Pressure, gallons, and chemistry

The workhorse chemical

Sodium hypochlorite, SH, the active in household bleach, is the backbone of exterior cleaning. Laundry bleach is about 6 percent SH with additives; the trade buys 10 to 12.5 percent SH in bulk from pool supply or chemical suppliers, and cuts it to the job. What SH does is kill: the algae, mildew, and moss colonizing siding and roofs are organisms, and dead organisms rinse away at garden-hose pressure. That is the whole secret of soft washing. You are disinfecting a surface, not sandblasting it.

Mix ratios from working operators: house washing typically lands at a 1 to 3 percent finished SH mix, depending on how dirty the siding is; roof cleaning runs stronger, typically 4 to 6 percent, because roof algae is stubborn (and note the shingle manufacturers' own bulletin prescribes a milder roughly 3 percent bleach mix, the safe floor). How you get there depends on your application method. A downstream injector, the $20 entry device, sips chemical from a tank into the water stream after the pump, at a roughly 10-to-1 water-to-chemical ratio, so pulling from a tank of bulk 12 percent SH delivers about a 1.2 percent mix at the wand, right in house-wash range. Dedicated soft-wash systems use metering valves so you dial the ratio without re-batching between a house mix and a roof mix on the same afternoon.

Two additives earn their shelf space. A surfactant (sold under various trade names as a "soap" or "slow" additive) makes the mix cling to vertical siding instead of sheeting off, so the chemical works during dwell instead of running into the dirt. A neutralizer is used after roof work to stop residual chlorine attacking landscaping and metal. Operators also keep a degreaser for concrete oil spots and a rust remover for the battery-acid and fertilizer stains concrete collects; those are specialty chemistry with their own instructions, and the label directions are the operating manual.

Dwell, rinse, and the dilution discipline

Chemistry does its work during dwell time, the minutes the mix sits on the surface. House washes dwell roughly 5 to 10 minutes; you soap a wall, watch the green start to gray and lift, then rinse before the surface dries. Dwell is why sectioning matters: soap half a house at a time, dirty side first, so nothing dries on and streaks. It is also why weather rules your calendar, since direct sun dries chemical fast and cold slows the reaction to a crawl.

The dilution discipline that prevents most damage claims: start weaker than you think you need, and only strengthen after you see the result. An operator-mentor pair in one training course put it bluntly: there is no undoing a mix that was too strong, and the damage often happens underneath where you cannot see it yet. Stronger means faster, and faster means etched concrete, bleached wood fibers, dead plants, and stripped paint. The professional sequence every job: pre-rinse the landscaping thoroughly with clean water before you spray a single ounce of chemical, keep misting plants during the work, and rinse everything again at the end. Dilution is plant protection. Saturated soil and leaves shed dilute SH; dry roots drink whatever lands on them.

What never gets pressure

The surface rules, learned by every operator the easy way or the expensive way: vinyl, stucco, and painted siding get soft wash only, because pressure forces water behind siding and into walls. Wood gets chemical and low pressure in single, smooth passes; too much pressure splinters the grain, and too strong a mix bleaches the wood white. Asphalt shingle roofs get chemical at low pressure and nothing else; the manufacturers' association says it in one line, that pressure washing shingles causes granule loss and likely premature roof failure (ARMA). Concrete tolerates the most pressure of anything you will touch, and even it etches under a zero-degree nozzle held too close.

That surfaces-for-chemistry map is the craft. Every job from here runs on it.