If you have the odd missing or severely rotted deck board, you might be able to replace it with new lumber. But large areas of rot in multiple spots could indicate that the deck has reached the end of its life.
If you have deck boards that are curving up and pooling water in the middle, remove and reinstall them so the hump is on the upper side and will help shed water. Splinters can be sanded down, but you should re-stain the deck boards so they don’t dry out and crack further.
In most cases, a deck is attached to the home or cottage with a ledger board — a horizontal piece of lumber that’s tied into the building’s framing. Inspect from above and, if possible, from below to make sure this key framing member is securely fastened to the structure.
Inspect your entire railing for missing vertical pickets or wobbly sections. Reinforce any loose sections. Note that the gaps between pickets should be no more than 4”-wide to prevent a young child being able to squeeze through.
Inspect each support post for signs of rot, and to make sure that the hardware connecting the post to the deck and footing is secure. If the deck is on concrete footings, check to make sure that frost hasn’t shifted them on an angle and that the supporting soil hasn’t eroded away.
Inspect each stair tread, section of railing, and the stringers — the angled framing that supports the stairway — for signs of rot and loose fasteners. Tighten or replace as needed. Each step should be of a uniform height (the rise) and depth (the run) within ¼” variation in order to avoid a tripping hazard.