Bathroom floor sagging

How do you fix a sagging bathroom floor. The floor is historic so my mom does not want to rip it up. She is fine with me just reinforcing it to prevent it from getting worse. I put the images in the link.
https://imgur.com/a/qImCG7O
It seems that some of the cross beams have rotted through and broke. The main beam there looks fine. The crawlspace is encapsulated so I am not sure if jacks can be used with out damaging the plastic. When I touch the fallen cross beams bits of the overlying concrete or mortar (what ever they used) crumbles and falls down. Any advise would be greatly appreciated.

 

Not much you can do if the floor isn't getting replaced. Any jacking / correcting you try to do is going to crack the tile. It likely needs more posts and beams underneath and they require footings. That would mean cutting and patching the encapsulation after work is done, just to stabilize what's there.

Typically post and beam houses were poorly built in the first place which is why it's in the shape it's currently in... and getting worse.

 

Welcome to the forums.

You have some very serious framing issues there. It looks like the foundation may be collapsing.

 

I hate to see that beautiful floor go but you've got major problems. The white salt deposits left on much of the subfloor boards and joists indicates that a lot of water has been going into that wood over time. I would be thinking this is a good time for a major bath remodel as the floor is going to probably need to be removed down to the joists, which also may need replacing or sistering.

The most common source of trouble is where the toilet connects to the drain piping. It can leak underneath the toilet, with the water seeping down to soak the surrounding wood. Often this is not visible from above. If repaired early more minor things like bracing can be done but I think you've waited too long and are in for a major repair.