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.
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.
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 forStreaking 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 forA 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 forA 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 forAn 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.
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 →
If the number is the problem, here are the ways people pay for it →
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When roof algae removal is the wrong call
When saying yes is the wrong move:
- 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.
Types of roof algae removal
| Option | What it means for you |
|---|---|
| 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 |
Can I do this myself?
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
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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Sources
Sources last checked
- Asphalt Roofing — algae and cleaning contextARMARetrieved 2026-08-04Manufacturer/trade guidance on cleaning methods that void warranties.
- Occupational Employment and Wages — Construction TradesU.S. Bureau of Labor StatisticsRetrieved 2026-08-04Labor cost context for home-services trades nationally.
Sources for figures and claims on this page.
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