The wood deteriorates and rots. You can replace or you can repair, if the damage is not too great. If the underside of the deck planks show deterioration up to a third of the plank thickness or less, and the joist the same degree of deterioration, restoration is possible. Sistering new wood next to the damaged joist is advisable. If you have questions or concerns, we strongly urge you to consult with a registered architect or professional engineer, and the repairs done in full conformance with building codes and existing regulations.
Restoration begins with the disassembly of the top of the deck.
Screws are removed, or if nailed the plank can usually be broken loose from below with a large hammer or maul. Sometimes a few hits from below to loosen and then a sharp strike on top will make the nail stand proud for pulling. Use a pencil to number the planks so that they can be replaced in the same order. Place the planks upside down and using a wire brush lightly scrub the area where plank and joist met to remove any loose, deteriorated wood. If this isn't done before the epoxy cures, there will be visible patches of epoxy on top.