The Science of Soft Wash Batch Mixing
In the exterior cleaning and pressure washing industry, batch mixing is the process of pre-mixing your Sodium Hypochlorite (SH), water, and surfactant into a single tank before applying it to a roof or siding. Unlike downstream injection (which draws soap into a high-pressure hose), batch mixing allows for hyper-accurate, high-strength chemical applications necessary for killing Gloeocapsa magma (roof algae).
However, if your math is wrong, you will either mix a batch that is too weak (wasting time and failing to clean the surface) or too strong (risking severe property damage to plants, siding, and paint). Our Soft Wash Batch Mix Calculator eliminates the guesswork.
How to Calculate SH Ratios Manually
The math behind the calculator relies on a simple cross-multiplication formula used by commercial roof cleaners:
For example, if you want to mix a 50-gallon batch at 3% strength using standard 12.5% pool shock: (3 ÷ 12.5) × 50 = 12 Gallons of SH. To find your water volume, simply subtract the SH from the total batch size (50 - 12 = 38 Gallons of Water).
Common Soft Wash Percentages
- 1% to 1.5% (Vinyl Siding / Painted Wood): Perfect for standard house washing. Strong enough to kill mildew but weak enough to prevent oxidation damage to siding or surrounding landscaping.
- 3% to 4% (Asphalt Shingle Roofs): The sweet spot for standard roof washing. This strength melts away black streaks (Gloeocapsa magma) rapidly without drying out the asphalt shingles.
- 5% to 6% (Heavy Moss / Concrete / Stucco): Reserved for extreme organic growth. Use extreme caution at these percentages, as runoff will quickly kill surrounding grass and plants if not properly watered down.
Don't Forget the Surfactant
Surfactant (soap) is critical in a soft wash mix because it breaks the surface tension of the water, allowing the bleach mixture to cling to steep roofs and vertical siding rather than immediately running off into the gutters. The industry standard is roughly 1 ounce of surfactant per gallon of total mix (e.g., 50 ounces of soap for a 50-gallon batch).
