What, if anything, should I do about all the moss that's growing on my roof?
—Dorothy Plummer, Dingmans Ferry, PA.
Tom Silva replies: Moss holds moisture against shingles, shortening their life. It also catches debris—twigs, leaves, dirt—that makes the moisture problem worse. But before removing the moss, grab a pair of binoculars and take a good look at your roof. If you see shingles with surface cracks, small blisters, curled-up edges, or areas where the surface granules have worn off, you probably need new roofing. But if the shingles are in good shape, here's how to get rid of the moss.
First, take these precautions: Spray water on all the plants under the section of the roof you will be working on or cover them with plastic tarps. The 50-50 mix of water and bleach that's used to kill moss can also kill or damage any foundation plantings it drips on. Also, because the roof will become very slippery when you spray the moss with the bleach mixture, stay off the roof and on a ladder during the entire process. An extension ladder with a roof standoff provides a secure platform.
Apply the bleach solution with a pump sprayer until the moss is saturated, then wait 15 minutes or so to rinse the roof thoroughly. If your foundation plants aren't covered, rinse them, too. Now wait a week or two for the moss to dry up. If you can, get on the roof to brush it off. If not, the dead moss will eventually wash away.
To prevent the moss from returning, install zinc or copper strips near the roof ridge, above the areas that tend to get mossy. Rain washing over the strips will leach out metal ions and send them down the roof, preventing new moss from taking hold. You can find these strips online at zincshield.com. Also, trim any overhanging tree branches as much as you can. Moss thrives in the shade that trees provide and the debris they drop.