Low pressure and non-pressure roof cleaning are the same idea: the solution does the cleaning and the water only carries it away. Here is what that looks like on shingles, metal, and flat roofs, and how long it holds in central North Carolina.
Low pressure roof cleaning, soft washing a roof, and non-pressure roof cleaning are three names for one method. A cleaning solution goes onto the roof at roughly the pressure of a garden hose. It sits long enough to kill what is growing there. Then it is rinsed off at the same low pressure, or left for the rain to finish, depending on the roof and the mix.
The point is that the chemistry does the cleaning. The water is only a delivery system. That is the opposite of how most people picture a pressure washer working, where the force of the water tears the dirt loose. On a roof, force is the thing doing the damage.
Most of what is on a roof in the Triangle and Johnston County is not dirt anyway. It is algae, moss, lichen, and the organic film they trap. Those are living things anchored to the shingle. You can kill them where they sit, or you can rip them off and take part of the roof with them. Low pressure cleaning is the first option.
An asphalt shingle is a fiberglass mat soaked in asphalt, with a layer of mineral granules pressed into the top. Those granules are what you see, and they are what protects the asphalt from sun and rain. When the granules go, the asphalt underneath bakes, dries, and cracks, and the shingle ages fast.
High pressure roof cleaning knocks granules loose. It does it at settings that feel gentle from the ground, and the evidence ends up in the gutter where nobody looks. It can also drive water up under the shingle courses and lift tabs that were sealed down. That is how a roof wash turns into a ceiling stain.
That is why the major shingle manufacturers, and the Asphalt Roofing Manufacturers Association they belong to, advise against pressure washing asphalt roofs and point owners toward a low pressure clean instead. Warranty terms differ by maker and by product line, so read your own. You will not find a shingle maker that recommends a pressure washer.
If you searched for high pressure roof cleaning because it sounds faster, it is not faster. It is the same visit with a worse result, and the damage shows up years later as a roof that aged early.
The standard roof wash in this trade is a diluted chlorine solution with a surfactant, which is a soap that helps it cling to a sloped surface instead of running straight off. The chlorine kills the algae, moss, and lichen. The surfactant holds it in place long enough to work.
Beds, shrubs, and grass under the eaves get soaked with clean water before anything goes on the roof. Wet leaves take up far less of what drips down.
The mix is sprayed from a ladder, the ground, or the roof edge. Nothing is scrubbed, and nobody drags a surface cleaner across the shingles.
The color change on black streaks often starts within minutes. Moss and lichen take longer, because there is more of them and they hold water.
The roof, the gutters, and everything below get rinsed. Downspouts are where runoff collects, so they get attention of their own.
Mix strength matters more than anything else in this job. Too weak and the growth survives and is back in a season. Too strong and it is harder on plants, metal, and painted trim than it needs to be. The right strength depends on what is growing, how thick it is, and what the roof is made of. That judgment is most of what you are paying a roof cleaning company for.
For what the streaks actually are and why they start at the ridge, see our guide to black streaks on a roof. This page is about the method.
Roof algae travels on the wind. Every roof in the Triangle gets new spores all the time. A proper treatment kills the colony that is there. It does not make the roof immune to the next one.
What decides how fast it returns is how long the roof stays wet. A sunny roof with no trees over it dries within hours of a rain. A north slope under pines in Clayton, or a wooded lot in Cary, can stay damp for a day or more after every storm, and that is the condition algae wants.
If a roof greens up within a year of a wash, the growth was not killed. It was knocked back. That is the most common sign a roof was pressure rinsed rather than treated. Trimming branches back and keeping the gutters draining both stretch the time between washes, because both help the roof dry.
Algae dies and lets go fast. Moss and lichen are tougher. Both grip into the granule layer, so the honest method is to kill them and let the weather release them over the following weeks. Scraping them off on the day takes granules with them.
That means a roof with moss looks better on the day and much better a month or two later. That is normal, and it is the right outcome. Moss is also more urgent than streaking, because it lifts shingle edges and holds water under them. Our roof moss guide covers why, and what to do about the shade that causes it.
Metal roofs grow algae and mildew too, mostly as a dull green or black film on the shaded side and along the seams where water sits. They have no granules to lose, so people assume they can take pressure. They can take more than shingles. They still should not get much.
If your painted metal roof leaves white powder on your hand, that is oxidation, not growth. It is a different problem with a different fix, and our oxidation removal service is built for it.
Flat roofs are almost never actually flat. They are low slope, and they drain slowly toward a drain, a scupper, or a gutter edge. Wherever water ponds after rain, dirt settles and algae follows. That is why a flat roof usually looks dirty in patches rather than evenly.
Around here you see two kinds. Residential low slope roofs cover porches, additions, and carports, often in modified bitumen or rolled roofing. Commercial flat roofs are usually single-ply membranes, the white or gray sheet roofs on offices, churches, and stores.
| Roof | What builds up | What to watch |
|---|---|---|
| White or gray single-ply membrane | Dirt and algae in the ponding areas, which cuts how well the roof reflects heat | Use only cleaners the membrane maker allows. Harsh solvents and high pressure can damage seams. |
| Modified bitumen or rolled roofing | Algae, moss at the edges, debris in the low spots | Granule surfaced like a shingle, so the same no-pressure rule applies. |
| Low slope porch or carport roof | Leaves, pollen, algae, and moss where trees hang over | Clear the debris and the drains first, or the wash just moves the mess around. |
| Metal low slope roof | Film at the seams and fastener lines | Low pressure, a gentle mix, and a careful rinse along every fastener row. |
A dirty white membrane is more than a looks problem. Those roofs are white so they reflect the sun, and a film of dirt and algae turns that off. On a commercial building, a clean roof is part of keeping the cooling bill where it was designed to be. We handle that work under commercial pressure washing, usually alongside the walls and walks.
Most of the risk on a roof wash day is on the ground, not on the roof. Everything that comes off the roof goes through the gutters and out the downspouts, and a downspout is a hose pointed at whatever is planted beside it.
You do not need to be home, as long as we can reach an outside spigot and the gates are open. Most people are surprised how quiet it is. There is no engine roaring for hours, because the heavy lifting is chemistry. When we finish, you get photos of the parts of the roof you cannot see from the ground.
Roof cleaning works most of the year here. Mild, overcast days are best, because the mix stays wet on the roof longer and does more work. Hot July afternoons dry it out too fast. A hard freeze is the only real stop.
Before you hire anyone, ask how they clean a roof. If the answer has the word pressure in it, keep looking. Ask how they protect plants, what they do at the downspouts, and whether they walk the roof. We work from ladders and the ground wherever the pitch allows, and our free drone inspection shows you the whole roof on a screen before you decide anything.
For what a roof wash typically runs, see what this work costs here. We do low pressure roof washing across Clayton, Garner, Knightdale, Wake Forest, Cary, and the rest of our area. Call (919) 698-3669 and a real person answers.
We do this work every week across the Triangle and Johnston County. Free quote, no obligation, and a real person on the phone.
“Did the roof and the gutters in one visit and showed me photos of everything I could not see from the ground.”
“The black streaks on our roof were embarrassing. Gone in one afternoon and the shingles were not touched.”
“Long driveway with years of clay stains. It came out even end to end with no stripes anywhere.”
A driveway clean is more than a pressure washer and an afternoon. Here is what a proper concrete cleaning service does, stain by stain, and what no amount of cleaning will fix.
Read the guide →ConcreteA patio is not one surface. Broom-finish concrete, stamped concrete, pavers, flagstone, and brick all clean differently, and the method that suits one of them can ruin another.
Read the guide →DecksA fence weathers two different ways at once. Here is how a fence cleaning service handles wood, vinyl, and aluminum, why pressure ruins pine, and what has to happen before a stain goes on.
Read the guide →We fly the roofline and show you the footage on the spot. You see your own gutters, joints, and downspouts on the screen instead of taking our word for it. It costs nothing and you are not committing to the cleaning.
Free for homes and businesses across the Triangle and Johnston County. No obligation to book any work afterwards.
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