Pressure is a tool
A pressure washer is a pump that turns garden-hose water into a jet of 1,500 to 4,000 pounds per square inch. Concrete shrugs that off. Softwood decking doesn’t: at full pressure with a narrow tip, the water cuts into the grain and leaves permanent stripes, a mistake you can see on half the decks in any neighborhood.
Three things set how hard the water hits: the machine’s pressure, the tip, and the distance. A wide 25- or 40-degree tip spreads the force; a 0-degree tip concentrates it into a drill. Start wide and far, a foot or more from the surface, and move closer only if the dirt doesn’t lift. Keep the wand moving. A jet held still on one spot is how holes happen.
- Concrete and pavers: high pressure is fine. Pre-treat oil stains; the washer alone won’t lift them. Rinse the sand out of paver joints only if you plan to re-sand.
- Wood decks and fences: low pressure, wide tip, with the grain. A cleaner and a stiff brush do more than pressure. Expect to sand raised fibers before staining.
- Siding: low pressure, pointing down, never up under the laps, where water goes behind the wall.
- Patio furniture, grills, trash bins: the easy wins. Wide tip and a few minutes each.
What not to wash
Some surfaces come out worse every time, whatever the setting.
- Old brick and stone. The mortar in a house built before the 1950s is softer than the brick. A pressure washer digs it out of the joints, and the repointing costs more than the wash saved.
- Painted wood. The jet lifts paint along every edge and forces water into the bare wood behind it. If the paint is failing, the job is scraping and repainting, not washing.
- Asphalt shingles. The granules that protect the shingle wash straight off. Roof cleaning is a low-pressure chemical job, not a pressure job.
- Windows, doors and seals. Water driven into a window seal or under a door threshold shows up later as a leak you can’t trace.
- Anything electrical. Outdoor outlets, light fixtures, meter boxes, the air conditioner. Water goes where you point it and also where you didn’t.
The gas engine problem
Most washers big enough for a driveway run on a small gasoline engine, and a small gasoline engine makes carbon monoxide fast. The CDC’s rule is plain: never run one indoors or in an enclosed space, and never within 20 feet of a window, door or vent, because the gas drifts inside. A garage with the door open doesn’t count as outdoors.
This warning isn’t theoretical. NIOSH, the federal workplace-safety institute, documented deaths and poisonings from people running pressure washers inside buildings, including a farmer cleaning out a shed and a worker cleaning the inside of a tank. Carbon monoxide has no smell and no color, and the first symptom is often a headache that gets blamed on the noise.
For a patio, a set of furniture or a few square feet of siding, an electric washer does the job with no exhaust at all. Save the gas machine for the driveway, and keep it in the open.
Before you stain a deck
Washing a deck before staining is the one case where pressure washing is a required step, and also the one where overdoing it costs the most. The goal is to lift grey, dead wood fibers and old finish so the new stain soaks in, not to carve the boards.
Use a deck cleaner, let it sit, then rinse at low pressure with a wide tip, moving with the grain. Then wait. Stain needs dry wood, which in a humid climate means three or four days and in Denver’s air is usually two. Sprinkle water on a board: if it beads, the wood is still too wet or still has old finish on it; if it soaks in, it’s ready.
Timing it
- Spring, once the nights stay above freezing: driveways, walks and patios after the winter’s sand and ice melt.
- Early summer: decks and fences, with time for them to dry before staining in the long dry days.
- Fall, before the first freeze: siding and gutters, so water has no chance to freeze in a crack you just filled.
- Never when the temperature will drop below freezing that night. Water in concrete and wood expands as it freezes and opens cracks.
Pressure washing can leave a satisfying result quickly. Damage can happen just as fast. Use a wide tip, keep it moving and keep the engine outside.