There are two natural elements that will age and degrade decks in a hurry. Ultraviolet damage from sunlight prematurely ages wood. Water, whether from melting snow or tropical rain, causes wood decks to rot, joints to loosen and nails to pop. Waterproofing a deck properly usually takes care of both of these problems in one step, and there are many deck waterproofing options.
Deck waterproofing makes a deck last longer by preventing rot and decay caused by water penetrating the wood. Redwood and cedar decks can oxidize and turn black if not sealed from water damage. Mildew takes hold in wet conditions and the resultant stains can be difficult to remove, even with wood bleach and power washing.
There are several types of waterproofing systems. Most commonly, clear or colored stains and sealers are used on residential decks. These can be either oil or water based. Solid stains and paints are another option, but they tend to be much higher maintenance as they will invariably start peeling within two years. For commercial applications, like apartment building decks, an epoxy coating is available. Although the time and cost of epoxy deck coatings are considerably more than other deck waterproofing applications, the investment can be worth it. Sealers and stains generally have to be redone every two to four years, while epoxy coatings can last almost indefinitely.
Local conditions may influence waterproofing choices. In very wet and humid climates, more frequent applications of sealer are necessary. If indigenous waterproof woods like teak and redwood are used as deck building materials, the deck may not need as much attention as cedar or pine decking. In hot, dry climates, ultraviolet protection will be more important than waterproofing. Generally, a tinted sealer or stain provides better ultraviolet protection.
Deck waterproofing should be done on clean, dry decks. New decks should be sealed immediately. If possible, the sealer should be applied to all six sides of each board (top, bottom, edges and end grains.) Railings should be treated the same way. Older decks may need to be scrubbed or power washed to remove dirt, mildew and stains. There are many excellent deck cleaners available. Brushing the sealer into the wood does a better job of penetrating and waterproofing decks than spraying because it works the sealer deep into the wood fibers. If the decks are sprayed, it's worthwhile to back brush to even out the sprayed coating.
Using solid stain or paint to waterproof decks is often a mistake. Because they form a solid, thicker film on the wood surface, these coatings will inevitably start peeling in a year or two. And because they peel instead of fading, once a deck is coated with an opaque finish, it either needs to be redone every two years or so, or a vast amount of work expended to remove the finish and start fresh. Only use a paint or solid stain to waterproof your deck if you are absolutely sure this is what you want for the life of the deck.
Epoxy deck waterproofing systems are best left to experts. These can involve as many as six applications, from an initial layer of galvanized mesh to a final layer of masonry sealer. Unless the homeowner is prepared to spend a lot of time learning how to apply each layer, a better option is to leave this task to reputable deck waterproofing professionals.