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Understanding Sodium Hypochlorite Mix Ratios for Soft Washing
By the Rent Wash Gear Team · 7 min read
Sodium hypochlorite, the active ingredient in bleach at professional strength, is the backbone of soft washing. It kills algae, mold, and mildew on contact, letting you clean green-stained roofs and siding with barely any pressure. But strength without math is how siding gets bleached and plants die. This guide covers the ratios, supporting chemistry, and safety practices that make soft washing predictable.
What Sodium Hypochlorite Is (and Is Not)
Sodium hypochlorite (SH) is sold to contractors as 10% or 12.5% liquid, often labeled pool shock. That is roughly double to triple the strength of household bleach. It degrades over time, especially in heat and sunlight, so a jug that sat in a hot garage all summer may test far weaker than its label. Buy fresh, store cool and dark, and use within a few months for consistent results.
SH is an oxidizer, not a detergent: it destroys organic growth but does not lift dirt or grease on its own. That is why mixes pair it with a surfactant. See our downstream injector detergent guide for the full chemistry picture.
Why Soft Washing Uses Chemistry Instead of Pressure
High pressure on asphalt shingles strips protective granules. On vinyl siding it forces water behind panels. On stucco it can carve lines into the finish. Soft washing flips the approach: apply chemistry at garden-hose pressure, let it dwell, and rinse gently. The cleaning happens through oxidation, not impact.
This is why soft wash rental gear uses low-pressure pumps or downstream injectors with soap nozzles. On a standard pressure washer, the black soap nozzle is your low-pressure applicator.
Mix Ratio Math: From Stock to Applied Strength
The number that matters is the applied strength at the surface, not the stock concentration in the jug. Work backward from your target:
| Surface | Target applied SH | Mix from 12.5% stock | Mix from 10% stock |
|---|---|---|---|
| Vinyl siding (light growth) | ~0.75% | 1 part SH to 15 parts water | 1 part SH to 12 parts water |
| Vinyl siding (heavy growth) | ~1% | 1 part SH to 11 parts water | 1 part SH to 9 parts water |
| Asphalt shingle roof | ~1.5% | 1 part SH to 7 parts water | 1 part SH to 5.5 parts water |
| Concrete (organic stains) | ~2% | 1 part SH to 5 parts water | 1 part SH to 4 parts water |
| Fence and deck (wood) | ~0.5% | 1 part SH to 24 parts water | 1 part SH to 19 parts water |
These are starting points. Old SH tests weaker, injectors add their own dilution, and cool weather slows the reaction. Start weak, test a patch for 10 minutes, and step up only if growth is not releasing.
Why Surfactants Matter as Much as Bleach
A surfactant is a soap-like additive that lowers surface tension so the mix clings to vertical surfaces instead of sheeting straight off. Without it, your expensive SH runs down the siding in seconds and dwell time collapses. With it, the solution hangs on the surface for the full 10 to 15 minutes the chemistry needs.
Professional soft wash surfactants include cling agents that make the mix visible as light foam, so you can see coverage. Add surfactant last at the label rate. Too much leaves a film; too little and you are back to runoff.
Dwell Time and Rinsing Technique
Apply bottom-up on siding, dwell 10 to 15 minutes, and never let the mix dry on the surface. On hot days, work in sections and mist to keep surfaces damp. Rinse top-down with low pressure, keeping the nozzle at least 12 inches away. If growth remains, reapply rather than reaching for a stronger mix.
Protecting Plants, Pets, and Property
- Pre-wet everything green before you start and rinse again immediately after. Wet foliage absorbs far less chemical.
- Cover delicate plantings with breathable tarps during application, then uncover and rinse.
- Divert downspouts away from beds and water features, or bag them during roof work.
- Keep pets and kids off treated surfaces until fully rinsed and dry.
- Rinse windows, vehicles, and metal fixtures that catch overspray, since SH can spot glass and dull some finishes.
Runoff management is part of the job. If your property drains to a storm drain, check local rules; some municipalities restrict wash-water discharge.
PPE and Critical Safety Rules
SH at contractor strength demands respect. Minimum PPE: chemical-resistant gloves, sealed eye protection, long sleeves, and closed shoes. A respirator is wise when spraying overhead or in enclosed areas, since atomized bleach irritates airways.
The hard rules: never mix sodium hypochlorite with acids (creates chlorine gas) or with ammonia (creates chloramines). Mix outdoors, label every container, and keep a water hose within reach. For gentler options, read our plant-safe detergent guide.
Frequently Asked Questions
Can I just use household bleach from the store?
You can, but the math changes: household bleach is 5 to 8 percent, so you need roughly twice the volume to hit the same applied strength, and scented or splashless versions contain additives that can leave residue. Pool shock at 10 to 12.5 percent is the standard for a reason.
How long does mixed soft wash solution stay good?
Use it the same day. SH degrades quickly once diluted, especially in sunlight and heat. Mix only what you will spray, and never store diluted mix in a sealed container where off-gassing can build pressure.
Will soft washing kill my lawn?
Overspray can burn grass and ornamentals if left on the leaves. Pre-wetting, covering, and prompt rinsing prevent nearly all damage. Established turf usually recovers from minor contact within a week or two.
Why did my siding turn white or chalky?
That is usually over-strength SH or letting the mix dry on the surface. Rewet, apply a weaker mix, keep dwell times honest, and rinse before drying. Chalky oxidation on old vinyl may need a dedicated oxidation remover, not more bleach.
Can I soft wash in direct sun?
It is harder, not impossible. Heat accelerates drying, which cuts dwell time. Work early morning or late afternoon, do smaller sections, and mist to keep surfaces wet. Overcast days are ideal.
Do I need a special pump, or will my pressure washer work?
A standard pressure washer with a downstream injector and soap nozzle works fine for siding and fences. For roofs and large commercial jobs, a dedicated 12-volt soft wash system gives better flow control. Both are available in our soft wash rental lineup.
Need the Right Gear for the Job?
Soft washing is a chemistry job first and an equipment job second. Rent the right low-pressure applicators and injectors from us, and we will walk you through the mix math for your specific surfaces.
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