Most people assume pressure washing a deck is simple and harmless. They simply go to their local hardware rental shop, pickup a power or pressure washer and go to it. Most people do not realize that a pressure washer can seriously damage and even ruin a deck and other wood structures. There is several things to take into consideration when pressure washing such as; pressure, fan width, and chemicals.
It's important to know that pressure washing isn't just about powering off dirt and grime with high pressure water. The process is actually a combination of several things, heat, water pressure, and the detergents or cleaning agents. If any one of the three are missing, the dynamics of the process is changed.
For the purposes of pressure washing a deck, let's look at the first element, pressure. For most wood and especially decks, 1500 PSI (pounds per square inch) is all that is needed to successfully clean most decks. Any higher then this and you run the risk of seriously damaging the wood. Even at 1500 PSI the possibility for damage is still there so you must be careful. Never engage the trigger while the nozzle is on or only a few inches away from the wood surface. Always engage the trigger with the nozzle facing away from the surface and move it into position slowly.
Tips come in different degrees and cause what is called the fan width of the spray coming out of the end of the nozzle. For cleaning a deck, you should use a fan width of 40' to 60' degrees. This should be sufficient in most cases. For reference, a degree of zero (0) would be a straight stream of pressurized water, which you never want to hit any wood surface with.
As far as detergents or cleaners go, check with your local big box hardware store or hardware rental shop. There are so many different types of cleaners it would be impossible to list them here. Although, one should always have a nice synthetic bristled brush for scrubbing the deck. Synthetic bristles are necessary due to the ingredients in many of the cleaning chemicals. Natural bristled brushed will break down from the chemicals and create a real mess. Always remember to read the manufacturers warnings and application directions on any detergents or deck cleaners.