Some older decks will begin to show signs of dry rot. The symptoms include spongy and discolored wood that may flake off and fall apart when wet. You can use a screwdriver to test the wood for soft spots. In certain conditions, even pressure-treated wood can rot and decay. Moisture and pooling water on decks can lead to rot and decay. Because dry rot is a fungus, it can spread like a cancer throughout a deck. Dry rot is caused by microorganisms that eat the cellulose from the wood leaving it brittle. Dry rot will also attract wood-eating insects such as termites, which will compound the problem.
Dry rot is a serious problem that can compromise the structural integrity of your deck. The most common and most serious rot issues involve an incorrectly installed ledger board that allows moisture to penetrate the house wall. Because the house framing is not treated, it is much more likely to rot. If your ledger board is infected with rot, you will likely need to replace the entire deck. You may also need to repair portions of the house wall. If a few joists or some decking is rotting, you may be able to remove and replace only the affected pieces.