Because the deck is made of concrete or stone, and the resistance of the stone is perfect for the power wash. The high pressure and detergent will eliminate any other material that sticks to the concrete, that make it look like dirty. When we finish it will look like the first day you got it installed. We have professional tools for pressure washing pool decks. I talk about those kind of tools that need to be installed on a truck and only a professional usually have. In conclusion any pool deck out there can be clean by Josh Pressure washing & Roof Cleaning whether its at a residential home, city facility or at a neighborhood.