To start off, you need a base layer of wood to serve as the foundation platform for the finish layer of hardwood paneling that is installed as the actual finish deck. This first layer goes over the top of the framing skeleton. Although interior-grade plywood is used for subfloors, when it comes to the areas around a tub, you need to use exterior-grade plywood, which uses special glues and preservatives to help the wood fend off water and thus rot over the years. A basic thickness of 5/8 inch does the trick, or use a 3/4-inch layer for extra durability.
Sealing and Caulking
While exterior-grade plywood provides an extra level of moisture protection in comparison to regular plywood, you still need to seal the wood to ensure water buildup and moisture saturation is kept to a minimum. On top of that, expansion joints must be provided at all perimeters -- such as against the wall and around the tub itself -- to account for the natural swelling of wood in the presence of humidity, moisture and heat. These joints cannot be left open, as they lead to water saturation; caulk them with a silicone-based caulk. Choose between water-based or oil-based penetrating sealer.
Wood Type and Installation
When it comes to choosing the right type of wood for a tub deck, stick to hardwood. This is because hardwood is more naturally resistant to moisture, and is less porous than the softer materials. Tongue-and-groove paneling works best for the deck as opposed to planks, because the tongue-and-groove system naturally lends itself to holding together over the years, rather than individual pieces popping up and warping. Keep a healthy expansion joint (1/4 inch or larger) around the perimeter of the walls where the wainscoting covers the gap, to allow for expansion in the presence of water, or the planks will buckle and pop up from the pressure. The perimeter around the tub deck should be no greater than 1/8 inch, and caulked. Top-mounted tubs should have the wood-to-tub joint caulked.
Just as you need to protect the base layer with sealer, you must protect the finish deck against moisture saturation, or you will face costly repairs in a matter of months. Regular sealing of the material is the only solution. Penetrating sealers bond within the wood itself to keep water from soaking down, while retaining the natural look of the wood; topical sealers leave a shiny, polished coat on the surface. Refer to manufacturer guidelines for recommended renewal time, and shave a few months off its suggestion for best results.