Do not pressure wash an asphalt roof. That is most of the advice on this page, and it is worth saying first because pressure washing is the most common way homeowners turn a cosmetic problem into an expensive one.
The Asphalt Roofing Manufacturers Association is explicit about it. Their technical bulletin on algae discoloration says, in capitals, do not use a power washer. High pressure strips the granules that protect the shingle from sunlight, and most manufacturers treat that as impact damage, which voids the warranty.
So the useful question is not how to clean a roof. It is whether yours needs cleaning at all.
Black streaks are algae, and they are cosmetic
Those dark streaks running down a north-facing slope are algae. They feed on the limestone filler in the shingle, they spread where the roof stays damp longest, and they look like the roof is rotting.
They are not doing structural harm. An algae-streaked roof is an ugly roof, not a failing one.
That matters because the risk of cleaning is real and the risk of not cleaning is mostly to your kerb appeal. If you are selling, clean it. If you are not, it is a genuine choice rather than maintenance you are neglecting.
Moss is a different matter
Moss is not a stain. It is a plant with a root system that lifts the edges of shingles as it grows, and it holds water against the roof surface indefinitely.
Lifted shingle edges let wind get underneath. Held moisture accelerates everything. Moss on a roof is worth dealing with, and worth dealing with before it thickens.
It grows where shade and damp persist, so it usually arrives on the north side and under overhanging branches. Cutting back the branches does more long-term good than any cleaning.
Lichen sits between the two. It bonds hard to the granule surface and pulling it off takes granules with it. Treat it chemically and let weather remove the dead material rather than scraping.
The method ARMA actually recommends
For algae, their technical bulletin “Algae Discoloration of Roofs”, revised June 2024, describes a solution of liquid household bleach and water, left on the roof for at least fifteen minutes and no more than twenty, then rinsed by gently spraying the surface with water.
Three details in that sentence do the work.
At least fifteen minutes, because the solution needs time to kill the organism rather than just bleach the colour out of it.
No more than twenty, because leaving it longer risks damage.
Gently spraying, not blasting. Professional soft-washing runs below about 100 PSI, which is closer to a garden hose than a pressure washer.
Do not scrub. Friction loosens granules just as effectively as pressure does.
What to protect before you start
Bleach solution runs off the roof and lands on whatever is below. Plants directly under the eaves need soaking with clean water first and covering, and rinsing again afterwards.
It also runs into the gutters and down the downspouts, so wherever those discharge is worth thinking about.
And it is slippery. A wet roof treated with a bleach solution is considerably more slippery than a wet roof, which brings us to the part most articles skip.
Whether to do it yourself
Roof falls are not a minor category of accident. A steep roof, a wet roof, or a two-storey house is not a sensible place to learn, and the cost of a professional clean is small next to the cost of the alternative.
A single-storey house with a shallow pitch, dry conditions, proper footwear and someone else at home is a reasonable DIY job. Anything else, pay someone.
If you do hire someone, ask one question: what pressure do you use? An answer involving a pressure washer is the wrong answer, and you now know why.
Stopping it coming back
Algae needs damp to establish, so anything that dries the roof faster helps. Trimming branches back to let sun and air reach the slope is the most effective single thing you can do, and it also stops leaf litter collecting in valleys.
Zinc and copper strips fitted near the ridge release trace metal ions when it rains, which suppresses algae on the slope below. They work, though the effect fades further down a tall roof.
If you are replacing the roof anyway, algae-resistant shingles have copper granules mixed into the surface and carry a separate warranty against streaking, usually ten years or more. On a shaded north-facing house in a wet climate that is worth specifying.
Clean gutters matter more than clean shingles
If you only do one thing from this page, clear the gutters twice a year.
Blocked gutters hold water. In autumn that water sits against the fascia. In winter it freezes, and frozen gutters make ice dams considerably worse by giving meltwater nowhere to go.
That is a genuine maintenance job with a genuine consequence, unlike algae streaks. It is also easier and safer than cleaning a roof, because you can do most of it from a ladder without stepping onto the slope.
Get someone to look at it
If you are unsure whether what you are seeing is algae, moss, or granule loss from hail, Proliance inspects roofs across the Chicago metro at no cost and will tell you which it is. Our inspectors are HAAG certified. Call 800-613-4373.
