The five products that do everything
The chemical wall at a supply house is intimidating and mostly redundant. Five categories cover essentially all exterior cleaning, and knowing which one a given stain wants is most of the skill.
| Product | Kills / lifts | Typical use |
|---|---|---|
| Sodium hypochlorite | Algae, mildew, moss, lichen | House wash, roof, concrete post-treat |
| Surfactant | Nothing alone — makes the mix cling | Added to every soft wash |
| Alkaline degreaser | Oil, grease, road film, exhaust | Driveways, dumpster pads, fleet |
| Sodium percarbonate | Grey lignin, mildew on bare wood | Decks and fences |
| Acid (oxalic, or masonry blends) | Rust, efflorescence, tannin, oxidation | Brick, gutter faces, wood brightening |
Sodium hypochlorite
The workhorse. Trade stock is 12.5%; household bleach is around 5–6%. It kills the biological growth that causes most of what people call “dirt” on exteriors, and the whole skill is dilution: roughly 0.5–1.5% at the wall for siding, about 3% at the shingle for a roof, about 0.75% for a concrete post-treat.
It degrades on the shelf and degrades faster warm and in light. Buy what you will use inside a few weeks, store it cool and dark, and set your mix calculator to the strength you actually have rather than the strength on the label.
Surfactant is not optional
Hypochlorite on its own runs off a vertical surface in seconds and does almost nothing. A surfactant lowers surface tension so the mix wets the surface and clings long enough to dwell — and a good one is thick enough that you can see where you have been, which is worth as much as the chemistry.
- Add it last, after the water and the hypochlorite, or the tank foams over.
- Two to four ounces per gallon is the usual working range; follow the product.
- Do not substitute dish soap. It is not hypochlorite-stable and it strips waxes off cars parked nearby.
Combinations that are genuinely dangerous
Surface damage risk
That includes accidental mixing: the same tank used for both without a full flush, a downstream injector line carrying residue, or two products meeting in a gutter as runoff. Colour-code your tanks, dedicate hoses, and flush properly between jobs.
- Hypochlorite + ammonia produces chloramine gas. Some window and general cleaners contain ammonia.
- Hypochlorite + hydrogen peroxide neutralises both and wastes a tank. Not dangerous, just pointless.
- Acid on a dry masonry wall burns the surface. Pre-wet, always.
Read the SDS for the drum you actually bought
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
Where it goes afterwards
Everything you apply ends up somewhere. On a residential drive it is usually diluted into a lawn and nobody asks; on commercial work it is the first question and the answer has to be a real one.
EPA’s stormwater guidance is that untreated wash water carrying soaps, solvents, detergents, sediment or grease should not be discharged to storm drains, surface water or groundwater — municipal permit holders treat it as an illicit discharge. The best-practice route is on-site treatment or discharge to the sanitary sewer with the authority’s agreement. State and municipal rules differ and are the operative ones, so ask your local water authority once and know your own answer.
- Wet the planting before, during and after. Dilution is the practical defence.
- Keep chemical off anything that drains directly to a watercourse.
- On commercial sites, price containment and recovery as a line item rather than absorbing it.