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How to Pressure Wash a Deck

You have rented a pressure washer from the local rental store. The sales clerk assures you that this machine will clean your deck “no problem.” You get home and now what? The “Do It Yourselfer” that you are figures that washing the deck is a task that can be achieved rather easily. This may or may not be the case. The next step is setting up the machine. You hook up the water supply lines. (NOTE: most require a ¾ inch inline hose.) You attach the high pressure hose and the gun, then turn on the water. After pulling the engine recoil cord, the machine roars to life. You approach the deck, point the tip at the deck, you nervously squeeze the trigger, and a jet of water comes forth. The next thing you know, you're cleaning your deck. Right? Maybe, then again, maybe not. You release the trigger and inspect your work realizing that you did in fact remove the dirt but you also removed ¼” of wood, leaving behind a board that looks more like a sea shell than the nice flat board you had only moments ago. The DIY’er deck cleaner you thought you were has mutated into a DIY wood engraver. The words "no problem" echo in your mind and you’re left with a damaged deck and a bruised ego.

Of all the questions I receive about cleaning a deck, the most often asked question is “HOW?” So, this is going to begin a series of articles on how to clean your deck. Whether your deck is wood or composite, periodic cleaning is a requirement if you want your deck to retain the beauty it had on day one. Periodic cleaning is preventative maintenance. This maintenance, done correctly, will do more than add years to your deck; it can save you valuable time and money in the long run.

As mentioned in previous articles, the pressure you need to wash your deck surface is rarely more than 1,500 pounds per square inch (PSI). This is still enough pressure to do damage to soft wood, and enough pressure to leave lap marks on most composite decking. Remember: less is more when it comes to pressure washing. The pressure washer serves as a mechanism to remove embedded dirt and other contaminants. Excessive forces on the wood will remove much more than those pollutants you want removed. The operator's hand will be the only measure as to when too much pressure has been exerted. Having said that, it is the cleaner that should do most of the work in removing the dirt.

Cleaners come in all shapes, claims and colors. Some are detergents, others are bleaches, and still others are chemical solutions designed to react with certain woods. There is no one clear answer as to what cleaner you should use. Whatever cleaner you do decide on, be sure to read the manufacturer’s directions carefully, and follow them to the letter. If you ARE going to use a pressure washer, remember that the job of the pressure washer is to remove the chemicals as well as the surface dirt. A pressure washer uses less water than a conventional hose and nozzle. Removing the residue of the cleaner is far easier with a pressure washer in that the cleaner is forced off the wood, and merely diluted. Allow the cleaner to do as much of the work as possible.

A stiff bristle brush should be a part of your deck cleaning arsenal. The brush should be synthetic, not natural bristle. Many cleaning solutions will deteriorate natural bristle brushes. Also, you will find that a good synthetic bristle will give you many years of performance. The brush should be outfitted to a long handle. It should be capable of reaching all the places you need it to reach. Rarely will one brush work for all your needs, so if you need multiple brushes, by all means get them. The right tool for the job can make all the difference.

Scrub the deck. Sometimes lightly watering down the deck can assist you in applying and spreading the cleaning solutions. Many solutions should not be allowed to dry on the wood so periodic spraying/misting may be required. Depending on how dirty the deck is, you may see the results immediately. But, often the cleaning compounds require a small period of time to break down the imbedded contaminants.

Once you have scrubbed the entire deck, you are now ready for rinsing. Again, a conventional hose and nozzle may work. But, if you are going to use a pressure washer, this next section will instruct you on the correct techniques to use. NOTE: Sometimes raising wood fibers is a direct result of applying pressurized water to a wood surface. Even the most cautious technique may leave behind these raised fibers. These raised fibers can be removed rather easily. More on that later. 

Always engage the trigger away from the surface, and away from anything that it may harm, including windows and people. The water that emerges from the tip is called a ‘fan.’ The tip is rated in degrees. The degree of angle indicates the fan size. Zero (0o) is typically a stream of water. You will never want to apply a stream of pressurized water to wood. A 40o to 60o (degree) tip size is standard for cleaning a deck. Bring the fan to the surface where you want to sweep the deck.

Begin “sweeping” the deck from the house side out. Be consistent in the length when you tip it from the surface. This constancy will prevent lap marks. The goal in sweeping a deck with a pressure washer is to remove the dirt, leaving behind no traces of the pressurized water. Too little pressure, or having the fan too far from the water, will result in a less clean surface. Too high pressure, or too close to the wood, will result in a stripped area. In sweeping the surface, you need to begin and end with the same pressure.

“Feathering” is a technique that may help you mask the starts and stops of the sweep. With this technique you want to overlap the areas previously swept, making sure that the point where the nozzle is closest to the wood begins at the point where the sweep ended on the previous stroke. Always working with the grain or the length of the board, this technique requires more strokes and is slower, but it does an exceptional job. It also ensures that as much of the cleaner is removed/diluted as possible. Excess cleaner left on the deck surface can have long lasting and detrimental effects. Feathering is the most efficient method for using a pressure washer on a deck surface.

The “long sweep” is another method. Using this method, you bring the fan to surface and walk the fan along the length of the board. The tip should be at the same distance from the deck from the beginning of the stroke through the whole length of the board. This method may require several passes. This method works fine for decks that have no railings or obstacles where starting and stopping can be problematic. If you use this method with railing structures, you will leave behind a line across the surfaces where the fan stopped. These lines will be difficult to remove as more pressure will have to be applied. Thus, starting a pattern of more and more pressure and risking damaging the surface.

Corners can sometimes be a challenge for pressure washer operators. The water has no direct place to travel and often results in the face of the user. Jettison debris and chemicals can be harmful. Always wear appropriate bodily protection. When approaching a corner, engage the fan and bring it into the corner first, spraying the debris out. Try not to work yourself into a corner, always work out of a corner. In doing so, you may find that you cross grain spray for a short time. This is fine as long as the distance is greater and pressure is lower on the cross grain than with the grain. The best way to remember this is the letter “L.” Long sweep into the corner, and a short sweep out of the corner with the grain.

Once you have completed the whole deck, put away the equipment and allow the deck to dry. Decks look very different from wet to dry. Small imperfections can often go unnoticed when the deck is wet. Also, raised fiber will be virtually impossible to remove from a wet surface. If the topcoat (sealant or stain) you have selected is a one-day application to be conducted after washing before the deck dries, it is still my recommendation that you allow the surface to dry a minimum of 24 hours. Once the deck has dried, inspect your work. The finished product should leave behind no lap marks, minimal raised fibers and clean wood. The surface should be consistent. No areas left unwashed. And no areas over washed. If your deck looks like this, pat yourself on the back as you have done a fine job. The next step is finishing prep and sealing. This is my next article. For now, be proud of your work. Washing a deck is far more difficult than engraving.

Cleaning a Deck
Cleaning a Deck

Use a cleaning solution to scrub your dirty deck before removing the suds and dirt with a pressure washer.