Pressure washing decks, fences and interlock without damage

Pressure washing will clean almost anything outside your house, and it will damage most of it too if you run the machine flat out. The trouble is nearly always the settings: wood, pavers and painted boards each want a different pressure, tip and distance.

A few minutes of testing on a hidden corner saves you from marks you can’t undo.

This guide covers wooden decks, wooden fences and interlock, the surfaces our pressure washing service handles around Markham and nearby. The step-by-step for a deck is further down the page.

What PSI should you use on a deck, fence or pavers?

About half what you would use on concrete pavers. Published guidance puts treated wood at 800 PSI or less and interlock at around 1,500 PSI.

This is what the manufacturers and industry groups say, surface by surface:

On tip colours, Alkota, a pressure washer manufacturer, lists the white 40° tip for sweeping large areas and rinsing, and the green 25° tip for decks and fences.

It warns that the red 0° tip and the rotary “turbo” nozzle can damage surfaces quickly. Consumer Reports goes further and advises getting rid of the 0° tip altogether.

Why does pressure washing damage wood?

Because water at high pressure cuts away the softer wood between the growth rings. The USDA Forest Products Laboratory’s Wood Handbook explains that power washing erodes the less dense earlywood more than the dense latewood, leaving ridges that are hard to paint.

That’s the rough, corduroy texture you see on a deck that was washed too hard.

The other common marks are fuzzy, raised fibres (sometimes called furring) and lines or arcs where the tip stopped or turned. The Wood Handbook treats raised grain from washing as something to sand off before refinishing, once the wood is dry.

Deep gouges and ridges usually can’t be fixed without sanding hard or replacing the board.

Cedar is a soft, low-density wood, and the Western Red Cedar Lumber Association says power washers can seriously damage cedar deck surfaces. It doesn’t recommend them for cedar decks or any other cedar product.

Its advice for yearly cleaning is warm soapy water for dirt and diluted oxygen bleach for mildew. Pressure-treated pine stands up better, but only at the low settings above.

Can washing bring back the colour of grey wood?

Not really. The Wood Handbook says weathered wood looks silvery grey because weathering removes the coloured parts of the surface, and that cleaning wood doesn’t add colour back.

Removing mildew reveals the original colour underneath. Some cleaners pulp the grey surface so the washer can strip it, which looks like restored colour but is really a thin layer of wood gone.

If you want the warm colour back, the answer is a stain after cleaning. Our deck staining guide covers when to stain and which kind, and fence painting vs staining helps if the fence is next.

When is the best time to pressure wash in Ontario?

Late spring through early fall, on a dry, mild day with more dry days to follow. The washing itself only needs weather above freezing, but what comes after needs dry weather.

In the GTA that rules out winter, when rinse water freezes on steps and paths, and makes late fall a gamble.

Spring is the natural time. The Western Red Cedar Lumber Association suggests cleaning a deck once a year, in the spring, and a winter of wet leaves, snow and road grit leaves decks and driveways at their worst by April.

Plan around the drying time. Each surface has its own wait before the next step:

Shady decks and the north side of a fence dry slowest, so give those spots longer.

The interlock figures come from Techniseal’s data sheet for its RG+ polymeric sand. Our interlock cleaning guide goes through that part of the job.

What mistakes leave permanent marks?

Most damage comes from a few habits, and nearly all of them are about wood.

On wood

• Running a deck at the machine’s full pressure because “it cleans faster”.
• Using the red 0° or yellow 15° tip, or a turbo nozzle, on wood or coloured pavers.
• Holding the tip still, or stopping and turning mid-board, which leaves arcs and stripes.
• Spraying across the grain instead of along each board.
• Pressure washing cedar at all, when soap, oxygen bleach and a brush would do.
• Trying to blast off paint that is still well stuck. The Wood Handbook warns this can easily cause deep erosion, and if paint only comes off at high pressure it probably doesn’t need to come off.
• Staining a deck before the boards have dried through.

On interlock

Spraying straight down into interlock joints blows the sand out of them. Unilock suggests holding the wand at knee height and angling it at 45°.

If a white film appears after you re-sand, see our guide on removing polymeric sand haze.

Chemicals, old paint and your own safety

Then there’s the risk to you. Consumer Reports reported that pressure washer injuries sent an estimated 6,057 people in the US to the emergency room in 2014.

Wear eye protection, long pants and sturdy shoes, and see a doctor about any skin break from the spray, even a small one.

How much does pressure washing cost?

HomeStars’ Canadian price guide, updated August 21, 2026, puts most residential pressure washing jobs at $150 to $600 or more, or about $0.15 to $0.75 per square foot. It doesn’t give GTA-only figures.

Its ranges for the surfaces on this page:

These are published market ranges, not our prices. The wash is often only the first half of the job.

Interlock usually needs new joint sand afterwards, and a deck or fence needs a finish once it has dried, so compare quotes on the whole job. Our guides on polymeric sand and deck staining costs cover those parts.

When it makes sense to call someone

A small, sound pressure-treated deck or a short run of fence is a fair weekend job with a rented machine, as long as you test first and keep the pressure down.

It’s worth getting help in a few cases:

• The deck is cedar.
• The boards are already splintering.
• There’s old paint you’re not sure about.
• The wash is the first half of a staining or re-sanding job, and you’d rather not time the weather yourself.

Our pressure washing service covers decks, fences and interlock, and once everything is dry we can follow it with new polymeric sand, paint or a waterproof coating.

Call (437) 684-2535 or text us a photo of the deck, fence or driveway, and we’ll come and look and give you a free quote.