Q. What causes those black stains that appear to streak down asphalt shingle roofs? Is there anything that can get rid of them?
A. If I took all the letters and emails Iíve received and stacked them in piles by category, the pile with questions about roof stains would be the tallest, edging out wet basements. Iíve discussed them before but due to the greater than average amount of warm and wet weather weíve had over the past few years conditions have been right for these roof stains to occur.The simple answer is itís a algae. The spores are always floating around in the air and if you looked at the surface of a roof shingle under a magnifying glass youíd see that itís a rough surface with lots of tiny crevasses into which these tiny creatures land, lodge and flourish. Moss seems to prefer shady, north facing or tree covered roof surfaces but algae doesnít seem to care. Moss can be destructive and troublesome to roof surfaces and create water back-ups and blockage s in valleys and at roof intersections but algae merely makes the roof look dirty and shabby without hurting it in any other way. Some try to power-wash these stains away. Powerwash and roof should never be in the same sentence. Too much nozzle pressure and you've blown your roof off in the worst case. At the bare minimum, high-pressure-- and by that I mean water nozzle pressure greater than what comes out of a household faucet-- will shorten shingle life. Roof shingles are more delicate than people think. The common household deck and siding wash recipe that I always recommend, consisting of one cup liquid laundry bleach with one quarter cup of automatic dishwashing detergent in a gallon of warm to hot water, will work too. Itís cheap and easy but be careful when doing anything up on a roof if working on a roof is something you donít do for a living. One small slip and your whole world changesóitís not worth the chance. If you try, use a garden sprayer and work from the bottom up. Be careful to be even. Iíve seen botched jobs and they look as bad as the original problem. Some area businesses offer roof cleaning and anyone with a cedar shingle roof Iím sure has contemplated hiring one from time to time. Now homeowners with asphalt shingles think about shingle cleaning. For future stain prevention there are strategies to consider. If the time for re-roofing is approaching y ou can specify a replacement roof shingle that is manufactured with copper and magnesium granules imbedded in the surface that will inhibit if not prevent the growths from getting started. Most roof shingle manufacturers offer them and the cost isnít that much more than regular shingles. They are very popular in the Deep South where roof algae and moss growth is relentless. If you've got problems now with a young roof you can buy metal strips which you place up near the peak of your roof for the length of the house and as the rain washes over the strips the oxides of the metals travel down the roof inhibiting growth. They can be obtained from the Chicago Metallic Co., South Austin Ave., Chicago Il 60638 for about $3.00 per lineal foot (www.chicagometallic.com/product). They come in three foot strips. Measure the length of your house and multiply that by two for the amount you'll need. Call the supplier for a quote to include shipping and handling. They can be reached at 1-800-638-5192. I'll bet these strips will take a couple of years clean the roof off if the staining has taken hold but will certainly work to prevent it if applied to a newer roof. This company also sells a cleaner called Shingle Shield Roof Cleaner that is essentially bleach based.