Yes, pressure washing a wood deck can be a good idea as it effectively lifts ingrained dirt, mold, and mildew, improving the deck's appearance and longevity. However, it must be done carefully, as it's easy to damage the wood, especially softwoods. To pressure wash safely, use a lower pressure setting and a wide, 40-degree fan pattern nozzle (often white). Keep the nozzle at least a foot away from the wood surface and maintain constant motion, avoiding hovering in one spot to prevent gouging.

Pressure-treated lumber is one of the most affordable types of decking upfront. This material is widely available, so you can usually find pressure-treated lumber at almost any local home improvement store you visit. Keep in mind that wood requires a lot of upkeep with repainting and restaining, so any kind of wood may be more expensive in the long run than something like PVC decking.

If you want to make a deck cleaner from products already in your home, there are various options. The most common is to use a gentle blend of 1 gallon of warm water and 2 tablespoons of ammonia-free dish soap. You can also mix water with distilled vinegar. Sprinkling on some baking soda can help to remove mold and mildew. You can add oxygen bleach (but never damaging chlorine bleach) to the water and ammonia-free dish soap solution for something a bit stronger. 

Yes, it is possible to add an under-deck drainage system to an existing deck by retrofitting it to the structure. This process may require some prep work. For example, many drainage systems are installed on top of the deck joists, which would require you to remove the decking boards and railings first. To avoid this, you could opt for a below-the-joist drainage system. For best results and to ensure proper installation, it is recommended to consult a professional. A pro can inspect your deck, make any necessary repairs, and then install the drainage system correctly.