Learn how to repair deck framing.
If you find several deck framing members that are substantially rotten, it's time to consider tearing down your deck and starting over. However, if the boards are not severely damaged or if you are certain that only a few are rotten, repairs may solve the problem. Consult with a professional deck builder if you are not sure.
Keeping it strong
If a deck feels spongy when you jump on it, it may be underbuilt, with joists or beams that are too small for their spans. If you have room to work underneath, it may be possible to shore it up by installing a new beam with posts and footings, though it can be slow, tedious work. To add a modest amount of extra strength, install a row of blocking in the middle of the joists and support it with a post or two.
A couple of hours for most repairs
Drill, hammer, circular saw, reciprocating saw, flat pry bar, cat's paw
Fastening with screws or nails while working in awkward positions
Inspect the entire deck to make sure the repairs will fix all the problems
Joist material, 2x4 for braces, screws, nails, carriage bolts with nuts and washers
A joist that is cracked and sagging can be splinted with a piece of pressure-treated lumber of the same size. Begin by placing a scrap of 2x lumber or a couple of layers of plywood on the ground beneath the damage. Cut a 2x4 long enough to be wedged beneath the joist -- it should be long enough to require a few whacks with a hammer to get it nearly upright. Cut a splint so it extends 2 feet on either side of the damage and set it in place above the support. Continue to tap the support upright until the cracked joist is even with the splint.
Fix the splint in place by drilling holes for two 1/2x8-inch galvanized carriage bolts at each end. Pound the carriage bolts in place, attach the washer and nut, and tighten. Remove the temporary support.