· by Michael Blair, Nashville's Roofers
Drive through any neighborhood in Middle Tennessee and you will see it: roofs with dark vertical stripes, heaviest on the side that gets the least sun. Homeowners often read it as mildew, soot or a roof going bad. It is almost always a type of algae that feeds on the limestone filler in asphalt shingles and thrives in our humidity. On its own it is mostly a cosmetic problem. Moss is a different matter, and the wrong cleaning method can do more harm in an afternoon than either will in years.
Algae, moss and lichen are not the same problem
The streaks are a blue-green algae that turns dark as it builds a protective coating against the sun. It spreads with wind and rain and runs downhill in lines, which is why the pattern looks like something dripped. It sits on the surface of the granules and does little direct harm, though a dark roof absorbs more heat, and a buyer will see it from the curb.
Moss grows in thick green cushions, usually along the bottom edges of shingles and in shaded valleys under trees. It holds water against the roof, lifts the shingle edges as it grows, and can let wind-driven rain work underneath. Lichen, the flat gray-green crusts that look painted on, grips the granules tightly and is the hardest of the three to remove without damage.
Why north slopes and tree cover make it worse
All three need moisture, and anything that keeps a roof damp longer after rain or morning dew invites them. A north-facing slope dries last. A roof under an oak or a big maple spends the fall covered in wet leaves and the summer in shade, and the debris that settles in valleys and behind chimneys holds moisture for days at a time.
This is the time of year to look, as the leaves start coming down and the roof shows what has been growing under them. Trimming branches back from the roof lets it dry and cuts the supply of debris. Keeping valleys and the uphill side of chimneys clear stops the damp pockets where moss usually gets its start.
Cleaning it without ruining the shingles
Never pressure wash an asphalt roof. The granules are the shingle’s protection from the sun, and a pressure washer strips them in stripes you will see for the rest of the roof’s life. It can also drive water up under the tabs, and it breaks the seal strips that hold shingles down in the wind. Many manufacturers say plainly that this kind of damage is not covered by the warranty.
The accepted method is a low-pressure application of a cleaner made for roofs, left to work and then rinsed gently, or simply left for rain to finish. Moss should be loosened with the cleaner and brushed off downhill, never scraped uphill against the laps. Rinse and protect plants below, since runoff from these products can burn foliage. Walking a wet, treated roof is dangerous, and that part is where most people are better off hiring it done.
- Low pressure only, never a pressure washer
- Brush moss downhill, with the shingle laps
- Wet down and cover plantings under the eaves
- Check the shingle maker’s cleaning guidance first
Keeping it from coming back
Strips of zinc or copper installed just below the ridge release a small amount of metal each time it rains, and that runoff discourages growth on the slope beneath. They work best on the area closest to the strip and are less effective lower down a long run. Many current shingle lines are made with algae-resistant granules that contain copper, which is worth asking for on any replacement here.
Read that part of the warranty carefully, because algae resistance usually has its own coverage, separate from and shorter than the shingle’s main term. When our crews look at an older roof with heavy moss, we check the shingle edges under the growth before recommending anything: if the tabs have curled or the mat is soft, cleaning will not bring them back, and the conversation moves on to repair or replacement.