Keeping Moss Off The Roof

Does moss actually do any harm to asphalt shingles?

Not according to the Asphalt Roofing Manufacturers Association (www.asphaltroofing.org) and two manufacturers I called. Unsightly, maybe, but moss apparently won't shorten the life of your shingles or your roof deck. I have a hard time believing a thick layer of moss is actually good for your roof, but suppose a few patches here and there are mainly an aesthetic problem and you'd like to get rid of them.

There are a variety of chemical treatments, but pesticides formulated to eliminate moss often contain zinc sulfate. As you suggest, zinc (as well as copper) is toxic to aquatic life. The city of Seattle takes the problem seriously enough to discourage the use of either zinc or copper roofing.

If an entire roof of copper or zinc carries too many environmental risks, what about the strips of metal you mention? One of them, called Z-Stop, is a strip of zinc 2 1/2 inches wide that's nailed to the roof at the ridge (on an existing roof, use roofing nails and neoprene washers). When it rains, very small concentrations of zinc carbonate wash down the roof and prevent the growth of new moss. A 50-foot roll costs about $50, so you could treat an entire roof inexpensively.

Z-Stop (www.z-stop.com) recommends cleaning the roof off and treating it with bleach, before you install the strips.

I spoke with Seattle's environmental office and asked specifically about products such as these and didn't get a definitive answer. But the risk seems slight.

Something else to consider: The problem is that runoff finds its way into major bodies of water through storm drains or streams very close to the house. If your gutters aren't connected to public sewers, but drain into a dry well or rain garden instead, the risk of harming fish and other aquatic creates seems remote.

You still have to get moss off the roof. If it's a steep roof (anything with a rise of more than 4 inches for every 12 inches of run) or you have a two-story house, hire someone else to do it. A broken leg would be a bigger problem than a little moss. Don't use a power washer unless you really know what you're doing. Try sweeping the roof instead.

Moss won't thrive in direct sunlight. So removing low-hanging branches over parts of the roof where moss grows can also help.

In the end, there's no perfect answer. The most benign thing you can do is clean the roof gently when it needs it, trim back tree branches, and not worry too much about the moss. If it really bugs you, though, I'd go with the zinc strips and try to divert the runoff so it doesn't get into storm sewers.