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There isn't a lot of information, so I'll make some assumptions. First the black "spots" are on the inside of the roof sheating, the black spots are assumed to be mold and you live in a cold climate?
If my assumptions are correct you probably have inadequate ventilation of the attic area causing moisture to accumulate, condense and probably freeze during cold weather (hence the spots being limited to north side where sun does not directly heat roof in winter) This water probably thaws in the spring and leaves enough moisture at the surface (water activity) for a long enough time with sufficient temperature to allow mold (probably Cladosporium) to grow.
This is probably either as a result of a moisture source, such as a bathroom vent or perhaps ambient moisture escaping the finished area at an attic access, can lights, etc. I suspect when you added the insulation, the temperature of the attic space was lowered during heating season as less heat was lost to the attic from the finished space.
Probably need to verify that there are no sources adding moisture load to attic, then add or increase the size of the venting or add a humidistat activated exhaust fan to the attic space to correct the cause and then properly remediate the (apparent) mold.
If the spots are on the outside, the addition of insulation may still have changed the balance of the attic temperature, resulting in moisture staying on the roof longer than in the past as the roof is not heated and dried as quickly by the "lost" heat from the inside. This may mean it is becomes a maintenance issue either by addition of a controlling agent in or on the roofing material or (annual) cleaning to remove the "spots" when they appear.
This type of staining (algae as the other posters have said) will usually only happen on the North facing surface of the roof. Same with condensation on your car glass. Suprisingly, the North facing sky will actually remove heat from a substrate and thus make the roof surface colder.
Colder roof surface = condensing surface. The moisture combined with the organic component will combine for a great growing surface (that the the rock dust organic component of the shingle).
It is not surprising that it is happeing more after you insulated the home. By insulating (a good thing), you have reduced the heat sink between the roof and the conditioned space below and therfore made the roof colder in the process. Colder roof...same as stated above.
Plenty of roof washes out there that will handled most of this stuff and it does not mean the roof is cooked.
Over a period of time, shingles may develop dark brown or black streaks that are sometimes mistaken for soot, dirt, moss or tree droppings. In actuality, this discoloration may be caused by algae growth. Although most
roofing systems are susceptible to algae discoloration, it is most readily visible on white or light-colored shingles.
Today many of the newer asphalt shingles have an Algae Resistant granular system that guards against roof algae. The system uses copper roofing granules from 3M to prevent algae growth‚ protecting the roof and preserving the home’s be auty. Here is a link to a good explanation.... http://roofpedia.com/algae-resistant-roof-shingles/
What kind of roof do you have? Is it an asphalt roof? The average lifespan of asphalt composite roofs is 10-15 years. Depending on how extreme the weather is in your area, some roofs made with temporary roofing materials may start to fail as soon as eight years.
The most common cause of this type of dark spots is a variety of blue/green algae called a Gloeocapsa Magna. Ths stains often evolve to a moss buildup, as the algae increases moisture. That is not only an appearance problem, as algae proliferation may shorten your roof life and increase energy costs. (You mentioned you recently added more insulation to your attic. Not only that was probably not the cause, but potentially, even the need for the extra insulation is already a sign of an underperforming roof.)
There are products on the market to clean it, but keep in mind the algae also reduces the composite integrity, so removing it may still keep your roof exposed.
It would be advised to call a professional inspector to do a roof and attic inspection. It can identify any other related issues and give you a timeframe of how much longer can your roof can hold.
Also refer to your warranty, keeping in mind each manufacturer have a different levels of protection.
When it's time to re-roof, you may want to consider other lifelong roofing materials (such as metal).
Feel free to contact us if you have any other question.
Using the information you have provided and not visually inspecting the roof, the black spots on your roof are more than likely algae. Most shingle manufacturers use copper or zinc coated granules to aid against the growth of algae. As the rain washes over your shingles, particles of the copper are dispersed helping the prevention of algae growth. Most manufacturers warranties will cover the first 10 years against streaking and staining, which helps to explain why you have not seen evidence of this issue before. There are low pressure solutions available to clean your roof but they will not prevent the stains from returning. The addition of copper or zinc strips under the first full course of shingles below the ridge cap could provide to be a longer lasting remedy. I suggest that you contact a reputable local roofing contractor for a complete roof inspection before you make a decision on the remedy, as the algae can hold moisture and can prematurely age your shingles. ( Damon Ward Project manager At Allstate Exteriors llc.)
Black spots on the roof usually form do on the Northern facing side of the home because it has less direct exposure to sun light.
Blue-Green algae is usually what the black spots are, however, depending on the age of the roof, sometimes the loss of granules also shows black streaking, which can be a sign that the roof is nearing the time for replacement.
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