Washing a house in spring without stripping the paint off it

Exterior washing is one of the few maintenance jobs where the wrong method causes more damage than doing nothing. I have been called out to reside walls that were fine in March and had water behind them by June, and in every case the homeowner had rented a pressure washer and done exactly what the machine appeared to invite them to do.

The professional version of this job is mostly chemistry and patience, and the machine is a rinsing tool. Once you understand that, the rest follows.

What is actually growing on the wall

The gray green film on a north facing wall is not dirt. It is biological: algae, mildew, and on shaded masonry often lichen. Blasting it off removes what is visible and leaves the root structure in the surface, which is why a pressure washed wall looks clean for one season and is gray again the next.

A detergent containing sodium hypochlorite, applied at low pressure and allowed to sit, kills it. That is the entire mechanism behind what the trade calls soft washing, and it is why a professional crew spends most of a job with a low pressure applicator and only a few minutes with anything forceful. The cleaning is done chemically. The pressure only rinses off what is already dead.

Dwell time, and why it is the whole job

Detergent needs time on the surface to work, generally several minutes, and it must not dry while it is working. That constraint sets the rules that look fussy from the outside.

Work in shade, or on the shaded side of the house, and follow the sun around rather than fighting it. Work in sections small enough that you can apply, wait, and rinse before anything dries. On a hot bright day, mist the section with plain water first so the detergent is not immediately absorbed. And rinse thoroughly, top to bottom, because detergent left in a lap joint keeps working on whatever it is touching.

Protect the things that will be damaged before you start

Twenty minutes of preparation prevents nearly all of the collateral damage on this job.

• Wet down every plant within reach, and wet them again during the work. Water dilutes what lands on them. Plastic sheeting sounds sensible and mostly traps concentrate against foliage, so wetting is better.
• Close every window and check the ones that do not latch properly. Water driven at an unlatched sash finds its way in.
• Cover exterior outlets, light fixtures and any opening that faces upward.
• Check for existing damage first: loose or cracked siding, failed caulk at trim, soft wood at the base of a column. Any of those will take on water regardless of technique, so fix them or work around them.

The rule about direction

Siding is designed to shed water running downward. Every horizontal lap, every shingle course, every course of trim assumes gravity. Spraying upward at a lap drives water behind the cladding and into the wall assembly, where it has no path out.

So rinse downward, or at worst level, and keep the wand moving. Stand back rather than close: distance widens the spray and lowers the force at the surface, which is what you want. If a spot will not come off from a reasonable distance, it needs more detergent or more dwell time, not a closer nozzle.

What should not see a pressure washer at all

Some surfaces will not survive it and the damage is not always visible on the day.

Older painted wood, particularly anything that may carry lead paint, should be washed by hand with a brush and a garden hose. Disturbing that paint creates a dust and debris problem with real regulatory weight, and it is not a homeowner project.

Soft mortar in older brick will wash out of the joints under pressure, and repointing costs many times what the cleaning saved. Stucco chips. Cedar shakes fur up and stay furred. Composite decking scars in stripes that are permanent. And a roof, of any material, should never be pressure washed at all; asphalt shingles lose granules, which is the layer that makes them last.

Where the line falls between doing it and hiring it

A single story wall, from the ground, with a garden hose and a pump sprayer of the appropriate detergent, is reasonable homeowner work and will produce a good result. Rent nothing. Buy a soft bristle brush on an extension pole for anything stubborn.

Hire the job out when the work requires a ladder, when the house is two stories or more, when the surface is old paint, soft mortar or stucco, or when a roof is involved. The market rate for a residential soft wash is low enough that the ladder time alone justifies it, and a crew arrives with the correct dilution already worked out, which is the part that is genuinely difficult to get right from a bottle label.

Ask a prospective contractor two questions: what they apply, and how they rinse. Anyone who answers in terms of pressure settings rather than detergent and dwell time is describing the version of this job that causes the damage. The right answer sounds almost boring, which is the point.