reinforcing a subfloor

[Nestor_Kelebay]

If you use a big steel plate to join two joists togther in the diagram, I'd expect much of the increase in strength is going to come from the steel, not the added wood.

What's critical is to ensure that you don't have any slippage at the joint between the old wood and the new wood, so I'd opt for a super strong glue rather than mechanical fasteners. I'd just use mechanical fasteners to hold the additional wood or steel to the joist while the clue cures.

And, of course, team up with a consulting engineer or architectural firm to ensure they put their stamp of approval on everything. Your insurance company might not want to cover you if they believe you've done something that affects the structural integrity of your house that wasn't approved by someone officially knowledgable about making such changes.

 

[Infinteriors]

Speaking as a 22 year contractor and having "Actual" experience in framing and tile, if you have the ability to add a joist in between then do it. 16 oc would be ideal so if you can easily add a joist you would have an even stronger floor. If not, then just make sure when you add extra ply that you glue in and nail every 12 inches. Don't use concrete board use "Ditra" instead, the extra weight of the concrete board will just add more flex to the floor. it will also keep floor level lower. "Ditra board is an uncoupler as well that is resistant to cracking. Also although Ditra costs a bit more than concrete board its 10 times easier to install and adds no weight to the floor. Oh and check out the band "Sevendust" Good remodeling music.