Pressure Washing Chemical Mixing Ratios: The Complete Guide

Ask five pressure washers what their mix ratio is and you'll get six different answers. Some learned from YouTube, some from a mentor, some are still eyeballing it. This guide ends the guesswork.

The foundation of every soft wash job is sodium hypochlorite (SH) — pool shock in various concentrations. Your delivered SH percentage at the surface determines whether you clean effectively, damage the substrate, or eliminate the surrounding vegetation. Getting it right is the difference between a job you're proud of and a callback that costs you the customer.

Understanding SH Concentration

Most pros source SH in bulk at 10–12.5% concentration (sometimes called "pool grade" or "commercial SH"). Your job is diluting that down to the right application percentage for each surface type.

The dilution formula is simple: (Desired % ÷ Stock %) × Volume = SH needed. If you're running 12% stock and want a 1% mix in a 100-gallon tank, you need 8.3 gallons of SH — the rest is water (with surfactant added).

The SH Dilution Chart by Surface

These are the working percentages — what actually hits the surface after dilution. Adjust based on stain severity, temperature, and dwell time.

Surface	SH % at Surface	Stock @ 12%	Notes
House Wash (vinyl/fiber cement)	1.0 – 1.5%	~8–12 oz per gallon	Low and slow — no pressure needed
Roof Wash (asphalt shingles)	3.0 – 4.0%	~28–36 oz per gallon	Eliminate algae at the root, let dwell 15–20 min
Concrete / Flatwork	2.0 – 3.0%	~18–28 oz per gallon	Pre-wet, apply mix, then pressure wash
Wood (decks, fences)	0.5 – 1.0%	~4–8 oz per gallon	Never exceed 1% — bleaches wood fibers
Stucco / Painted surfaces	0.75 – 1.25%	~6–10 oz per gallon	Test patch first — older paint is porous
Brick / Masonry	1.5 – 2.0%	~13–18 oz per gallon	Wet surrounding plants, rinse thoroughly

House Wash Ratios (1–1.5% SH)

House washing is almost always a soft wash application — low pressure, chemistry does the work. Your goal is to eliminate organic growth (mold, mildew, algae, lichen) and rinse clean without pushing water behind siding or into soffits.

At 1–1.5% SH, you're effective against most organic stains on vinyl, fiber cement, and Hardie board without risk of streaking or over-oxidizing the surface. Push toward 1.5% for heavily neglected homes or north-facing walls with heavy black streaks.

Surfactant tip: Always add a surfactant (Elemonator, Simple Cherry, or similar) at 1–2 oz per gallon. It keeps the mix on the surface longer, penetrates biofilm, and reduces the rinse pressure needed.

Roof Wash Ratios (3–4% SH)

Roof washing requires the highest SH concentration in your arsenal because you're dealing with Gloeocapsa magma — the black algae that roots into asphalt shingles and returns within months if you don't treat it at the spore level.

Apply at 3–4% and let it dwell 15–20 minutes before rinsing. On a badly stained roof you may need two applications. Never use pressure on asphalt shingles — you'll void warranties and knock off granules. Soft wash only.

Concrete & Flatwork Ratios (2–3% SH)

Driveways, patios, pool decks, and sidewalks are typically hot water + pressure wash applications, but a pre-treatment with 2–3% SH helps break down oil stains, algae, and dark oxidation before the surface cleaner runs.

For routine maintenance washes, 1–2% pre-treatment is sufficient. For neglected driveways with heavy organic growth, push to 3% and let it dwell 10 minutes. Follow with your surface cleaner at appropriate PSI for the concrete type (2000–3000 PSI for standard residential, lower for stamped or decorative).

Wood Surface Ratios (0.5–1% SH)

Wood is the most forgiving surface to damage and the hardest to fix. Keep SH under 1% — exceeding this bleaches the wood fibers, raises the grain, and leaves you with a gray, weathered look that's difficult to stain over correctly.

For deck washing, the sequence matters: soft wash to treat organic growth, light pressure (under 1500 PSI) to clean, let dry fully, then sand and stain. Rushing the dry time is the #1 cause of stain failure on deck jobs.

Quick Dilution Calculator Tips

If you're mixing in a batch tank (not a proportioner), here's how to calculate volumes quickly:

1. Know your stock concentration — measure with a chlorine test kit or order from a supplier who labels it consistently
2. Formula: (Target % ÷ Stock %) × Tank Volume = SH gallons needed
3. Example: 100-gallon tank, 12% stock, want 1.5% → (1.5 ÷ 12) × 100 = 12.5 gallons SH
4. Fill order: Add water first to half tank, add SH, add surfactant, top off with water
5. Test: Use a chlorine test strip after mixing — verify you're in range before applying

Common Mixing Mistakes (And How to Fix Them)

Mixing too hot: Going above the recommended percentage doesn't just risk damage — it can accelerate corrosion on your equipment seals, o-rings, and pump components. More SH is not always more effective, it's just more aggressive.

Skipping surfactant: SH alone doesn't stick. Surfactant gives it dwell time. Roof mix without surfactant is money down the drain — your mix runs off before it treats anything.

Not accounting for dilution downstream: If you're using a downstream injector, your pump is pulling SH at a much lower concentration than your batch tank. Most downstream injectors dilute 10:1. If you want 1% at the surface, you need ~10% in your chemical tank going through the injector.

Eyeballing instead of measuring: "A splash" is not a mix ratio. Invest in a chemical metering pump or use a calibrated bucket. Consistency matters when you're doing 4 jobs a day.

The Bottom Line on Mix Ratios

Get the chemistry right and the cleaning takes care of itself. Use the chart above as your starting point, adjust for severity and conditions, and always test before you spray anything irreplaceable.

The pros who build lasting businesses aren't the ones with the most pressure — they're the ones who understand their chemicals well enough to quote confidently, mix accurately, and stand behind their results.

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