Soft wash roof cleaning removes roof algae, moss, and lichen with a low-pressure chemical mix instead of high-pressure water. The chemistry does the work. On a shingle roof, pressure is the fastest way to ruin the surface, so a roof soft wash runs under about 100 PSI at the nozzle and lets sodium hypochlorite (SH) kill the growth at the source.
The roof mix is where operators get burned. Run it too weak and the black streaks live through the wash, so you eat a callback. Run it too hot and the customer's beds and grass pay for it. We build the 12V systems, proportioners, and surfactants that meter these roof mixes, so dialing in that number is a problem we hear about constantly.
What Is Soft Wash Roof Cleaning?
Soft wash roof cleaning is the low-pressure application of an SH mix, usually with a surfactant, under roughly 100 PSI at the nozzle. The chemical kills the algae, moss, and lichen on the roof at the source. It doesn't blast the growth off. You rinse the roof at low pressure once the mix has done its work, and the dead growth keeps weathering off over the following weeks.
Those black streaks aren't dirt. They're Gloeocapsa magma, a blue-green cyanobacterium that feeds on the limestone filler in modern asphalt shingles. The black color is a UV-protective sheath the organism grows. It's airborne, so it spreads roof to roof, and it's worst in humid climates. The general method behind all of this, across siding and concrete too, sits in our top softwashing tips for beginners. On a roof, the specifics change, starting with pressure.
Why You Never Pressure Wash a Shingle Roof
Pressure strips the granules off a shingle. Those granules are the roof's protective coating, and once they're gone they don't come back. ARMA, the Asphalt Roofing Manufacturers Association, is blunt about it: never use a pressure washer to clean an asphalt shingle roof, because it causes granule loss and very likely premature failure of the roof system (Source: ARMA, Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems).
Soft washing sits at the other end of the scale. A roof soft wash runs under about 100 PSI, the range of a garden hose, against the 3,000 to 4,400 PSI a pressure washer puts out. Even a hard rinse from a standard nozzle can knock granules loose, which is why the water pressure is kept low and the chemistry carries the load. We go deeper on the two methods in our breakdown of pressure washing vs soft washing. The short version for a roof is that pressure is the wrong tool.
Why Roofs Run a Hotter Mix Than Siding
If you already wash houses, you run vinyl siding around 1 to 2% SH. A roof runs hotter. The applied mix at the shingle usually lands somewhere in the 3 to 6% range, and field practice tends to sit in the upper half of that, roughly 5 to 6.25% on heavy, shaded, or north-facing growth and closer to 4% on lighter, sunny sections. Our sodium hypochlorite guide breaks down the mix math and the dilution off a 10 to 12.5% stock.
There's a genuine spread in the numbers, and you should know it before you dial anything in. ARMA's consumer guidance is more conservative than field practice: a 50:50 mix of laundry-strength bleach and water, which lands around 2.5 to 3% at the surface, with a 15 to 20 minute dwell before a low-pressure rinse (Source: ARMA). Independent pro sources cite 3 to 6% at the shingle, and some go as high as 3 to 8% (Sources: J. Racenstein and National Roof Cleaning Authority). The honest read is a band, not one magic number. Too weak and the streaks don't die, so you eat a callback. Too hot and you're risking the landscaping and wasting chemical.
Surfactant is the other half of the setup. On a steep pitch it's your runoff-control tool, not the throttle. It thickens the mix so it clings to the shingle and dwells instead of sheeting straight into the gutter. Plenty of operators skip it on walkable roofs with gutters, where the gutters catch the runoff, and that's a fair call. SH also works better warm, so time roof jobs for the warmer part of the day when you can.
Streaks Don't Vanish the Moment You Spray
SH kills the growth and changes its color, but it doesn't remove it. On a light-colored roof, a full color change can make the surface look clean while the dead organic material is still sitting there waiting to weather off. Green turning brown or tan means it's dying. White means it's dead. It still has to come off.
Black algae streaks weather away over about two to four weeks of sun and rain. Moss is different. It sits loose on top of the shingle, and it washes off after about two good rain events. Lichen is the stubborn one. It roots down into the granules, so it can take a month or more to release, sometimes 6 to 12 months. Pull a piece of lichen off by hand and it tears granules out with it, which is a useful thing to show a customer who wants you to make it disappear today.
So set the expectation before you spray. The streaks aren't permanent, but they don't vanish the second the mix hits them. Don't promise a date. On a moss-heavy or lichen-heavy roof, tell the customer it'll take a few heavy rains to fully shed the dead growth, and put that in writing.
Will It Damage the Roof or Void the Warranty?
Done at low pressure, soft washing is the method the roofing industry itself points to. ARMA describes low-pressure chemical cleaning, a 50:50 bleach-and-water mix with a 15 to 20 minute dwell, worked from a ladder, no pressure, as the recommended way to clean algae and moss off asphalt shingles (Source: ARMA). ARMA also states there's no scientific evidence that algae damages shingles, though it does hurt the roof's appearance and reflectivity. Moss is another matter: ARMA notes moss can lift and curl shingle edges and raise the risk of blow-off (Source: ARMA).
Warranty is where you have to be careful with your words. The line you'll see everywhere, that soft washing is "ARMA-approved" and "won't void the warranty," largely traces back to one company's marketing, not to ARMA. What ARMA actually does is describe a compatible cleaning method and defer to the individual shingle manufacturer on warranty terms. GAF's technical bulletin R-102 tells owners not to power wash because it can dislodge granules, and treats stain removal as temporary (Source: GAF). So don't tell a customer that a cleaning guarantees anything about a warranty. Give them the accurate version: ARMA describes the method, and the warranty terms are the shingle manufacturer's to state.
The manufacturers also sell algae-resistant shingles with their own warranties, which is worth knowing when a customer asks how to keep the roof clean:
- GAF StainGuard Plus: a 25-year limited warranty against algae discoloration. StainGuard Plus PRO carries 30 years, available only on Timberline UHDZ (Source: GAF).
- CertainTeed StreakFighter: copper-infused granules with a 10- or 15-year base algae warranty, extending to 25 or 30 years only when paired with matching algae-resistant hip-and-ridge accessories (Source: CertainTeed).
- Owens Corning StreakGuard: a 25-year algae-resistance warranty on Duration-series shingles, conditioned on installing approved OC hip-and-ridge shingles. Without them it reverts to 10 years (Source: Owens Corning).
Matching the Mix to the Roof Material
Not every roof takes the same mix, and one of them shouldn't be cleaned with SH at all. Here's how the common materials break down.
| Roof type | Soft wash it? | Approach | Watch out for |
|---|---|---|---|
| Asphalt shingle (architectural & 3-tab) | Yes, soft wash only | 3 to 6% SH, no pressure, rinse at low pressure after dwell | Complete coverage matters; one missed spot leaves one visible streak |
| Clay or concrete tile | Yes | Around 5 to 5.25% SH; double-tap porous thin-set and mortar; rinse after dwell | Fragile and slippery underfoot; walkability is limited |
| Standard or painted metal | Yes, but rinse it | 3 to 4% SH, then rinse the whole surface | SH left to dry on metal attacks fasteners and coatings |
| Bare Galvalume | No | Don't clean with SH | SH corrodes the coating; use manufacturer guidance instead (Source: Southeast Softwash) |
| Cedar shake | Special case | Milder oxygen or fatty-acid cleaners; SH can be harsh | Expect some shake replacement; test first |
| Slate | Yes, from a ladder | Low-pressure chemical from a ladder | Never walk a slate roof; it cracks and it's dangerous |
The rule that catches operators out is metal. Rinse every roof at low pressure once the mix has done its work, but on metal it isn't optional and it can't wait, because SH left to sit and dry goes after the fasteners and the coating. And bare Galvalume is a hard no with SH. When in doubt on a material you haven't done, test a small section and tell the customer it may not respond before you commit.
How Long Does a Soft-Washed Roof Stay Clean?
Plan on about three to six years before a roof needs cleaning again (Sources: Benz Softwash at 3 to 5 years and Trotta's Power Washing at 3 to 6 years). It's a kill, not a permanent fix. ARMA says the same thing plainly: the effect is temporary and the discoloration may recur (Source: ARMA).
What moves the number is exposure. Shade, heavy tree cover, north-facing slopes, humidity, and a neighbor's dirty roof upwind all shorten the cycle. Under heavy tree cover it can come back within a year or two, and one contractor reports 6 to 9 months on roofs under overhanging trees (Source: Benz Softwash). To slow regrowth, zinc or copper strips along the ridge release ions when it rains that inhibit growth on the slope below, and the algae-resistant shingles above use copper granules for the same reason. If a customer wants fewer cleanings, that's the honest lever.
Plant, Pet, and Runoff Safety
SH is an EPA-registered antimicrobial, and in the presence of oxygen it reacts with organic matter and converts readily into salt and water (Source: EPA). That's the reassuring part, and it's true. The part that needs discipline is the concentrated runoff. Don't send SH down a storm drain. Discharge to waterways runs into Clean Water Act problems, so you capture and dilute it instead.
On the job, the routine protects the property:
- Soak every plant, bush, and bed before you start, and keep watering through the whole job. The salt left behind when SH dries is what burns plants, so don't let it dry on vegetation. St. Augustine grass is especially touchy.
- Bag the gutters and downspouts to catch the strongest slug of runoff, and flush the downspout discharge points before you spray.
- Neutralize residual SH where it collects. Our bleach neutralizer guide walks through how and when.
- Keep pets inside or off treated areas until everything is rinsed and dry.
When a customer asks whether it's safe for their plants, that routine is the real answer. It's safe because the operator makes it safe, not because the chemical is harmless.
Working at Height Without Getting Hurt
A wet, chemical-covered roof is slippery, and roofs are where this trade gets people hurt. One high-pressure worker fell off a steep roof and broke both ankles when the surface got slick. OSHA sets the baseline: crews doing residential construction work 6 feet or more above a lower level need fall protection, guardrails, a safety net, or a personal fall-arrest system (29 CFR 1926.501). The general-industry threshold is 4 feet (29 CFR 1910.28) (Source: OSHA).
The safer method is to stay off the roof. Most pros apply from the ground or a ladder using a 12V or gas pump and telescoping gear, and a ladder puts down less chemical with less runoff than climbing on and walking it. Wear a respirator, because SH gases off a hot roof and you don't want to breathe it, which we cover in our guide to soft washing health risks. And don't do a roof alone. A second person pre-wets the plants, watches the runoff, and is there if the ladder goes. The gear a roof job actually needs is worth setting up before your first one.
What Roof Cleaning Pays
A roof soft wash runs about $0.30 to $0.75 per square foot (Source: LandscapeUnite), or roughly $300 to $1,050 on a 1,500 square foot roof (Source: This Old House). National guides put the average near $460 to $700, with Angi citing about $460 and a range of $150 to $1,000, and Fixr putting typical jobs at $450 to $700 (Sources: Angi and Fixr). That's the range you quote against. What moves your number: stories, roof pitch, how heavy the growth is, roof type, and whether there are gutters to manage runoff.
The margins are the reason to add the service. The chemical is cheap. A $700 single-story roof might use 7 to 10 gallons of bleach at around $2 a gallon, so $14 to $20 of product on a job that clears several hundred dollars. Roof cleaning is a separate service from a house wash, never folded into it, and it's often the highest-margin line an early operator can run. Our guide on how to price a soft washing job gets into measuring off satellite and factoring pitch. One more thing worth knowing: insurers increasingly push owners to clean algae-stained roofs to avoid a cancellation, which keeps the demand steady.
When Soft Washing Is the Wrong Call
Cleaning can't fix a roof that's finished. If the shingles are at the end of their life, a wash won't bring back granules that are already gone, and it won't make a worn roof last longer. A good tell: if dark spots stay after two proper treatments, that's usually permanent granule loss showing the darker asphalt underneath, not living algae. No amount of SH changes that.
The other wrong call is a stain that isn't biological. Rust, exhaust soot, and tar don't respond to a biocide, because there's nothing alive to kill. Those need a different chemical or a different trade. And bare Galvalume, again, stays off the SH list entirely. The operators who say this out loud, who tell a customer the roof needs replacing instead of cleaning, are the ones who get the referral anyway.
Before Your First Roof
Dial the mix in before you're on a ladder over someone's landscaping. Our sodium hypochlorite guide has the roof ratios and the dilution math off a 10 to 12.5% stock, so your first roof runs at the right percentage the first time.
FAQ
Does soft washing damage a roof or shingles?
Not when it's done right. Soft washing runs under about 100 PSI, so it doesn't strip the protective granules the way pressure washing does. ARMA specifically warns against pressure washing shingles because it causes granule loss and premature failure (Source: ARMA). The chemistry kills the growth, and the low pressure protects the surface.
Will soft wash roof cleaning void a warranty?
ARMA describes low-pressure chemical cleaning as the recommended method for asphalt shingles, but defers to the individual manufacturer on warranty terms (Source: ARMA). Treat any flat "won't void the warranty" claim as marketing, not fact. The accurate answer for a customer is to check the shingle manufacturer's written warranty for the terms.
How long does a soft-washed roof stay clean?
About three to six years for most roofs (Sources: Benz Softwash and Trotta's Power Washing). Results are temporary, and ARMA confirms the discoloration can recur (Source: ARMA). Shade, heavy tree cover, north-facing slopes, and humidity shorten the cycle. Under dense trees, algae can come back in under a year.
Does soft washing kill algae and moss permanently?
It kills what's there, but it isn't permanent. SH kills the algae, moss, and lichen on contact, and the dead growth weathers off over the following weeks. New spores land and the growth returns over a few years (Source: ARMA). Zinc or copper ridge strips and algae-resistant shingles slow the regrowth.
