ROOFTOP MOSS: Does It Damage My Roof?

A reader emailed me with this question . . .
Q: Is moss on a house roof a serious problem? About 20% of the roof area of a house we’re looking to buy has moss, mostly near a large maple tree. Is there a proper way to remove this moss without damaging the roof? Should we get rid of the tree if we buy the place?

Moss on Roof

Notice the lack of moss downstream of the chimney and flashing. This happens because tiny amounts of zinc get dissolved from the chimney. It doesn’t take much, but a little zinc stops moss and lichen from growing on roofs.

A: Moss and lichen on roofs are not uncommon and it doesn’t necessarily happen only in shady areas. I’ve seen quite thick growths in full-sun locations. This is especially true with lichen. The good news is that there are ways to remove the existing growths and prevent re-growth. Don’t cut down the tree if it’s healthy. Different products are sold for cleaning up roofs. I’ve used an oxygen-based stainer remover, a plastic bristle brush and a garden hose to remove moss. Some moss removers promise scrub-free results, but these take time to work.

Zinc Strip Installation

Here a strip of zinc is being installed under the cap shingles of a new roof installation. This detail will keep roofs moss-free for 20 feet downstream of the zinc. If your roof is longer than this, install another strip of zinc half way down the roof.

To prevent moss re-growth, install strips of zinc underneath the top course of shingles. Building supply outlets everywhere sell it in rolls. It’s not cheap (about $1 per foot), but it does work. Small amounts of zinc will be dissolved as rain runs down the roof, preventing new moss and lichen from getting a foothold. It’s completely safe for people, pets and plants. You can see this effect happening accidentally around certain kinds of metal chimneys and rooftop vents that have zinc coatings. The moss-free effect is really quite striking.

To install zinc strips on roof you’re reshingling, nail down the zinc so an inch or two is exposed after the cap shingles get installed. If you’re adding zinc to an existing roof, you’ll find it easiest to pry up the bottom edge of the top-most coarse of shingles, slip the zinc underneath, fasten it with roofing nails every 3 or 4 feet, then place a blob of roofing tar from a caulking gun every 8 to 12 inches to hold the shingle down in the wind.

Posted on August 24th, 2018