My son bought a new (10 yr old) repo last year. Place is pretty well constructed, but the builder did not install cross bracing on the floor joists, so there is a "bounce" in one area of the living room. Our plan was to install metal cross bracing, but were told that to do so after the fact would not be effective, and we should install wooden braces. Is this accurate?

Far more likly to long a span, under sized floor joist is causing the bounce.
Cross bracing just keeps the joist from twisting.
Easyist way to fix it would be to sister another joist to what's there now.

Rare for me but I am going to disagree with Joe. Cross bracing joists (or blocking) can do a lot to stiffen up a floor. Recommended blocking is every 8' or less. Basically the blocking keeps each joist from twisting as a load is applied, making the whole floor stiffer. Any floor system with a span greater than 8" will benefit from blocking.

It is true that you cannot use the metal cross bracing straps as they are designed for the tops to be attached to the top of each joist and then covered by the subfloor. While the movement range of a floor system is very small, the forces applied are very great - so any nails or screws into the sides of the joist will give too much 'wiggle' room to be effective.

Instead, install solid blocking between each joist. Stagger them along a center line so the each one is offset by 1-1/2" to the other. You'll have to measure and cut each block individually to get a really tight fit. I think you will be amazed as to how much a run of blocking will stiffen up your floor.

You don't mention what's under the floor, but if it's crawl space or unfinished basement, you could also install a new center beam and posts. Since it's just one room with the bounce, a 4x4 beam with support every 6' will give you direct bearing to ground.

Thanks. The house has a semi-finished basement, and support posts are not preferred. We'll try blocking the joists first. Span is about 12' so we'll make a run down the centerline and see how that works out.

If yo are even going to finish that basement and put a ceiling in then do this to save yourself some trouble later: If your existing floor joists are 2x10, then make your blocking out of 2x8. Put the bottom side of the block flush with the bottom side of the joists, leaving a small gap at the top of each block. Otherwise you'll find that using the same size block will probably hang down below each joist by 1/4" or so.

Thanks for the tip.