Soft washing works by chemistry, not by force. A diluted bleach mix kills the algae, mold, and lichen on the surface, a surfactant holds it there long enough to act, and a high-volume, low-pressure stream carries the solution on and rinses the dead growth off. The pressure never does the cleaning. For the full definition and where the term came from, our what is soft washing guide covers that.
We manufacture the proportioners and pumps operators run, so most of what's below comes from what we see in the field and in customer calls every week.
How Soft Washing Works, Start to Finish
Soft washing cleans in four moves. Sodium hypochlorite (SH) oxidizes the algae, mold, and lichen and kills it at a cellular level. A surfactant lowers the water's surface tension so the mix clings to vertical and sloped surfaces instead of running off. A high-volume, low-pressure stream, usually under 100 PSI at the gun, carries the solution onto the surface. The mix dwells, then a rinse washes the dead growth away.
That's the whole process. Everything below is how each step works and why each one matters.
The Chemistry Does the Cleaning, Not the Pressure
SH works by oxidation, not force. Its hypochlorite ion (OCl−) attacks the chromophores, the color-producing molecules inside the growth, and renders them colorless (Source: J. Racenstein Co.). The active chlorine also denatures proteins and disrupts the organism's DNA (Source: American Cleaning Institute, citing EPA). So it does two jobs at once: it kills the growth down to the root so it regrows slowly, and it changes the color of the stain.
That color change is your working gauge. Green algae turning brown or tan means it's dying and ready to rinse. Black still showing after a coat means the mix was too weak for that growth, and the spot needs another pass. Our sodium hypochlorite 101 guide goes deeper on the chemistry.
Bleach only kills what it touches, so it won't crawl into a porous surface to reach roots inside. And it needs oxygen to react, so a thick flooded coat slows the kill. A moist, even coat beats a soaking one.
What Goes Into the Mix
Three ingredients. Each carries a job.
Sodium hypochlorite (SH). The active ingredient, same chemical family as pool chlorine. Operators buy it at 10% or 12.5% and dilute it for the surface. What matters is the strength that lands on the wall, not the strength of the jug in the truck. Working applied ranges run roughly like this:
- Vinyl and aluminum siding: about 1% to 1.5% applied SH
- Stucco, brick, and masonry: about 2% to 4%
- Asphalt shingle roofs: a wider band, roughly 3% to 6% depending on growth and temperature
- Wood decks and cedar: 0.5% to 1%, or sodium percarbonate (oxygen bleach) on raw wood
Roof strength is where the trade genuinely disagrees. ARMA's published 50/50 mix assumes laundry-strength bleach and lands around 1.5% to 3% applied, while many roof operators run 5% to 6% on heavy north-side growth (Sources: ARMA and field practice). Defer to ARMA and the shingle manufacturer on any roof, and start low.
Surfactant. Soap that drops the water's surface tension so the mix clings to a wall or roof slope long enough to work, instead of sheeting off. Worth knowing: plenty of operators run no surfactant on production house washes because it speeds the rinse, while steep roofs are where cling earns its keep. Never run it on screen enclosures. Our surfactant guide covers when it helps.
Water. Dilutes the SH down to the applied strength the surface needs. Getting that number right is the whole game, and it's where most operators get tripped up.
How the Solution Gets Applied at Low Pressure
The machine is the delivery system. The chemical does the work. A few ways get the mix onto the surface at low pressure.
Downstream injector. The most common method for house washing. The injector sits on the outlet side, after the pump, so chemical never runs through the pump and corrodes it. It works on the venturi effect: water flowing through a small orifice creates a low-pressure zone that pulls chemical up a siphon hose into the stream. The catch is that it only draws when a large-orifice "soap" tip is on the gun. That tip drops machine pressure by about 300 PSI, which opens the draw. Put a small high-pressure tip on and the draw stops. Roughly 9 of 10 "my injector won't draw" calls turn out to be the injector installed backward.
12V and air-diaphragm pumps. Dedicated soft wash pumps that apply a stronger mix than a downstream injector can reach. A 12V diaphragm pump runs off a battery and pushes a controlled stream up and over a two-story roofline from the ground. Air-diaphragm pumps are the most dependable for daily use. Our 12V vs gas soft wash pumps guide breaks down the tradeoffs.
Proportioner. A manifold that meters water, SH, and surfactant independently before the mix reaches the pump, so you dial the applied strength on the fly moving from siding to soffit to roof. Our soft wash proportioner guide walks through the setup.
How the Mix Hits the Right Strength at the Surface
The strength that cleans is the strength at the wall, and a downstream injector dilutes the stock heavily on the way there.
Run straight 12.5% SH through a standard downstream injector and you land at roughly 1% applied at the surface (Source: field practice). The draw ratio moves with machine flow: a 4 GPM machine with a 2.1 injector pulls about 7:1 and lands a bit over 1%, while an 8 GPM machine pulls closer to 10:1 and lands near 1%. Lower flow means a stronger mix. Hose length works the same way. You lose about 1 PSI per foot, and every foot shaves a little strength off, so a 150-foot run hits the wall stronger than a 300-foot run (Sources: Pressure Tek and field practice).
A downstream injector realistically tops out around 1% to 2% applied, with one operator's measured maximum at about 1.87% (Source: field practice). That's the ceiling that trips operators up. Fine for siding and most house washes. Well short of the 3% to 6% an asphalt shingle roof needs. Treat these as an engineering model, not lab measurements, and bucket-test your own rig. Our soft wash vs downstreaming guide covers where the injector runs out of room. When the surface needs more, you step up to a dedicated pump or a proportioner that dials the exact strength.
GPM vs PSI: Why Volume Cleans and Pressure Damages
For soft washing, water volume (GPM) is the asset and pressure (PSI) is the liability. Volume carries the chemistry onto the surface, spreads it, and rinses the dead growth off. High pressure strips granules off shingles, forces water behind siding panels, and chips brittle stucco. That's why the rule among working operators is to buy a machine for its GPM, not its PSI.
| Soft Washing | Pressure Washing | |
|---|---|---|
| Pressure at the nozzle | Under 100 PSI, often far less | 1,500 to 4,400 PSI |
| What cleans | Bleach kills the growth at the root | Water force shears dirt off |
| Best for | Roofs, siding, stucco, screens, wood | Concrete, driveways, sidewalks |
ARMA is direct on roofs: do not use a power washer, because high-pressure systems are likely to damage asphalt roofing (Source: ARMA). Our pressure washing vs soft washing guide lays out the full contrast.
The Soft Wash Job Sequence
A soft wash runs in a set order. The sequence is most of the job.
- Walk the property and protect. Pre-wet every plant and the soil out two to three feet from the wall until the ground is soggy. A saturated plant resists chemical uptake. A dry plant pulls runoff in fast. Divert runoff away from beds, tape light fixtures and lock cylinders, and confirm windows and doors are closed.
- Apply the mix at low pressure. Soap siding bottom-up so clean lines don't streak through dirty sections. Wash brick top-down so upper runoff doesn't drip onto rinsed areas. On roofs, lay a moist medium coat, not a flood, since the chemistry needs oxygen to react.
- Let it dwell. Hold the mix on the surface while it kills the growth and changes its color. Keep the surface wet the whole time.
- Rinse. A low-pressure rinse carries the dead growth off siding and most surfaces. On asphalt shingle roofs, many operators leave the mix and let the next rain finish the rinse.
The pro move is staggering the work. Soap two to four sides, then rinse the first while the others dwell. Save the dirtiest, most shaded side for last so it gets the longest dwell.
How Long Does the Solution Dwell?
Dwell is the window where the chemistry kills. For house washing, 5 to 10 minutes is standard. For asphalt shingle roofs, ARMA specifies leaving the solution on for at least 15 minutes and no more than 20, then rinsing with low-pressure water (Source: ARMA).
Temperature moves the clock. SH reacts faster when it's hot, so a sunny day shortens dwell and you rinse sooner. Cold slows it, so you give it longer or run a second pass rather than mixing the SH hotter. The rule that doesn't bend: never let the mix dry on the surface. As the water evaporates, the applied concentration climbs from under 1% toward the stock strength of 12% or higher, and that's what chalks vinyl. Pre-wetting a hot wall before you soap it keeps the mix from flashing off.
Why It Doesn't Damage the Surface
Two things keep soft washing safe on the surfaces it's used on. The pressure stays low, so the shingle granules, the panel seals, and the stucco finish stay intact. ARMA recommends low-pressure chemical cleaning over pressure washing for this reason, and high-pressure washing voids most shingle warranties (Source: ARMA). And the applied concentration is matched to the surface, so the bleach is strong enough to kill the growth without harming the substrate.
Surroundings are where careless operators cause damage. SH is plant-toxic at the concentrations used near roofs, with kill risk starting around 1.5%. The protection that works is covering plants and flooding their roots with water, not spraying a neutralizer afterward. Bleach burns foliage within a couple of seconds, faster than any liquid neutralizer can stop, so a neutralizer is a backup for the soil, not the primary shield. Our bleach neutralizer guide covers that chemistry. SH also corrodes metal and off-gasses chlorine, so wear a respirator once you can see vapor rising or you're working at 3% to 5%, and read the SDS for anything you mix.
When Soft Washing Is the Wrong Call
Soft washing isn't the answer for everything.
- Growth heavier than downstream can handle. A downstream injector caps out around 1% to 2% applied. A roof thick with black streaks needs a dedicated pump or a proportioner, not more coats from a weak mix.
- Chalky oxidation on old vinyl or aluminum. That's a degraded coating, not biological growth. Bleach won't touch it. It needs a metasilicate or oxidation-specific cleaner.
- Bare wood at high strength. High SH fuzzes raw wood. Use a lower percentage or switch to sodium percarbonate, then brighten.
- Inorganic stains like efflorescence or rust. SH is for organic growth. These need different chemistry, and you never mix an acid with bleach.
- Clean shingles. If there's no algae, there's nothing for the SH to react with, and a dry coat on bare shingle only dries it out.
If you're not sure what you're looking at, spot-test first. It saves a callback.
Next Steps
If you're building toward running this work yourself, the next step is the rig that makes pro-grade mixing possible. Our soft wash proportioner guide walks through how a proportioner meters water, SH, and surfactant so you can hit the exact applied strength every surface needs.
Frequently Asked Questions
What's the difference between soft washing and pressure washing?
Soft washing applies a bleach-and-surfactant mix at low pressure, usually under 100 PSI, and the chemistry kills algae and mold at the root. Pressure washing uses high-pressure water, 1,500 to 4,400 PSI, to shear dirt off by force. Different tools for different surfaces and different problems.
What chemicals are used in soft washing?
Three things. Sodium hypochlorite (SH), the same family as pool chlorine, kills the growth by oxidation. A surfactant lowers surface tension so the mix clings and dwells instead of running off. Water dilutes the SH down to the applied strength the surface needs, from about 1% on siding to 3% to 6% on a roof.
How long does the solution sit on the surface?
It depends on the surface. House washes dwell 5 to 10 minutes. For asphalt shingle roofs, ARMA specifies 15 to 20 minutes before a low-pressure rinse. Heat speeds the reaction and cold slows it, but one rule holds everywhere: never let the mix dry on the surface.
Does soft washing damage roof shingles?
No, when it's done right. It's the low-pressure chemical method ARMA and major shingle manufacturers recommend, and it falls within most warranty parameters. High-pressure washing is what damages shingles, stripping the ceramic granules and voiding the warranty. The bleach kills the algae without harming the granule surface.
How long does a soft wash last?
Because the chemistry kills the growth at the root instead of only rinsing the surface, regrowth is slow. A properly soft-washed roof can stay clean two to five years, and siding one to three, before re-treatment. Shaded, north-facing surfaces re-stain fastest. Sun-exposed surfaces hold up longest.
