WashQuoter guide
Soft Washing vs Pressure Washing: When to Use Each
Soft washing applies a diluted sodium hypochlorite solution at low pressure (under 500 PSI) to kill algae, mold, and lichen on siding and roofs. Pressure washing applies high-pressure water (2,000 to 4,000 PSI) to physically remove surface debris from concrete and other hard flatwork. Almost every professional pressure washing route uses both, with soft wash accounting for a majority of house-and-roof revenue.
The core difference in one sentence
Soft washing uses chemistry (sodium hypochlorite plus surfactant) to kill the biology that is staining the substrate. Pressure washing uses water velocity to physically knock debris off the substrate. Most exterior stains a homeowner sees on siding or a roof are biological (algae, mold, lichen). Most stains on concrete flatwork are a mix of biological (mildew) and physical (tire marks, oil, tannin, rubber). That is why the trade uses different tools on different substrates.
Substrate-by-substrate table
| Substrate | Recommended technique | Working pressure | Why |
|---|---|---|---|
| Asphalt shingle roof | Soft wash only | Under 100 PSI | High pressure blows shingle granules off. ARMA (Asphalt Roofing Manufacturers Association) recommends against high pressure on asphalt shingles. |
| Metal roof | Soft wash preferred | Under 500 PSI | Chemistry kills algae without denting or leaving pressure marks. |
| Tile roof (barrel, flat, concrete) | Soft wash preferred | Under 500 PSI | High pressure fractures tiles. |
| Vinyl siding | Soft wash | Under 500 PSI | High pressure forces water behind the panels and into wall cavities. |
| Fiber-cement siding (Hardie) | Soft wash | Under 500 PSI | James Hardie explicitly recommends low pressure and no direct high-pressure spray on painted board. |
| Painted wood siding | Soft wash | Under 500 PSI | High pressure strips paint. Test first; aged paint may fail even at low pressure. |
| Stucco | Soft wash | Under 500 PSI | High pressure erodes finish coat. |
| Brick (masonry) | Low to moderate pressure | 1,000 to 2,500 PSI | Sound brick tolerates moderate pressure; deteriorated mortar joints do not. Test. |
| Concrete driveway | Pressure wash with surface cleaner | 3,000 to 4,000 PSI | Physical removal of tire marks, tannin, mildew. SH pre-treat improves result on biological staining. |
| Concrete pool deck (broom finish) | Pressure wash, careful with pattern | 2,500 to 3,500 PSI | Surface cleaner produces even result; direct wand leaves lines. |
| Wood deck (raw or lightly stained) | Chemistry + low pressure rinse | Under 800 PSI | Pressure gouges grain. Percarbonate cleaner or dilute SH plus oxalic brightener is standard. |
| Composite decking (Trex, TimberTech) | Soft wash | Under 500 PSI | Manufacturer warranties are voided by high-pressure spray. |
| Paver patio | Pressure wash then re-sand | 2,500 to 3,500 PSI | Removes weed growth and old sand; polymeric sand goes back after. |
| Fleet trucks and equipment | Hot water pressure wash | 2,000 to 3,000 PSI with heat | Grease and road film respond to heat and detergent. |
How soft-wash systems are actually configured
Two mechanical setups dominate:
Downstream injection
A downstream injector is a small brass fitting installed after the pressure washer pump. When the operator switches to a low-pressure soap tip (black 65-degree), the pressure drops to about 100 to 200 PSI at the tip, and the venturi in the injector pulls chemistry from a jug into the water stream, diluting it in the process. Typical downstream ratio is 10 to 12 parts water to 1 part concentrate. If the operator feeds the injector with 12.5 percent SH plus a surfactant, the applied working solution is about 1 percent SH at the tip. Simple, cheap, and reliable. Every route contractor has this on the truck.
Dedicated 12-volt soft-wash pump
A separate 12V diaphragm pump (Delavan, Everflo, or ProPortioner) pushes premixed soft-wash solution through its own hose reel and a low-pressure wand. This lets the operator apply higher working strength SH (up to 3 to 4 percent for heavy roof jobs) without cycling the main pressure washer pump. Shore Flex hose and a Suttner or Kingfisher wand round out the setup. Typical cost for the 12V system: $400 to $1,200 depending on flow rate and reel size.
Almost every professional route business runs both: downstream for house washes, dedicated 12V for roofs.
Where the techniques overlap
Concrete flatwork is the most common overlap. The correct method for a mildew-stained concrete driveway is:
- Pre-treat the surface with a diluted SH plus surfactant solution downstream at 1 to 2 percent. Let dwell 5 to 10 minutes.
- Surface-clean with a rotary surface cleaner at 3,500 to 4,000 PSI and 4 GPM. This scrubs the loosened biofilm off.
- Rinse and post-treat any remaining staining with a second SH pass if needed.
The SH kills the biology that surface cleaning alone would leave behind as a shadow. Skip step 1 and the customer complains a week later when regrowth reappears.
Why soft wash costs more per square foot despite lower pressure
Three reasons:
- Chemistry. Sodium hypochlorite is a real cost of goods (2025 to 2026 wholesale pricing around $3.20 to $3.90 per gallon of 12.5 percent from regional suppliers). A single roof soft wash can consume 20 to 30 gallons.
- Slower work. Soft wash requires application, dwell, and rinse. Concrete flatwork with a surface cleaner is faster per square foot even accounting for pre-treatment.
- Site protection. Landscaping wetting, tarp deployment, and neighbor coordination take real time.
Common questions
Can I do a soft wash with just bleach from the grocery store?
Not for professional work. Grocery bleach is about 5 percent SH; concentrated pool-supply and janitorial 12.5 percent SH exists specifically because at working dilutions you need enough active ingredient to kill the biological load quickly. Diluting 5 percent household bleach down to 1 percent for a house wash leaves too little active chlorine. See our SH safety guide.
Does soft wash damage plants?
SH will burn foliage at working strength. The standard mitigation is to pre-wet all landscaping before application, cover sensitive plants, and continuously rinse during and after the work. Done properly, damage is rare. Done sloppily (no pre-wet, spray in high wind), plant kill happens. See the site protection section of our SH safety guide.
Does soft wash damage roof shingles?
The Asphalt Roofing Manufacturers Association (ARMA) has published guidance recommending against high-pressure cleaning of asphalt shingles and endorses low-pressure chemical cleaning methods as the safe approach for algae removal. That is why the industry moved to soft wash for roofs over the last 15 years. See asphaltroofing.org for the trade association's published position.
What about a "no-pressure" house wash spray-and-rinse?
Same technique. What contractors sometimes market as "no-pressure" is a downstream-injected soft wash at soap-tip pressure (about 100 to 200 PSI at the substrate). Not literally zero pressure, but well below what would drive water into the wall assembly.
Can I pressure-wash my composite deck?
Composite deck manufacturers (Trex, TimberTech) publish care and cleaning instructions in their product manuals that specify low pressure (typically under 1,500 PSI) and a soft brush for mildew removal. Higher pressure damages the top layer and voids the warranty. Contractors treat composite decks as soft-wash substrates.
Working solution recipes contractors actually use
Reference points, not prescriptions. Test on a small area before you commit to a whole substrate. Adjust for water hardness, ambient temperature, and product-specific SDS instructions.
| Job | Downstream mix (from 12.5 percent SH) | Working SH at tip (approximate) | Dwell before rinse |
|---|---|---|---|
| Standard vinyl house wash, light algae | 1 gallon SH + 4 gallons water + 4 oz surfactant, injector set at 10 percent | 0.5 to 0.8 percent | 10 to 15 min, keep wet |
| Fiber-cement house wash, moderate algae | 2 gallons SH + 3 gallons water + 4 oz surfactant, injector at 10 percent | 1.0 to 1.3 percent | 10 to 15 min, keep wet |
| Concrete pre-treat before surface cleaner | Downstream at 10 percent from 12.5 percent SH | 1.2 percent | 5 to 10 min, then surface clean |
| Roof soft wash via 12V system, moderate load | 50/50 SH and water plus 2 oz surfactant per gallon of mix | 6 percent applied, dilutes on shingle | No rinse if per applicator practice; light rinse landscaping |
| Roof soft wash via 12V system, heavy lichen | Straight 12.5 percent plus surfactant; two-pass application | 12 percent applied | Slow rinse landscaping heavily |
Surfactant is dosed by volume per gallon of applied mix, following the surfactant manufacturer's instructions. Non-ionic surfactants (Elemonator, Slow Down, Simple Cherry) are the trade standard because they hold up in high pH environments.
Where to use pressure alone
A short list of jobs where high-pressure water without SH is the right call:
- Post-construction concrete cleanup. Grout haze, mortar drips, paint splatter on flatwork respond to pressure and a good surface cleaner. Chemistry does not help.
- Loose gravel or sediment on driveways. Rinse only. No biology to kill.
- Paver joints being re-sanded. Blast old sand out with high-pressure water and a directional tip; refill with polymeric sand.
- Deck stripping before re-stain (paired with a stripper chemical, not SH). Sodium hydroxide-based deck strippers plus moderate pressure.
- Equipment and construction gear washing. Grease and mud yield to pressure, especially with heat. See our equipment guide.
Related guides
Sources
- Asphalt Roofing Manufacturers Association (ARMA), Technical Bulletin on Algae Discoloration of Roofs. asphaltroofing.org.
- James Hardie, care and cleaning of fiber-cement siding (manufacturer maintenance documentation). jameshardie.com.
- Trex Company, composite decking care and cleaning guidance. trex.com.
- TimberTech (AZEK), care and cleaning instructions for composite decking. timbertech.com.
- Power Washers of North America, soft washing safety and best practices. pwna.org.
- OSHA, 29 CFR 1910.132 PPE requirements. ecfr.gov 1910.132.
- CDC NIOSH, sodium hypochlorite pocket guide. cdc.gov npg 0573.
- US EPA, NPDES stormwater compliance. epa.gov/npdes.
- Suttner America, downstream injectors and soap tip specifications. suttner.com.
Last reviewed 30 June 2026. See methodology and sources.
Educational content for pressure-washing contractors and their customers. Not legal, insurance, or engineering advice. WashQuoter is not affiliated with or endorsed by PWNA, UAMCC, IWCA, OSHA, EPA, or any regulatory or trade body. Confirm licensing with your state contractor licensing board and coverage with a licensed insurance broker before relying on any figure above.