Algae stain on roof can be cleaned, prevented

Staff Writer
Mount Shasta Herald

Q: I was looking at my roof, and I noticed a dark streak where two roofs meet and form a valley. I had a contractor friend check the shingles, and he said I had a couple of years left before the roof would need to be replaced.

He also showed me a picture of the rubber flashing at the plumbing pipes that stick up out of the roof. The rubber is almost totally gone and, luckily, we have not had that much rain this year. There are no water stains inside the home below theses pipes. My friend said he could caulk the flashings, but it might not last the rest of the year. Is there a way to repair the flashings without removing some of the shingles?

A: I often see the rubber flashings damaged like this. Trying to caulk such a large opening would be difficult, and any protection it might offer against the elements would not last very long. What I recommend is to purchase a vent boot at a home or hardware store, and simply slide it down over the pipe to cover the damaged boot, and secure the boot’s metal rim to the roof using galvanized roofing nails.

The original boot’s metal frame will protect the home from the weather, and the second rubber ring will stop water from running down the pipe to the attic. It is likely the damaged boot is leaking to the attic, but you may not see a stain directly below the pipe.

A plumbing vent originates above a bathroom or kitchen and runs across the attic to the back of the home where it exits the roof. Rainwater can run down the pipe and across the attic until it meets a pipe joint, insulation or other obstruction where the water will then accumulate and drip to the attic floor below. Eventually, you will notice a ceiling stain or a stain on a wall somewhere in the home.

The dark streak on the valley shingles is algae, which grows on the surface of the shingles, usually on the north side or shaded sides of homes. Algae stains can be removed using a roof-cleaning solution available at home and hardware stores. When working on a roof, falling is a real safety concern. Unless you are very skilled at working on a slippery wet roof surface, the cleaning should be left to the professionals. Once the roof has been cleaned, place thin strips of zinc near the peak of the roof. The rain oxidizes the zinc, and the oxidized metal helps keep the algae from returning.

When you decide to replace the roof covering, ask the dealer for algae-resistant shingles, contain small granules of copper to protect the shingles against algae and stains.Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, Ind. 47702 or e-mail him at Scripps Howard News Service