The joists supporting the main floor of the house were probably undersized for the room span, so the best thing is to double up, or sister, the joists for better stability.
The result is great - now our floors no longer bounce in the middle.
Unfortunately though, the plumbing isn’t holding up to the movement and is now leaking in no fewer than 4 places.
This little project has now spawned Scope Creep into Plumbing.
To further improve our House Stability experience, Carpenter Jim has also faced the interior Pony Window Wall with plywood (after first running electrical wiring behind it and insulating).