TL;DR
Roofing, siding, stucco, wood decking, old mortar, windows, screens, air conditioner fins, and electrical fixtures should never be cleaned with high pressure. Each fails in a way that costs far more than the cleaning saved, and most of them clean up better with a detergent and a soft brush anyway. Concrete and other hard flat surfaces are the real pressure washing jobs.
Roofing: the most expensive mistake on the list
Asphalt shingles are protected by a layer of mineral granules bonded to the surface. Those granules block ultraviolet light and are the reason a roof lasts decades instead of years.
High pressure sweeps them off. You will see them collecting in the gutter and at the bottom of the downspout, and once they are gone the exposed asphalt ages very quickly. Most shingle manufacturers state plainly that pressure washing voids the warranty.
Source: shingle manufacturer care and warranty guidance
Pressure also drives water up and under the shingle courses, which are designed to shed water downward only.
Use instead: a low pressure soft wash treatment applied by a professional. Roof work also involves a fall risk that no cleaning result justifies for a homeowner, so this one belongs to a contractor with proper access equipment.
Vinyl, lap, and fiber cement siding
Siding is a shedding system, not a sealed one. Each course overlaps the one below so water runs down the face and off. It was never designed to resist water pushed upward.
A pressure stream aimed up or into a seam sends water behind the panel, onto sheathing and housewrap that dry slowly if at all. The result is rot and mildew you cannot see until it is a repair rather than a cleaning.
Fiber cement adds a second problem: most manufacturers cap cleaning pressure very low and specify a garden hose and a soft brush, so exceeding it puts the warranty at risk.
Use instead: a soft wash. Apply a detergent at low pressure from the bottom up, let it dwell, rinse from the top down with a wide fan aimed downward. Soft washing and when it applies covers the method in detail.
Stucco and synthetic stucco
Stucco is a porous cement-based finish coat, and synthetic stucco systems put a thin coating over foam insulation board.
Pressure erodes the finish, opens hairline cracks into water paths, and leaves permanently blotchy patches where the wand lingered. On the synthetic systems, a breach of the coating lets water into the assembly, which is a serious moisture problem rather than a cosmetic one.
Use instead: low pressure and a mild detergent, with a soft brush on stubborn areas. Cleaning stucco without stripping it walks through it step by step.
Wood decking, fencing, and siding
Wood grain is not uniform. Dense annual rings alternate with much softer growth between them, and pressure erodes the soft part first.
What is left is a raised, splintery, fuzzy surface called furring. It is permanent short of sanding the whole deck, it holds dirt worse than the original surface did, and it accepts stain unevenly forever after.
Painted or stained wood has a second failure mode: pressure gets under any coating that is even slightly loose and peels it in sheets.
Use instead: a deck cleaner, a stiff brush on a pole, and a rinse. What high pressure does to a wood deck explains why the wide-tip compromise still is not worth it.
Mortar joints and older masonry
Brick itself is usually hard enough to take pressure. The mortar between the bricks often is not, especially on older homes built with softer lime-based mortar.
A narrow stream held on a joint will hollow it out in seconds, and a wall with eroded joints admits water directly into the structure. Repointing costs many times what the cleaning did.
Use instead: low pressure with a wide tip, a masonry-safe detergent, and dwell time. Test one small area first, and if sand comes off the joint, stop. Cleaning brick without blowing out the mortar covers the test.
Windows, screens, and skylights
Window glass is strong but the seals and glazing are not. Pressure forces water past weatherstripping and into the frame, and on older units it can break the seal on an insulated glass unit, which shows up later as permanent fog between the panes.
Screens are worse. The mesh stretches or tears, and the spline pops out of the frame.
Thermal shock is a real risk too. Cold water hitting hot glass in direct sun can crack it.
Use instead: a bucket of mild soap, a soft brush or sleeve, and a squeegee. Screens come out of the frame and get laid flat for a gentle wash.
Electrical fixtures, meters, vents, and AC units
Anything with wiring, an opening, or thin metal fins belongs on the do-not-spray list.
Outdoor light fixtures, outlets, meter bases, and doorbell hardware all have gaskets rated for rain falling down, not for water driven sideways at pressure. Water inside an enclosure is a shock and corrosion hazard.
Air conditioner condenser coils are made of aluminum fins thin enough to fold flat under a garden hose held too close, and bent fins cut the unit efficiency.
Dryer vents, soffit vents, and attic vents are all openings by design. Spraying into them puts water inside the building.
Use instead: hand cleaning, with power cut at the breaker for anything electrical. How outdoor light fixtures should be cleaned covers the sequence.
Two safety rules that override everything
Never operate a pressure washer while standing on a ladder. The wand recoils, and there is no way to brace against it on a ladder. Anything out of reach from the ground gets an extension wand or a contractor.
Never mix a bleach-based exterior cleaner with an acid such as a rust remover, a masonry etcher, or a descaler. That combination releases chlorine gas. Finish one product, rinse thoroughly, and only then move to another.
FAQ
What should you never pressure wash?
Roofing, vinyl and fiber cement siding, stucco, wood decking and fencing, older mortar joints, windows and screens, air conditioner fins, and any electrical fixture, meter, or vent. Each of these either absorbs the force and breaks down, or admits water into a place that is not built to get wet.
Can you pressure wash a house exterior at all?
The walls of a house should be soft washed rather than pressure washed. A detergent applied at low pressure, given time to work, and rinsed with a wide fan aimed downward cleans better and does not risk driving water behind the siding, which is the failure that turns a cleaning into a repair.
What happens if you pressure wash a roof?
The mineral granules that protect the shingles from ultraviolet light are stripped away, shortening the roof life substantially, and water is driven up under the shingle courses. Most manufacturers treat pressure washing as grounds to void the warranty. Roofs are soft washed only, and by a professional.
Is it safe to pressure wash a wood fence?
Not at high pressure. The soft growth between the grain lines erodes first and leaves a raised, splintery surface that will not take stain evenly afterward. A wood cleaner, a stiff brush, and a plain rinse produce a better result and do not damage the boards.
Can I pressure wash my windows?
No. Pressure drives water past the weatherstripping into the frame and can break the seal on double-pane glass, which shows up as permanent fogging between the panes. Wash glass with mild soap and a squeegee, and take screens out to lay flat for gentle cleaning.
If this keeps landing on one person, regular house cleaning services cover the whole-home version of the job.
Matching the method to the material is most of what separates a good exterior wash from an expensive one. If you are not sure what your siding, roof, or masonry can take, get a quote and we will look at the surfaces before anything gets sprayed.