Moss should come off a roof once it’s thick enough to see from the ground, and it should come off gently. Left in place, it holds rainwater against the shingles all winter and slowly pries up their edges. In Santa Rosa, shaded roofs under oaks and redwoods can stay damp for months, which is exactly what moss needs. Here’s what it does, why it grows where it does, why pressure is the wrong answer, and how to keep it from returning.
What does moss do to a shingle roof?
It keeps the roof wet and slowly lifts the shingles. IKO, a shingle manufacturer, explains that moss soaks up rain and holds it, keeping the surface damp, and that over time it can work its way under shingles and lift them. It also gets in the way of water running down to the gutters.
Lifted edges are the real trouble. Once a shingle no longer lies flat, wind-driven rain can get under it, and the nails and underlayment below were never meant to stay wet. Moss that sheds into the gutters adds to the clogs there too, so a mossy roof and overflowing gutters often show up together.
Is lichen on a roof the same problem as moss?
Not quite. Lichen is the flat, crusty growth, often gray-green, yellow or orange, that grips the shingle surface instead of forming a spongy cushion. It holds less water than moss, but it clings so tightly that scraping it off tends to pull the protective granules away with it.
That’s why Dave Stankas, in his Diamond Certified expert video on pressure washing, talks about the chemical put in the machine “to remove lichen, moss, etc.” rather than about more pressure. Letting a cleaning solution loosen the growth, instead of forcing it off, is what protects the shingle surface.
Why does moss favor the north side of a roof?
Because it gets the least sun. In Sonoma County, a north-facing slope stays in shade much of the winter, so dew and rain dry slowly, and moss thrives on that steady dampness. Trees make it worse: a roof under redwoods, oaks or bay trees stays cool and wet, and fallen needles feed the growth.
Steve F. saw it on a roof with many levels: “Moss & staining was rampant especially on the north sides.” IKO’s guidance points the same way, noting that moss prefers shade and suggesting owners trim trees that shade out the roof. The rain pattern adds to it. The NOAA normals for the Santa Rosa station put about 26 of the year’s 31.5 inches between November and March, a long, wet growing season.
When should roof moss come off?
As soon as you can see it from the street, and ideally during a dry stretch. A dry roof is safer to work on, and removing moss before the winter rains means it isn’t spending months soaking up water against the shingles. Waiting rarely helps, since the cushions grow thicker each wet season.
Fire season is another reason not to wait. CAL FIRE’s home hardening guide asks homeowners to clear leaves, needles and other debris off the roof regularly so embers have nothing to catch, and moss tends to hold that kind of debris in place. A late-summer cleaning handles both jobs before the first storm.
Why is a pressure washer the wrong tool for a shingle roof?
Because the force that strips moss strips the shingle too. The Asphalt Roofing Manufacturers Association warns that high-pressure washing systems are likely to damage asphalt roofing, and IKO tells owners never to put a pressure washer or a pressurized stream of water on the roof. The mineral granules are what wash away.
Those granules shield the asphalt from the sun, and once they’re gone they don’t come back. IIBEC, a building enclosure consultants’ group, says pressure washing an asphalt shingle roof causes granule loss, and a strong stream aimed up the slope can also drive water under the laps. ARMA adds that scrubbing can loosen granules and encourages using professionals to clean shingles. Hiring someone? Our post on hiring a pressure washing company lists the questions to ask.
How do you keep moss from growing back?
Treat the roof after cleaning, let in more sun, and keep debris from piling up. A treatment applied once the moss is off slows its return, trimming shading branches dries the roof faster, and clear gutters and valleys stop water from backing up. IKO also lists zinc or galvanized metal strips along the ridge.
Ridge strips are prevention only. IKO notes they won’t handle moss that’s already growing, so the roof has to be cleaned first. Results can last: Steve F. had his roof cleaned and treated in June of 2022 and wrote, “Four years later, and still no sign of the moss coming back!” Shadier roofs may need a yearly look, which is what the Peace of Mind Program is for. Our roof moss removal page shows how the crew handles it: only the loose clumps come off by hand, and a treatment and the winter rain take care of the rest.
Questions we hear about this
Can I take moss off my own roof?
Small patches you can reach from a ladder at the eave, possibly, but walking a mossy roof is risky because wet moss is slippery. Stay away from pressure washers and hard scrubbing, and follow the cleaner’s label. ARMA encourages hiring professionals to clean shingles, and a steep or two-story roof is a good place to take that advice.
Are the black streaks on my roof moss?
Usually not. Long dark streaks running down a roof are typically algae, which ARMA describes as discoloration that shouldn’t be confused with moss or tree droppings, since those leave more localized marks. Algae shows up as a stain on the surface, while moss forms raised green cushions that hold water. Neither needs a pressure washer.
How long does a moss treatment keep a roof clear?
It depends on shade, trees and which way the roof faces. A sunny roof with few trees over it can stay clear for years after one treatment, while a north slope under redwoods may show new growth sooner. Checking every fall, when the gutters get cleaned, catches regrowth early.
Sources
- ARMA (Asphalt Roofing Manufacturers Association): Algae Discoloration of Roofs
- IKO: How To Deal With Green Moss on Roof Shingles?
- IIBEC: How to Remove Moss From Shingles
- CAL FIRE Ready for Wildfire: Hardening Your Home
- NOAA NCEI 1991-2020 climate normals, Santa Rosa station USC00047965
- Diamond Certified Experts: 4 Tips for Hiring a Pressure Washing Company (David Stankas)