What causes moss to grow on a roof and what’s the best way to get rid of it? -B.V.

Moss thrives in a damp, shady environment. For this reason it often occurs on the north side of a roof—since it receives the least amount of sun—or under overhanging trees that provide shade. Over time it can cause roofing to degrade.

You can physically remove moss from your roof with a long handled scrub brush if you’re careful not to overdo it. While a pressure washer can be used, the powerful jet of water could damage asphalt shingles. With either method, work down the roof to keep from lifting and breaking shingles.

There are also several chemicals on the market specifically made to kill moss. Diluted bleach will work as well, though the runoff can damage plants. Be sure to wear rubber gloves, protective clothing, and eye protection when working with chemicals.

To prevent moss from returning, cut back any limbs that overhang the roof, or install strips of copper or zinc along the ridge. Since the treatment for moss is similar to that for algae stains, refer to our article on How to Remove and Prevent Algae Stains on Asphalt Shingle Roofs for more details.