I have a 1920s house that developed a water leak that destroyed a beam,
a joist, and subflooring that is used to support a 2" thick concrete
slab that is the original tiled bathroom floor. I contracted out for
about $1000 to have this repaired, but foundation companies in Texas
seem to be generally crooked and the check was cashed but no work has
or will be done. I went through over 20 businesses to find a small
handful that would even show up at all, and of those most didn't think
the job would make them enough money to even tackle.

Regardless, I can't easily get my money back nor have the repairs made
so I am forced to attempt it myself before the toilet falls through the
floor.

The subfloor consists of 1x6s between the joists and they are rotted
and the concrete cracked and coming down an inch, the supporting joist
is rotted, and the beam as well. How do I safely replace the beam,
sister the joist (it also has numerous cutouts for the plumbing) since
the wood floor is nailed to the original, and fill in and fix the
subfloor to support the cracked concrete?

Step by step, if you please, and I am on an extreme budget, especially
after being ripped off. No, I can't find an honest nor competent
contractor (have tried for over a month), and collecting damages in
Texas requires an act of God to succeed because of debtor law.

I have two jacks of unknown tonnage, and can buy wood as needed if I
know how to do this job. I am hoping to have one helper. I have never
done foundation work. I studied engineering in college. The crawl
space is about 18" at best.

Thanks,

-Kirk