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.
Checklist
Time
A couple of hours for most repairs
Tools
Drill, hammer, circular saw, reciprocating saw, flat pry bar, cat's paw
Skills
Fastening with screws or nails while working in awkward positions
Prep
Inspect the entire deck to make sure the repairs will fix all the problems
Materials
Joist material, 2x4 for braces, screws, nails, carriage bolts with nuts and washers