Dirty siding, a green-streaked roof and a stained walkway all invite the same quick fix: blast it with water. Sometimes that works. Sometimes it strips wood fiber, drives water behind panels or sends a stream of soapy runoff into a storm drain. This page explains the two main approaches to exterior washing, pressure washing and soft washing, what each one does to a surface, and the safety and environmental rules that sit around both. It is written for people who look after a home or a small commercial property and want to understand the field before anything is done to a building.
We lean on public sources throughout: a federal property agency's masonry specification, a university safety office, a Forest Service wood laboratory, a shingle manufacturers' association and an EPA-funded stormwater manual. None of them is a sales document, and where they disagree or fall silent we say so. Nothing here recommends a method, product or provider.
What the terms actually mean
Pressure washing uses a pump to push water through a narrow nozzle so that the force of the water lifts dirt off a surface. Two numbers describe a machine. PSI, pounds per square inch, is how hard the water pushes. GPM, gallons per minute, is how much water moves. A machine with high PSI but low GPM hits hard over a small area, while higher GPM rinses away loosened material faster. One university safety procedure for a gasoline-powered unit lists a maximum pressure at the nozzle of 3400 psi, which shows how much force is available in ordinary equipment (University of Memphis EH&S).
Soft washing is a different idea. Instead of relying on force, it applies a cleaning solution at low pressure, lets it sit, and then rinses gently. The solution does the loosening, so the water does not have to. The shingle manufacturers' association describes this logic for roofs when it recommends a household bleach (sodium hypochlorite) and water solution that is dispersed gently, left on for a set time and rinsed, rather than blasted off (Asphalt Roofing Manufacturers Association). Our page on roof and siding soft washing goes through that case in detail.
Pressure, nozzles and distance
The nozzle shapes the water into a fan, and the fan angle changes how the force is spread. A narrow fan concentrates the energy on a small strip. A wider fan spreads the same flow over more area and feels gentler on the surface. Distance matters as much as angle, because force drops quickly as the nozzle moves away.
The clearest published figures come from a federal specification written for historic masonry, where the goal is the mildest method that works. It calls for a 45-degree fan-tip nozzle at about 4 gpm with no more than 400 psi for a rinse, starting at 100 psi or below, and it keeps the nozzle at least 18 inches from the wall. Water washing in the same document is not to exceed 300 psi, and any cleaning product is applied by natural fiber brush or very low-pressure spray not exceeding 100 psi (U.S. General Services Administration). These are limits for fragile historic stone, not a universal setting, but the document also notes that most commercial pressure washing systems operate at significantly higher pressures than those it recommends. That gap is worth knowing about: a machine's top rating is not a setting anyone should assume is safe for a given wall.
The same specification says high-pressure spray equipment should not be used to apply any cleaning product, and that the lowest possible pressure should be used to get the result. It also asks for a test patch no larger than 6 inches by 6 inches in an inconspicuous place, so that the mildest successful method is found before the whole surface is treated. Testing first is the habit that carries over to any building.
Some machines heat the water or use steam, and the university safety procedure cited above tells operators to use extreme caution around high-pressure steam tips and nozzles. We found no public agency page comparing hot and cold water for exterior cleaning, so we make no claim about which suits a given soil.
What washing removes
Most exterior cleaning deals with a handful of soils. Loose dirt and dust are the easy ones, and plain water often moves them. Algae are different. The dark streaks on many roofs are usually algae, with the species Gloeocapsa magma named as a common contributor by the shingle association, which says the organisms travel through the air and settle on roofs, and that their natural pigments can turn light roofs brown or black. The same page points out that algae are easily mistaken for fungus, soot, dirt, moss or tree droppings, and that moss and droppings typically cause only localized discoloration (Asphalt Roofing Manufacturers Association).
Mildew is the other common one on wood. The Forest Products Laboratory notes that wood decks without proper maintenance can develop problems including checks and cracks, raised grain and mildew, and that deck cleaners and brighteners remove mildew and dirt so the natural color of the wood can show. It also gives a mildew treatment of liquid household bleach diluted with 3 parts water plus some detergent, followed by rinsing (USDA Forest Products Laboratory).
What it can damage
Force is the usual culprit. The Forest Products Laboratory warns that aggressive scrubbing with a caustic cleaner or power washing can remove wood from the surface, particularly on softer wood such as western redcedar, and that the silvery fibers left on weathered wood should not be stripped away because they absorb stain well (USDA Forest Products Laboratory). On asphalt roofing, the shingle association says plainly not to use a power washer or any type of brush or broom to clean algae from the roof surface, and that high-pressure washing is likely to damage asphalt roofing.
Siding has its own failure mode, which is water forced behind panels and into openings. Our page on roof and siding soft washing gives the vinyl siding trade association's guidance on wand angle, manufacturer limits and caution around windows, doors and wiring.
Chemistry carries its own risks, covered more fully in our guides to reading product labels and safety data sheets and to mixing chemicals safely. One example from the wood laboratory: do not use a detergent containing ammonia with bleach, because ammonia reacts with bleach to form a toxic gas. Finally, older paint is a special case, and our page on pressure washing safety and suitable surfaces covers the lead paint rules that apply to pre-1978 buildings.
Runoff and wastewater
Wash water does not vanish. It flows to a drain, a lawn, a neighbor's boundary or a storm sewer, and storm sewers usually lead to streams and lakes without treatment. EPA's illicit discharge guidance defines the problem in general terms: illicit discharges include any discharge into a storm drain system that is not entirely composed of stormwater, with limited exceptions such as water from firefighting and discharges from facilities already covered by a permit. EPA notes that these discharges often contain pathogens, nutrients, surfactants and various toxic pollutants (U.S. EPA). Surfactants are the cleaning agents in soaps and detergents that help water spread and lift soil, so a soapy rinse falls squarely into that description.
Scale and soil decide whether washing is a concern. An EPA-funded guidance manual from the Center for Watershed Protection says outdoor washing may or may not be an illicit discharge depending on the site producing the wash water. Hosing off individual sidewalks and driveways may not generate significant flows or pollutant loads, whereas routine power washing of fuelling areas, outdoor storage areas and parking lots may result in unacceptable pollutant loads. The manual also lists building maintenance and parking lot maintenance by power washing among typical commercial sources (Center for Watershed Protection and R. Pitt, EPA-funded IDDE manual). The manual carries a notice that it does not necessarily reflect Agency views, and local rules vary.
A practical example of how a rule can read comes from a university campus. Its power washing guidelines say that washing without soap on unpainted building surfaces, brick and wood decks calls for screening runoff for solids and discharge to landscaping or the street. Washing with soap means blocking storm drains, containing the runoff, screening out solids, confirming the pH is between 6 and 10 and pumping the water to the sanitary sewer. The same page warns that state and federal laws dictate the use and disposal of certain cleaning chemicals and that failures can lead to significant fines for the university and its contractors (UNC Environment, Health and Safety). That is one institution's policy, not a national rule.
Protecting plants, people and property
Plants suffer when solution drips onto them. The masonry specification above requires surrounding vegetation to be protected from runoff. The shingle association advises eye protection and rubber gloves when handling bleach, and the label and safety data sheet of any product take precedence over general advice.
Equipment injuries are the more immediate danger. The CDC warns that the spray from a pressure washer can cause serious wounds that might first appear minor, and that delayed treatment raises the risk of infection, disability or amputation. It also says never to point the washer at yourself or others and never to use the spray to push or move objects, since thrown objects can injure bystanders. For electric units it advises testing the ground fault circuit interrupter before use and keeping extension cord connections out of standing water, and for gasoline units it says never to run them in an enclosed space or within 20 feet of a window, door or vent (CDC: Pressure Washer Safety). Our page on pressure washing safety takes these hazards one at a time.
When a surface needs a different method
Some surfaces should not be washed at pressure at all. Asphalt shingles are the clearest case according to their manufacturers' association. Soft or historic masonry is another, where the federal specification keeps pressures low and sets a 100 psi limit for applying products. Softer woods such as western redcedar can lose surface material to power washing, per the wood laboratory. Where growth rather than loose dirt is the problem, the cleaning solution and its contact time are doing the work, and extra force only adds risk.
Where this fits in property maintenance
For a home, exterior washing is an occasional task within a larger maintenance cycle. For a shop, office or multi-tenant building it usually sits in a maintenance plan with defined areas and frequencies, alongside the indoor routines described in commercial and office cleaning explained. If you are deciding how interior and exterior upkeep relate, how cleaning works sets out the underlying principles of soil, method and product.
People who commission exterior work for a building can ask better questions once they know the vocabulary above: what pressure, what nozzle, what solution, where the water goes, how plants and people are protected. Our checklist of questions for evaluating a cleaning provider turns those into practical items that apply to any outside cleaning or washing provider.
Frequently asked questions
Is soft washing the same as using a low-pressure nozzle?
Not quite. A low-pressure nozzle only changes how the water leaves the machine. Soft washing, as described by the shingle association and the masonry specification, adds a cleaning solution that is applied gently and left to work before a light rinse. The solution and its contact time do the cleaning, which is why product labels and safety data sheets matter.
Does the wash water go down the storm drain?
It can, but that is exactly what stormwater programs try to prevent when the water carries soap, oil or other pollutants. EPA describes any discharge to a storm drain that is not entirely stormwater as illicit, with listed exceptions, and local rules differ. Checking the city or county stormwater office is the reliable way to learn what applies.
How high is too high for a PSI setting?
There is no single answer, because it depends on the surface. The federal masonry specification caps water washing at 300 psi and rinses at 400 psi for fragile historic stone, while the equipment in one university procedure reaches 3400 psi. The practical rule in the sources is to use the lowest pressure that gets the result, and to test first.
Can a pressure washer injure you even if the skin looks fine?
Yes. The CDC says the spray can cause serious wounds that might first appear minor, and that delayed treatment raises the risk of infection, disability or amputation. For any such injury, get medical attention as soon as possible.
The short version
Pressure washing relies on force and soft washing relies on a gentle application of a cleaning solution. Every surface, from vinyl to shingles to soft stone, has its own tolerance, and the public sources agree on the habit that protects it: use the lowest pressure and mildest method that works, test in a small hidden spot, and keep wash water, plants and people in mind throughout. Labels, safety data sheets and equipment manuals have the final word.