Soft-Wash Ratio
Soft-wash ratio refers to the dilution of concentrated sodium hypochlorite (typically 12.5%) with water to achieve a target application-strength solution. The standard industry mix for house washing is a downstream application of 0.5–1.5% sodium hypochlorite at the surface. Since downstreaming dilutes the mix approximately 10:1 at the injector, a tech starts with 3-5% SH in the chemical bucket to hit a 0.5-1% surface concentration. For roof cleaning (where a slightly higher kill concentration is needed for deep-seated algae), the surface concentration target is typically 1.5-3%, requiring a stronger bucket mix.
In depth
Mix ratio also varies by ambient conditions: on a hot sunny day, chemistry dries faster and requires a stronger mix or more frequent re-application; in cool humid conditions, dwell is more effective and a lighter mix will achieve the same kill. Inexperienced operators use the same mix regardless of conditions and produce inconsistent results — either burning landscaping with too-strong chemistry or achieving no kill with too-weak chemistry. Dialing mix ratio is one of the fundamental craft skills in soft washing.
How this shows up on our jobs
Before every house wash and roof clean in Madison, we check the ambient temperature, substrate condition, and test the downstream draw rate to set the correct mix. Chemistry this precise is the difference between one visit and two.
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Related terms
Chem Ratio
Chem ratio refers to the final dilution of cleaning chemistry at the surface — the percentage of active ingredient after the downstream injector, surfactant, and rinse water have all combined. For soft-wash work, the target ratio at the surface is typically 1-2% sodium hypochlorite for siding and 2-4% for roofs, down from a 12.5% concentrate in the bucket. Achieving the right ratio depends on injector flow rate, hose length, nozzle size, and pump pressure — and it is rarely a fixed number across all rigs.
Downstreaming
Downstreaming is the technique of injecting soap or cleaning solution into the pressure washer's water line on the low-pressure side of the pump — downstream from the pump, hence the name. A chemical injector pulls detergent from a bucket through a siphon hose whenever a low-pressure (soap) nozzle is attached. When the technician swaps to a high-pressure nozzle, the injector automatically stops drawing chemical, allowing for a clean rinse without switching lines.
Downstream Injector
A downstream injector is a small one-way check valve installed on the low-pressure outlet of a pressure washer pump that automatically draws cleaning solution from a bucket whenever a low-pressure nozzle is attached. When the operator switches to a high-pressure nozzle, back-pressure closes the check valve and chemical flow stops, leaving the machine in rinse mode. This automatic switching is the foundation of a professional exterior-cleaning setup — no hand valves, no extra hoses, just a nozzle swap.
Sodium Hypochlorite
Sodium hypochlorite (NaOCl) is the active ingredient in household bleach and the primary disinfectant used in professional soft washing. It is sold to exterior cleaners in 12.5% concentration (commonly called "SH" or "pool shock"), roughly twice the strength of consumer bleach. When applied in a properly mixed soft-wash solution, it kills algae, mold, mildew, lichen, and bacteria at the cellular level within minutes and breaks down into salt water and oxygen.
Dwell Time
Dwell time is the interval between applying a cleaning chemistry to a surface and rinsing it off. It is the single most misunderstood variable in exterior cleaning. Chemistry does not clean instantly; it needs time to break chemical bonds, kill organisms, or dissolve minerals. Too little dwell and the chemistry is wasted. Too much dwell — especially in direct sun — and the chemistry can damage the surface, bleach landscaping, or drive the stain deeper.