How to Pressure-Wash a Wood Deck

pressure water cleaning a deck
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Deck-Washing Basics

Pressure Washer Cleaning a Weathered Deck
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When it's time to clean your wood deck in preparation for refinishing, a chemical deck brightener applied by hand-scrubbing may not do the job of deep-cleaning, but a power washer will. Often a pressure-washer can effectively clean a deck with just plain water. But a pressure washer can ruin your deck if it's not used properly. This is a tool that is powerful enough to etch brick, but in the right hands it can also be gentle enough to wash siding. If you use it with too much jet pressure or place the tip too close to the surface of your deck, you can deeply etch the wood and cause the fibers to splinter.

There are three basic steps to doing it right:  

  • Pressure and Tip Selection
  • Power Washing 
  • Sanding After Washing

Tools and Supplies You Will Need

  • Pressure washer with a fan tip or rotating tip
  • Deck stripper/brightener (optional)
  • Sheets of plastic (optional)
  • Random orbit sander
  • Sandpaper (60- or 80-grit, 100-grit) 

Note: A pressure washer has many uses around the home, but if you use the tool only rarely, an option is to lease it from a home improvement center or tool rental outlet. It costs about $40 for four hours of use. 

Pressure and Tip Selection

Power Washer on a wet wooden deck
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Choose the appropriate pressure setting and spray tip for your application:

  • Use the lowest possible pressure that is still effective. Pressure for soft wood (like cedar or pine) should be at about 500 psi to 600 pounds per square inch (psi); harder woods may tolerate more pressure, but you should not go higher than 1,200 psi.
  • Use a fan tip with a 40- to 60-degree spread. A rotating tip is also suitable if you use it carefully. 
  • Test your pressure and its effectiveness in an inconspicuous area, such as a stair tread. If you find it necessary to replace a piece of wood because you etched it, a stair tread is easier to replace than a surface deck board.

Power Washing

Close up of a wooden deck
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Follow a few tips and safety rules for proper power washing technique:

  1. Start with the pressure at 500 to 600 psi and test it on an inconspicuous area. Increase the pressure incrementally as needed to clean the deck without damaging or etching the wood surface.
  2. Engage the trigger a couple of feet away from the deck surface and then lower the wand to about 12 inches above the surface.
  3. Clean the deck with a sweeping motion of the wand. As you sweep, avoid the tendency to pivot with your arm, which will result in ​an inconsistent distance between the spray tip and the deck surface. Try to maintain a consistent distance by moving your arm laterally back and forth.
  4. Clean from the house outward.
  5. Work with the grain by feathering the spray lengthwise, parallel to the deck boards, and overlapping each area slightly. The goal is to achieve even cleaning with no "hot spots" or visible "cleaning edges."

Using Chemical Stripper/Brightener (Optional)

Pressure-washing with plain water often cleans a deck sufficiently, but if the wood is badly darkened or stained with mildew, washing with a deck-cleaning solution that included sodium hydroxide can help brighten it.

  • Look for a product designed for brightening deck wood and for use in a pressure washer. 
  • Make sure to use a pressure washer that allows for the intake of cleaning solutions. Mix up the solution as recommended by the manufacturer.
  • The chemicals in deck strippers/brighteners are toxic to plants, so make sure to shield shrubs and garden plants around the deck with sheets of plastic before using stripper with a pressure sprayer. 
  • Apply the stripper solution in the same manner as when pressure-washing with plain water (see above), then rinse by pressure-washing again with plain water.
  • Stripper/brightener products tend to roughen up the wood fibers, so you will generally need to sand the deck completely before re-staining or sealing the wood. 

Sanding After Washing

sanding a deck
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Ideally, power washing a wood deck will not ruin the wood and etch or erode the softwood fibers. However, when wood gets wet, the fibers may become raised, creating a rough surface. This is a particular problem on handrails, where the raised fibers can lead to splinters. *** Make sure the wood is dry before sanding.***

You have a couple of options for remedying this problem. If you did a great job of power washing with minimal damage and you just want to clean the deck, you may be able to get away with a spot sanding here and there. But if you are going to refinish (re-stain or reseal) the deck, you will need to sand the entire deck to create smooth boards with little to no splintering. Sanding also opens up the wood's pores so the wood soaks up the sealer or stain consistently.

When sanding, do not use very fine sandpaper, since this can clog the pores of the wood and prevent the stain/sealer from readily soaking in.  For the handrail, use no finer than 100-grit sandpaper. For the deck surface itself, use a 60- to 80-grit sandpaper. The best sander to use for a wood deck is a random orbital sander with a 5-inch sanding pad.