How can this be prevented?

A.    No, it isn’t caused by interior moisture but by an algae that grows onto the mineral granules of the asphalt shingles because the shingles do not dry fast enough after getting wet or after snow has melted off the roof. Observe the fact that this algae grows on unheated buildings like garages and barns.

There are several ways to remove algae — the growth responsible for the black streaks. You can spray the shingles with Wet N Forget or spray them with a solution of equal parts fresh Clorox bleach (it contains some effective chemical ingredients not found in most other bleaches) and water. Use a plastic garden sprayer and apply the solution on a windless day at the rate of 1 gallon per 50 square feet of roof. Wear rubber gloves and old clothing as well as eye protection. Spray the solution from a ladder as it is dangerous to walk on a roof, and it may void any warranty left on the shingles.

If you have metal gutters, run water in them while applying the solution because it is corrosive. Soak all vegetation before spraying the roof, cover it with plastic and rinse it thoroughly when you are done and there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

You can also have a contractor specializing in roof cleaning do the job for you. Installing special galvanized or copper strips at the ridge (or replacing an existing ridge vent with one specially made for this purpose) should prevent recurrence.