Roof Cleaning · Service

Those black streaks are alive, and that changes the fix

The dark stains running down a roof are not dirt and not asphalt bleeding through. They are a colony of algae, which is why washing harder does nothing and why the streaks always run downhill. Here is what actually removes them and what keeps them off.

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In short

What is roof algae removal?

The black vertical streaks on an asphalt roof are a colony of algae — most commonly the cyanobacterium Gloeocapsa magma. It feeds on moisture and on the limestone filler in the shingle, and it travels downward with rainwater, which is why the marks always run down-slope and start under the ridge. Because it is living, the fix is a cleaning solution applied at low pressure, not scrubbing. Force removes the algae and the shingle granules together.

The job

How roof algae removal actually works

Confirming it is algae and not something else

A clean stripe running down from a metal vent or a length of flashing is the diagnostic: trace metal in the runoff suppresses the growth in that band. If you can see those clean stripes, you are looking at algae.

Watch for: Streaking that is uniform rather than vertical, or concentrated around one penetration. That can be a flashing or ventilation problem wearing an algae disguise, and washing it will not help.
Protecting planting, paint and hardscape below

The solution runs off the eaves. Beds get soaked before application and kept wet, sensitive plants are covered, and everything is rinsed at the end.

Watch for: A crew that starts before wetting anything down. Runoff damage is the most common complaint on this job and it is a preparation failure, not an accident.
Low-pressure application across the stained slopes

The solution is applied broadly and gently and left to work. Applying from the bottom of the slope upward avoids clean solution running over dry stained shingle and drying into new streaks of its own.

Watch for: A pressure washer of any kind on asphalt shingle. It will clear the streaks, and it will take the granule layer that protects the shingle from UV.
Rinse, then patience

A gentle rinse carries away the dead colony. Residual shadowing usually keeps fading over the following weeks as rain works on it — algae staining is one of the jobs that genuinely looks better a month later than on the day.

Watch for: An operator escalating to pressure to chase the last of it. That is where the damage happens, and it is entirely avoidable.
Deciding what, if anything, to do about regrowth

Algae returns wherever the conditions that produced it persist. Cutting back the shade is the free option. Metal strips near the ridge are the mechanical one. Algae-resistant shingles at the next re-roof are the durable one.

Money

What does roof algae removal cost?

Algae removal is generally the least expensive roof-cleaning job, priced per square foot of roof or as a flat job price, because it is a single treatment with no hand removal. Height, pitch and access matter more than the size of the stain. The Cost Guide carries current ranges.

See the full roof algae removal cost guide — by material, size and region →

Straight answer

When roof algae removal is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

You are within a few years of replacing the roof. Algae staining does not shorten a roof's life in any meaningful way. If a re-roof is close, spend the money on algae-resistant shingles instead and skip the interim wash.
What you have is actually moss. Moss needs treatment, waiting and hand removal, and it is the growth that lifts shingles. Booking an algae wash on a mossy roof gets you a lighter-looking mat and nothing else.
Nobody can see it. If the stained slope faces a blank wall, a hedge, or nothing at all, you are paying to solve a problem that has no consequence. Algae is a cosmetic issue on asphalt — treat it as one.
You want a permanent result from the cleaning itself. Cleaning removes the current colony. It does not change the shade, the humidity or the shingle it grows on, and no operator can honestly promise otherwise.
The streaks came back within months of a previous wash. That is a conditions problem, and washing it again gets you the same outcome. Deal with the tree cover, or plan for algae-resistant shingles at replacement.
The only quotes you can get involve a pressure washer. Trading permanent granule loss for a temporary appearance fix is a bad trade on a cosmetic problem. Wait for someone who soft washes.

Your options

Types of roof algae removal

Preventive vs reactive treatment

Treat now or prevent later, compared

Soft wash roof cleaning

The application method used on algae

Preventive roof treatment

Metal strips and inhibitors after the wash

Roof moss removal

If what you have is thicker than a stain

Compare roof cleaning options side by side →

Straight answer

Can I do this myself?

Never scrub or pressure wash asphalt shingle to remove algae. The stain is only microns deep; the granule layer that protects the shingle from ultraviolet light is barely thicker. Removing the first inevitably removes the second, and the consequence — a roof that ages years faster — does not show up until long after the contractor has gone.

Reasonable to DIY

Applying a labelled, ready-to-use roof cleaner from the eave or the ground with a hose-end applicator, following the label exactly
Wetting down your own beds and shrubs before any treatment is applied
Thinning the branches shading the stained slope — the only genuinely durable free fix

Call a licensed pro

Anything requiring you to be on the roof surface rather than at the edge of it
Two-storey and steep roofs, where the ladder is the actual hazard
Mixing concentrates yourself, where getting the strength wrong damages plants and paint
Roofs where the staining is mixed with moss or lichen and needs more than one approach

Before you sign

What to check when hiring for roof algae removal

Are you treating this as algae, and what tells you that?

A pro should be able to point at the clean bands under the flashing and explain them. If the diagnosis is "it's dirty", the plan will be wrong.
What pressure will you apply at on my shingles?

On asphalt the answer must be low. The streaks are shallow; so is the granule layer that protects the shingle.
Will you work up the slope or down it when applying?

Applying from the bottom up stops clean solution running across dry stained shingle and drying into fresh streaks. It is a small detail that separates operators.
What should I realistically expect the roof to look like the day you leave?

Honest answer: much better, with some shadowing that fades over weeks. An operator promising a perfect finish that afternoon is planning to use force.
Do you recommend metal strips, and where would they go?

Near the ridge, so runoff carries over the stained slope. If the recommendation is vague about placement, it is a product sale rather than a plan.
Is there any moss or lichen mixed in that needs different handling?

Mixed growth needs a mixed plan. Finding out mid-job is how a fixed price becomes a change order.
How are you protecting the plants directly under the eaves?

Those beds take the concentrated runoff. Pre-soaking and rinsing should be described specifically.

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Local pros

Roof Algae Removal near you

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Related work

Often done at the same time

Roof Moss Removal

The growth that damages rather than stains

Soft Wash Roof Cleaning

How the treatment is applied

Preventive Roof Treatment

Keeping the streaks from returning

Roof Cleaning

The service overall and how to hire

All roof cleaning services →

FAQ

Roof Algae Removal — questions people ask

Still have a question? Talk to our team →

What causes black streaks on a roof?

A colony of algae, most commonly the cyanobacterium Gloeocapsa magma. It is airborne, it settles on roofs, and it feeds on moisture and on the limestone filler used in asphalt shingles. The dark colour is a protective sheath the organism develops against ultraviolet light. It spreads downward because rainwater carries it, which is why the streaks always run down the slope rather than across it.

Why is there a clean stripe under my roof vents?

Because rain running over galvanised or copper flashing picks up trace metal, and that trace metal suppresses the algae in the band directly below. It is the clearest confirmation that the staining is biological rather than dirt, and it is also the entire principle behind fitting zinc or copper strips near the ridge as a preventive measure.

Is roof algae damaging my shingles?

The consensus is that it is primarily a cosmetic problem on asphalt shingle, not a structural one. It is worth keeping this in proportion: moss lifts shingles and traps water, which is a real failure mechanism, while algae mostly makes a sound roof look like a failing one. That perception matters at resale, which is a legitimate reason to clean it — but it is a different reason from preventing damage.

Will the streaks come back?

Yes, unless something about the conditions changes. Cleaning removes the colony; it does not alter the shade, the humidity or the shingle composition that made your roof hospitable in the first place. The three things that genuinely change the outcome are reducing tree cover, fitting metal strips near the ridge, and choosing algae-resistant shingles the next time the roof is replaced.

What are algae-resistant shingles?

Shingles manufactured with copper-bearing granules mixed into the surface, so the roof continuously releases the trace metal that suppresses growth. They are widely available and usually a modest upgrade at the point of re-roofing. If you live somewhere streaking is normal on every house on the street, this is the decision that actually solves it — and it is a re-roof decision, not a cleaning one.

Can I use a household bleach mix on the streaks myself?

Homemade mixes are what most DIY guidance describes, and the risks are real rather than theoretical: getting the concentration wrong damages plants and metal, chlorine must never be combined with anything containing ammonia or acid, and applying it means being at height on a slippery surface. A labelled, ready-to-use roof product applied from the eave is the sensible version of doing this yourself.