Wood floors, whether on decks, porches, balconies or landings, are going to rot eventually. Rotting floors can be a safety hazard. People often discover rot by putting a foot through a floor board.
Walk the entire surface of deck and porch floors. Bounce on the floors near the midspan to check for excessive deflection. Check for rot with a screwdriver or awl, particularly at vulnerable places. By now you know that those are horizontal joints where water might get hung up (at a railing post, for example) or where end joints of wood are exposed.
Most decks have adjacent boards butted tightly end to end. This detail always occurs over a joist in the deck so that both boards will be supported. Rot is common here because the vulnerable end grain is exposed. The situation is worsened by the joist below which holds water that can be absorbed into the end grain. Some deck builders protect against rot in this areas by leaving a slight space between deck boards that are butted end to end. Some go further, using double joists under this detail so that the adjacent deck boards are supported by different joists spaced an inch or two apart. This allows water to fall past the end grain of the boards. Sealing the end grain with stain is helpful, but difficult to do once the deck is built.
Pneumatic nailers can lead to premature rot. If the nail heads are driven well below the wood surface (as is common), little cups in the deck boards are created at the nail heads. Each of these cups fill with water every time it rains, and the water has no place to drain. To make matters worse, the nail heads have usually broken several fibers and there is end grain exposed here. Decks often rot around their nails because of this. We prefer to see deck nails driven flush, not below the wood surface.
Deck boards should be installed crown up so that if they cup slightly as they dry, the cup will be on the bottom of the board, and water will run off the crowned top. Again, we want to avoid holding water against the wood.
Another cause of rot on decks and porches is carpeting. Carpeting holds water against the wood and prevents it from drying. Carpet will shorten the life of wood decking.
You have to be careful not to mistake polyvinylchloride (PVC) roof membranes for carpeting. A roof membrane will protect the wood below. Some of these single-ply membranes have a carpet-like surface. The PVC roof membrane will be flashed at the edges and will extend up walls and down over edges. It is a continuous, single-ply membrane. The PVC is a watertight skin, while carpeting allows water to flow through it.