This is a good question. Most commercial power washers are designed to deliver very high pressure, sometimes exceeding over 4000 PSI. You should never need this much pressure when cleaning wood. In most cases, 1000 PSI to 1800 PSI will be sufficient. If you use too much pressure, you will damage the wood usually beyond repair. Before power washing a deck, an experienced contractor will in all cases use an environmentally safe deck cleaner. This is a must when cleaning a deck. Power washing wood is not as easy as some home owners think. This process is best left to a professional.