I just bought a 1956 ranch house in Portland, OR and am facing an issue. I've gutted the bathroom for a remodel and it's down to the studs. There was a bit of water damage to the subfloor around the shower/tub so I decided to replace the whole thing.
In the process, I discovered the floor joists are approximately 5' on center 4x8 which is certainly not up to modern day code. On top of that is tongue in groove 2x6 decking for the subfloor. The floor is surprisingly sturdy but many people have already suggested that it would be smart to add more joists, especially since I'll be tiling the floor.
The old joists abut against the concrete foundation, kind of like a mortise and tennon joint (not sure what it's actually called). The foundation is in good condition and doesn't have any cracks and looks dry. The joists span what looks to be the entire length of the house with 4x4 pier supports every 6' or so.
My question is what's the best way to add new joists to the space between the old ones? I guess I need to make a sill against the concrete for which I can attach the new joists. I can't see myself running new 2x8 joists across the entire length of the house, so I was thinking I could go a few feet past the length of the bathroom, using pier supports along the way with concrete pier blocks beneath, and then block off their ends between the original joists. Is that a structurally sound way to do this?
It sounds like my whole floor will need more reinforcement at some point but like I said, it all seems really sturdy for the time being. There aren't any significant sags or bounciness that I can find.
Here's a short video of a walk through the bathroom if it helps: https://www.youtube.com/watch?v=Fb59H0MT3bo (sorry for a couple f bombs! my dirty rotten mouth)