Your roof is dirty, you can see black stains, moss, lichen or maybe some other type of fungus. Now the question is "how do I clean it?"
You are not alone, in fact I'm sure that you have noticed the same problem on other roofs in your neighborhood. If you have done any research you will have noticed that the problem is in every town and in every state.
What is it? The black stains that you see is a form of black algae, it is a living organism feeding off of your roof. It generally can only survive on the north facing roofs or on heavily shaded sections of your roof, that is because the ultra violet rays from direct sunshine will kill the spore and direct sun light dries out your roof not allowing moisture to congregate. The exact opposite happens on the north facing and shaded sections of your roof. Moss, algae, lichen or any fungus needs three things to survive, air, food and water. We discussed the water source being on the north side or under shade, the food source comes from the shingle contents, if you have wood shingles the wood is the obvious food source, if you have asphalt shingles, then the limestone that is used as a thickening agent is the food source.
Is it damaging my roof? Yes, I have seen moss eat through two layers of shingles right into the wood underlayment, black algae is a surface feeder, eating the shingle granules, while spreading over the roof. Just look in your gutter of the infected side, I guarantee it is full of discarded shingle granules. So, yes it can cut a 40 year shingles life expectancy in half or worse.
What can I do about it? 3 choices here, clean it yourself, hire a professional roof cleaning company or replace it. Let's examine these three options.
Replace it - If you have that kind of money, great! here is the bad news, even though the new shingles on the market are algae resistant, buyer beware! read the warranty they only guarantee them for an average of 10 years, it takes that long before visible signs, from the ground, confirm that algae is present. If you have the money and want to buy yourself 10 years of a clean roof, go for it.
Hire a roof cleaning company - Good choice but who? they are around, usually listed in the yellow pages under "roof cleaning" or "power washing". What to look in a roof cleaning company is very important. 1. references 2. specific insurance coverage, a painters, carpenters or any construction general liability policy does not cover roof cleaning, check it out 3. inquire about their cleaning product, there are a lot of products out there that will clean a roof but if they heavily chemical laden they will dry out the roof shingle to such a point that a few months down the road on a windy day, shingle pieces will be flying. These product will also stain the face of your home, kill plants and grass, you want them to use something "environmentally safe", not just "bio-degradable" that only means that eventually it will disappear, there are bio-degradable plastics available but you wouldn't want them laying on your lawn for the next 20 years, would you? so make sure the product is environmentally safe 4. portfolio, they should have before and after pictures of jobs that they have done and you should take the time to drive by one of the homes and make sure that they didn't just download pictures off of the internet 5. do they have a home improvement license? they should. If they can't provide those 5 things, move on to the next company.
Clean it yourself - Dangerous work! but achievable if you consider yourself the "handyman" type and the most cost efficient solution. Buying the right product is the trick, don't even consider bleach it is a disaster waiting to happen buy a product specially designed for roof cleaning. Don't fall for the "just spray it on and nature will clean it" product sales pitch, that's BS, it might work but it would be years before it would be clean and you would have to keep reapplying it. No, there is only one way, get up there, stage the roof safely along the bottom even if it is walk able the roof will become incredibly slippery, spray the product on, usually through a "garden style" sprayer, take out a medium bristle scrub brush and scrub the infected area keeping the area moist while scrubbing then take a regular garden hose and wash it off, it may only become 80% clean at first but all the loose, dead fungus will wash off with the next rain.
After you have successfully cleaned your roof, had it cleaned or replaced it, I have one very important suggestion. Apply a strip of zinc flashing across the ridge line, it is the only protection your roof will have against fungus growth. The effectiveness of zinc is easily proved, just look around any metal object on your roof, pipes, copula, or chimney and you should notice that the area below it is clean. The way it works is quite simple, when it rains, metal ions wash out of the metal and coat the roof below it, the metal ions will not allow growth to occur. Zinc has the best run-off of all metals and is the most cost efficient, you may know that zinc is used on boat bottoms and around boat props to keep algae from growing. I recommend that the zinc should be at least 3 1/2" wide, zinc has a run-off coverage area of about 10 feet for every inch the zinc is wide. The zinc remains actively releasing ions through out it's life.
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