Water is a deck's greatest enemy, finding and penetrating any weak spots or openings in the wood, such as exposed edges, nail or screw holes, splits, knots, and cracks. If the wood isn't protected by a waterproofing sealer, it's prone to rot. Though cedar, redwood, and pressure-treated pine resist rot better than other woods, all decks eventually show some water damage. If water seems to soak into your deck after a rainstorm instead of beading, it probably needs to be sealed. Most decks need to be resealed every year or two.
Even if the wood is sound, decks in damp climates or heavy shade can take on a mossy patina. Dirt, leaves, needles, and other material accumulate on the deck, allowing slippery mold and mildew to form. You can usually remove this layer of mold by scrubbing with a deck cleaner or mild bleach solution. To prevent further buildup, sweep the deck often and use a crevice tool to remove debris from between boards. You can use a garden hose with sprayer to rinse off dirt, but be careful with power washers -- they can damage wood.
Baking sun not only fades wood, but also dries it out, making it susceptible to water retention and rot. If your deck is dry and faded, a deck-brightening solution can help rescue the wood's natural color. Once your deck regains some color, protect it from further damage by applying a penetrating deck finish containing UV inhibitors, compounds that disrupt the chemical reactions cause by ultraviolet light.
Freeze-thaw cycles in winter and extreme heat in summer put stress on wood, causing boards to expand, contract, and shift. The most common signs of this movement are popped-up nails. If pounding nails down doesn't keep them from popping back up, try replacing them with new, longer nails or deck screws. Always choose corrosion-resistant fasteners for deck boards.