Moss and Algae on Your Roof: Why Insurers Care and How to Remove It

The short answer

Black streaks on a shingle roof are usually algae, which roofing makers say is cosmetic. Green, fuzzy moss is different: it holds water and can lift shingle edges. Insurers looking at aerial photos sometimes flag both. A gentle cleaning (never a pressure washer), plus dated before-and-after photos sent to your insurer, has helped some homeowners keep their policies.

Is this you?

Your insurer’s letter mentions moss, algae, mold, or “discoloration” on your roof.
You see dark streaks or green patches, often on the shady side of the roof.
An insurance inspection is coming and you want the roof to look its best.
Someone offered to pressure wash your roof and you are not sure it is safe.

Algae or moss? How to tell

	Algae	Moss
What it looks like	Dark or black streaks running down the roof	Green, raised, fuzzy clumps, often near edges and seams
What it is	Mostly a blue-green algae called Gloeocapsa magma, spread by wind	A small plant that soaks up water through its leaves
Where it grows	Humid and coastal areas, where moisture lingers	Shady, damp roofs, often the north side or under trees
Does it harm the roof?	ARMA says there is no scientific evidence it damages asphalt shingles	Yes, it can. It can lift or curl shingle edges and hold moisture against the roof.
Insurance view	Often cosmetic, but can still show up in aerial photos	Can point to a real condition problem, since it can damage shingles

ARMA is the Asphalt Roofing Manufacturers Association. It says moss can make lifted shingles more likely to blow off in wind, and in severe cases can lead to wet decking (the wood boards under the shingles) or leaks. Algae mostly affects looks, though ARMA notes it can reduce how well a reflective “cool roof” works.

Why insurers care

Some insurers now review roofs with aerial or satellite photos, and streaks or moss can look like wear from above. Learn more in how insurers use aerial roof photos. Three public cases show how this plays out:

Liberty Mutual, San Francisco (2024). ABC7 reported that a non-renewal (a decision not to continue the policy) cited aerial footage showing “moss, mildew, algae, and mold.” A roofer found no problems. The policy was reissued after the station contacted the company.
Clovis, California (2025). ABC30 reported that a homeowner’s notice said she had to remove algae to be renewed. She paid about $1,000* for removal, and her policy was renewed.
Connecticut (2024). The state insurance department told insurers that discoloration and streaking are “cosmetic roofing conditions” that do not support non-renewal. Insurers need evidence of real damage to the roof.

Rules differ by state, so do not assume your insurer will treat algae as cosmetic. More real cases are in our roof letter examples. Also, standard policies do not pay for slow wear or long-running leaks, so moss damage that builds over years is usually your cost. See what a healthy roof looks like.

How to remove it safely

ARMA gives clear guidance for asphalt shingle roofs. Hire a pro for the work itself. ARMA says homeowners “should never climb onto a roof.”

No pressure washing. ARMA says, “Never use a pressure washer to clean an asphalt shingle roof.” It strips granules (the gritty coating) and can shorten the roof’s life.
Use a mild bleach mix. ARMA suggests a 50:50 mix of laundry-strength liquid chlorine bleach and water, sprayed on.
Let it sit, then rinse gently. Leave it 15 to 20 minutes, without letting it dry, then rinse with low-pressure water.
Protect plants and people. ARMA says to protect landscaping from the bleach and to wear protective gear.
Be patient. Algae usually washes away with later rain. Dead moss loosens over time and can be blown off. Heavy moss may need more than one treatment.

Ask any cleaner how they plan to clean your roof. If the answer is a pressure washer, find someone else. Tile, wood, and metal roofs have their own rules. HomeAdvisor notes clay tile “can’t take pressure-washing,” so follow the roof maker’s instructions.

Cleaning and prevention options

These are general service and product types, not brands. Costs are national estimates.

Professional low-pressure roof cleaning

What it does: A crew applies a cleaning solution and rinses at low pressure, often called soft washing.
Pros: No ladder work for you. Fast results before an inspection.
Cons: Growth can come back over time, and some crews still use high pressure.
Typical cost: About $296 to $626*, with an average of $461* (HomeAdvisor). Angi gives $300 to $620*.
Best for: Visible streaks or moss, especially with an insurance deadline.

Moss and algae treatment only

What it does: A spray treatment that kills growth, which then washes or blows away over time.
Pros: Lower cost than a full cleaning.
Cons: Results are slower, and the roof may look patchy for a while.
Typical cost: About $100 to $250* (HomeAdvisor).
Best for: Light growth with no deadline.

Tree trimming

What it does: Cuts back branches so the roof gets more sun and less debris. ARMA lists this as a key prevention step.
Pros: Treats the cause, not just the symptom. Also removes limbs that could fall on the roof.
Cons: Branches grow back. HomeAdvisor suggests checking yearly for limbs too close to the house.
Typical cost: About $270 to $2,000*, average $460* (HomeAdvisor). A single branch over a roof is about $50 to $175*.
Best for: Roofs under shade trees, or with moss on one side.

Zinc or copper strips

What it does: Metal strips near the ridge release small amounts of metal in the rain that discourage growth.
Pros: Low material cost.
Cons: ARMA says adding them to an existing roof is “not recommended,” since exposed nails can cause leaks and lifting shingles can lead to wind damage. Runoff can also harm plants.
Typical cost: Zinc is about $40* per 50-foot roll for materials (Fixr, 2022).
Best for: A new roof, installed by the roofer at replacement time.

Algae-resistant shingles

What it does: Shingles with copper granules mixed in that slow algae growth.
Pros: Long-term protection built into the roof.
Cons: Only makes sense when you are already replacing the roof.
Typical cost: Ask your roofer to price it in your replacement quote.
Best for: Humid or coastal areas. See asphalt shingle roofs.

Prove it to your insurer

Take before photos. Ask the cleaner to photograph each side of the roof, with the date.
Get it in writing. Ask for an invoice that lists the method used, such as “low-pressure, bleach solution.”
Take after photos from the same spots once the growth is gone.
Add a roofer’s note if you can. A short statement that the shingles are sound and how many years they have left is strong evidence.
Send it all to your agent or insurer before the deadline, and ask them to confirm in writing that the issue is resolved.

If the insurer still refuses, read how to appeal a roof non-renewal.

Good to know

If your letter mentions algae or discoloration, ask the insurer for the photo it used and the date it was taken. An old image may show streaks that were cleaned long ago.

Want a pro to clean and check your roof?

A local pro can clean your roof safely, check the shingles underneath, and give you dated photos to send your insurer. Free, with no obligation.

Talk to someone about my roof

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Common questions

Is the black staining on my roof mold?

Usually not. ARMA says the dark streaks people often call mold or mildew are usually algae. It is mostly a looks problem on asphalt shingles.

Can I just pressure wash my roof?

Not an asphalt shingle roof. ARMA says never to use a pressure washer, because it removes the protective granules and can shorten the roof’s life.

Will cleaning the roof save my policy?

Sometimes. In the Clovis case, removing algae led to renewal. If the insurer also cites worn or damaged shingles, cleaning alone may not be enough. Ask exactly what they need, in writing.

How fast does algae come back?

It varies by climate and shade. Trimming trees helps, and algae-resistant shingles are an option when you replace the roof.

Where to go next

Sources (10)

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* Prices are estimates and could vary depending on roof size, labor, and materials used.