What it is, at what strength
Sodium hypochlorite is the active ingredient in bleach, and it is what actually cleans most exterior surfaces. It oxidises the cell walls of algae, mildew, moss and lichen and kills them; what looks like dirt washing away afterwards is dead organism.
| Source | Typical strength | Notes |
|---|---|---|
| Household bleach | 5–6% | Fine for spot work; uneconomic in volume |
| Trade stock | 12.5% | What this site’s calculators assume |
| Pool chlorine, liquid | 10–12.5% | Same chemical, check the label strength |
Every dilution figure on this site is expressed as percentage at the surface, not as a ratio, because a ratio is meaningless without knowing what you started from.
It gets weaker while you own it
Hypochlorite decomposes continuously, and three things accelerate it: heat, light, and metal contamination. A drum bought at 12.5% and left in a hot van through July can be several points weaker by the time it is half empty. Nothing about the label changes.
- Buy small and often. Two weeks of stock beats a season of it.
- Store cool and dark. Inside, off concrete, out of the sun.
- Test if you buy in volume. Strips are cheap and tell you the truth.
- Set your calculator to reality. A mix computed from 12.5% when you are holding 9% is a wasted trip.
A soft wash that has stopped working is at least as likely to be aged stock as a broken injector. Check the chemical before you buy parts.
Dilution by surface
| Surface | At the surface | From 12.5% | Dwell |
|---|---|---|---|
| Vinyl or fibre-cement siding | 0.5–1.5% | ~1:8 to 1:12 | 5–10 min |
| Asphalt shingle roof | ~3% | ~1:3 | 15–20 min (ARMA) |
| Concrete post-treat | ~0.75% | ~1:16 | Leave on |
| Painted trim and wood | Avoid, or below 0.5% | ~1:24+ | Short, watched |
Add a surfactant to anything vertical. Without it the mix runs to the ground before it has dwelled and you will conclude, wrongly, that the strength was too low.
Where the trade disagrees
Read three trade sources on house-wash strength and you will find two answers. Some quote 0.5–1.5% at the wall for vinyl and fibre cement; others describe a “standard house wash mix” of one part 12.5% stock to three or four parts water, which lands near 3–4%.
This site publishes 0.5–1.5% for siding and says so plainly. The reasoning is that the higher figure is a roof strength applied to a wall: it works, and it also takes sheen off painted trim, stresses planting that pre-wetting will not fully protect, and leaves a smell on the property for days. If a wall genuinely will not clean at 1.5%, the answer is a longer dwell and a second application rather than a stronger tank.
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
The safety rules
Surface damage risk
- Eye protection every time. Not sunglasses. A splash in an eye is an emergency.
- Chemical-resistant gloves and long sleeves. It bleaches clothing on contact and irritates skin quickly.
- Add bleach to water, never water to bleach. The other order splashes concentrate.
- Ventilation. Never mix in a closed van or garage.
- Clean water on the truck. For eyes, for skin, and for the customer’s plants.
- Read the SDS for the drum you bought. Strengths and first-aid measures vary by supplier.
Plants and pets