Moss on a roof is more than a cosmetic problem. It holds water against shingles and tile, lifts shingle edges, and pushes water under tile laps. The safe way to remove it is gently: a moss treatment that kills it, followed by soft brushing or letting rain wash it off over time. Never pressure wash a roof. To keep it from coming back, let in more sun, keep the roof clear of debris, and consider zinc or copper strips near the ridge.
Moss is most common on the Ridge and in the foothills, but it shows up in Chico too. Here is what to know.
Why moss grows on Butte County roofs
Moss needs three things: shade, moisture, and something to hold onto.
- The foothills get far more rain. Paradise, Magalia, and Forest Ranch typically get more than twice the annual rainfall of Chico. Roofs stay wet longer through winter and spring.
- Tree cover. Ponderosa pine, cedar, and oak keep roofs shaded and drop needles and leaves that hold moisture.
- North-facing slopes get the least sun and the most moss.
- Chico's creeks and shaded streets. Homes near Big Chico Creek, Little Chico Creek, and under big valley oaks can grow moss on fairly new roofs.
We touched on this in trees and your roof in the City of Trees. This article goes deeper on removal and prevention.
Moss, algae, and lichen: what is on your roof?
- Moss is green and fuzzy, grows in clumps, and is thickest along shingle edges and tile laps.
- Algae shows up as dark streaks running down the roof, often on north slopes. It is mostly cosmetic, though it can make a roof absorb more heat.
- Lichen is flat, crusty, and gray-green, and grips the surface tightly. It is the hardest to remove without damaging the roofing.
What moss does to a roof
On asphalt shingles
Moss roots into the granule layer and grows under the bottom edges of shingles. As it thickens, it lifts the shingle edges, which lets wind-driven rain get under them. It also holds water against the shingle surface for days after a storm, which speeds up wear. On the few nights each winter when temperatures drop below freezing in the foothills, that trapped water expands.
On tile
Moss grows in the laps between tiles and in the channels water is supposed to drain through. It dams the water and pushes it sideways, where it can reach the underlayment. Since the underlayment is the real waterproofing on a tile roof, that matters.
In gutters and valleys
Moss and the debris it traps wash into gutters and valleys, where they block drainage. We explain why that matters in when to clean your gutters in Chico.
How to remove moss safely
Do not pressure wash
Pressure washing strips the protective granules off shingles and can force water up under them. On tile, it can crack tiles and blast water under the laps. It also puts someone on a slippery, wet roof with a high-powered hose. It does far more harm than good.
Do not scrape hard
Scraping thick moss with a hoe or stiff tool tears up shingles and breaks tile. Light brushing of loose, dead moss with a soft brush, working from the top of the roof down, is the most you want.
Use a moss treatment
Roof moss treatments kill the moss so it dries out and releases its grip. Look for products labeled for use on roofs and follow the label exactly. Some things to keep in mind:
- Apply in dry weather with no rain in the forecast for a day or two.
- Protect plants below. Some treatments can harm landscaping, and runoff goes into your gutters and downspouts.
- Be patient. Dead moss often takes weeks to loosen, and rain will wash much of it off on its own.
- Avoid harsh chemicals that are not labeled for roofs. Some can discolor or damage roofing and metal flashing.
Leave steep and tile roofs to a pro
Wet roofs with moss are slippery, and tile breaks underfoot. If your roof is steep, two stories, or tile, have a roofing professional look at it rather than climbing up yourself.
How to keep moss from coming back
- Let in light. Trimming branches that shade the roof is the single most effective step. It also reduces debris and the risk of limbs falling on the roof.
- Keep the roof clear. Needles and leaves that sit on the roof hold moisture and feed moss.
- Keep gutters and valleys flowing so water drains off quickly.
- Consider zinc or copper strips. Installed near the ridge, these release small amounts of metal when it rains, which discourages moss growth on the roof below. They work best as prevention on a clean roof.
- Choose algae-resistant shingles on your next roof. Many manufacturers offer shingles with copper-containing granules designed to resist algae streaking.
Moss and fire season
In the foothills, a roof covered in moss, needles, and leaves is carrying material that dries out every summer. Keeping the roof and gutters clean is part of keeping your home wildfire-ready. See what a wildfire-ready roof looks like in Butte County.
When moss means it is time for more than cleaning
If moss has been on the roof for years, the shingles underneath may be worn, lifted, or brittle, and tile underlayment may be compromised. Signs it is time for repair or replacement:
- Shingle edges that stay curled or lifted after the moss is gone
- Bare spots with missing granules
- Leaks or stains inside
- Soft spots in the decking
We can tell you whether the roof has years left or whether it makes sense to plan for a new one. Learn about our roof repair services and residential roofing, or request an estimate. We serve Paradise, Magalia, Forest Ranch, Chico, and the rest of Butte County.
Frequently Asked Questions
Is moss bad for my roof?
Yes. Moss holds water against the roofing, lifts shingle edges, and on tile roofs it can dam water in the laps and push it toward the underlayment. Over time that shortens the life of the roof.
Can I pressure wash moss off my roof?
No. Pressure washing strips granules from shingles, can force water under them, and can crack tile. Use a moss treatment labeled for roofs and let the dead moss loosen and wash off.
What kills moss on a roof?
Moss treatments labeled for roof use kill the moss so it dries out and releases. Follow the label, apply in dry weather, and protect plants below the roofline.
Do zinc strips stop moss?
Zinc or copper strips near the ridge can help prevent moss from coming back on a clean roof. They work best combined with trimming trees and keeping the roof clear of debris.
Why does my roof have moss on only one side?
North-facing and heavily shaded slopes get less sun and stay damp longer, so moss grows there first. Trimming trees to let in more light is the best way to slow it down.
