Pressure washing looks like the most straightforward outdoor job there is. Point the wand, pull the trigger, watch the dirty surface turn clean. That impression is exactly why so much of the damage we get called out to look at was done by someone who had rented a machine for the weekend and used one setting on everything they owned.
The useful way to think about it is that a pressure washer does not clean a surface so much as remove a thin layer of it. On concrete, the layer being removed is grime, and the concrete underneath does not care. On a cedar deck, on vinyl siding, on mortar between bricks, that same layer is the surface itself. The machine cannot tell the difference. The person holding it has to.
Concrete driveways and walkways genuinely can take high pressure, and this is where the dramatic before-and-after videos come from. Even here, though, technique shows: hold the wand too close or move it too slowly and you leave visible wand marks — a permanent striped pattern etched into the surface that nothing will take back out. Consistent distance and overlapping passes at a steady speed is all that separates a clean drive from a striped one.
Wood decks are where most of the damage happens. Timber is soft, and softwoods like cedar and pine are softer still. Too much pressure raises the grain, splinters the board edges and gouges the soft growth rings out from between the hard ones, leaving a washboard texture that only sanding will fix. A deck should be cleaned at low pressure with a wide fan tip, with a cleaner doing the actual work and the water only rinsing it away. If a deck comes up looking furry when it dries, it was washed too hard.
Vinyl and aluminium siding has a different failure mode, and a worse one, because you cannot see it happening. Siding is designed to shed water running down it, not water driven upward at pressure. Aim up under the laps and you push water behind the cladding and into the wall cavity, where it has no route out. The visible result — if there is one — arrives months later as a mould smell or a stain on an interior wall. Siding gets washed from above the line you are cleaning, downward, at low pressure, with detergent doing the work.
Brick and masonry look invulnerable and mostly are, but the mortar between them is much softer than the brick itself. High pressure will erode mortar joints and blow out any that were already failing, and repointing costs far more than the wash did. Older mortar, on older buildings, is softer again.
Roofs deserve a plain statement: do not pressure wash a shingle roof. Asphalt shingles are protected by a granular surface, that surface is what stops UV destroying the shingle, and pressure strips it off. A roof that is washed at pressure may look better and will have lost years of its service life. The black streaking people are trying to remove is algae, and it is dealt with chemically at low pressure, not mechanically.
The underrated part of the whole job is detergent and dwell time. People reach for more pressure when a stain will not lift, when what the stain usually needs is the right cleaner and ten minutes to work. Organic growth — the green film on a north-facing patio, the black streaks on a roof — is biological, and it responds to chemistry far better than it responds to force. Letting a cleaner sit and then rinsing at modest pressure will consistently beat blasting a dry surface, and it will not damage anything.
Last thing, and it is the one people forget until it is a problem: where the water goes. Everything you lift off the surface leaves in the run-off, along with whatever detergent you used. Point that at a flower bed and you will find out which plants object. Point it at a doorway or a basement window well and you have moved an outdoor problem indoors. Planning the run-off before starting is not a nicety — it is part of doing the job properly.
Want the outside of the property done without any of the above happening to it? Call (647) 975-0222 for a free quote.