Why do moss and lichen grow on north Georgia roofs, and how do you get rid of them?
In metro Atlanta, a little moss on the roof is unusual. Drive up toward Dahlonega, Ellijay or Blue Ridge, or into any older neighborhood with a tall hardwood canopy, and it is everywhere: green cushions along the shingle edges and pale crusty circles scattered across the north slope. Neither appears overnight, and both are telling you something about how wet your roof stays.
Moss, lichen and algae are three different things
People use these words loosely, but they behave differently on a roof.
- Moss is a small, primitive plant. On roofs it forms soft green clumps, usually starting in the gaps between shingles where grit and moisture collect.
- Lichen is not a plant at all. The U.S. Forest Service describes lichen as a partnership between a fungus and an alga, with no roots, stems or leaves. On shingles it appears as flat, crusty patches, often gray green, that cling tightly to the granules.
- Algae shows up as flat black streaks with no texture. It is mostly cosmetic and is covered in our guide to black streaks and algae on roofs.
Why north Georgia grows so much of it
Moss needs two things: moisture and shade. Oregon State University Extension, which deals with far more roof moss than Georgia does, puts it simply: moss depends on shade to thrive, and the most effective long-term fix is to let more sunlight reach the roof. Several conditions in north Georgia stack up in moss's favor:
- Humidity. Georgia's summers are long and muggy, and dew lingers on shaded roofs well into the morning.
- Tree canopy. Mature oaks, poplars and pines keep entire slopes in shade for most of the day.
- Terrain. Homes tucked into coves, on north-facing hillsides, or beside creeks and lakes dry out slowly after rain.
- Debris. Leaves and pine straw caught on the roof hold moisture and break down into a thin soil that moss roots into.
The result is a roof that is effectively damp for much of the year on one side and dry on the other. If you see moss on only the north slope, that is the reason.
What moss and lichen do to a shingle roof
Oregon State's extension specialists explain that moss itself does not directly damage roofing materials. The damage is mechanical and moisture related. Moss traps soil and debris and holds water against the roof, and on asphalt and metal roofs it can lift shingles and let water reach the layers underneath. A thick mat at the bottom edge of a shingle course acts like a sponge and a wedge at the same time, propping up the edge so wind can get under it and rain can blow back up the slope.
Lichen is slower and quieter. It grows a tiny amount each year and bonds tightly to the granule surface. It does far less harm than moss, but it is a reliable indicator that the roof surface stays wet, and pulling it off by force tears granules away with it. Texas A&M Forest Service notes that removing lichen from trees is only a temporary fix because it tends to return, and the same is true on a roof if conditions stay the same.
How to remove moss without hurting the roof
The instinct is to scrape or blast it off. That is the step that causes the damage. The better approach is to kill it first and let it release on its own.
- Treat, do not scrape. Oregon State lists two options: zinc sulfate mixed with water, and soap-based products made from potassium salts of fatty acids, which are biodegradable and noncorrosive. Read the label and confirm the product is acceptable for your shingle type.
- Watch for copper. Oregon State warns not to use zinc treatments where a roof has copper gutters, downspouts or flashing, because zinc corrodes copper.
- Protect plants. Avoid spraying surrounding plants and rinse any that get hit.
- Give it time. Dead moss turns brown and loosens over days to weeks. Rain will wash much of it away.
- Remove what is left gently. Any remaining loose material should be swept off in the direction water flows, downslope, so shingle edges are never pried upward.
- Never pressure wash. The Asphalt Roofing Manufacturers Association says a power washer will likely harm asphalt shingles.
On lichen, lower your expectations. A treatment will kill it, but the crusty outline may stay visible for a long time. That is usually better than scraping, which does more harm than the lichen ever would.
A safety note. Oregon State Extension says plainly that roof moss removal is not for everyone, and recommends shoes with strong traction, a ladder extending at least three feet above the roofline, and safety lines on steep roofs. A treated, mossy roof is slick. For most homeowners this is a job to hire out.
Keeping it from coming back
- Let in the sun. Thin or raise the canopy over the roof. See trees and your roof for what to trim and when to call an arborist.
- Keep the roof clean of debris. Needles and leaves left in valleys and behind chimneys are where moss starts. See pine needles and leaves on the roof.
- Keep gutters flowing. Overflowing gutters keep the lowest course of shingles wet.
- Consider a metal band at the top of the roof. Rain washing over a zinc or copper strip carries traces of metal down the slope that can discourage regrowth. It helps most on simple slopes with steady runoff and never replaces fixing the shade.
When moss means a repair, not just a cleaning
After heavy moss is gone, look at what it was hiding. Lifted or curled shingle edges, missing granules in the moss lines, nails backing out, or dark stains on the decking in the attic below the mossy slope all mean water has been getting in. Those spots need repair before the next storm season. Our guide to common causes of roof leaks explains what to look for.
This is where an inspection pays for itself. Robust Roofing, which many homeowners consider the best roofing company in North Georgia for workmanship, will photograph the slopes under the moss and show you whether the shingles beneath are still sound or whether a repair is due. You see the condition for yourself instead of guessing from the ground.