My son in law's house has a rough in for a bathtub/shower drain in the floor in a hole filled with gravel, so don't know what's down there yet, but he thinks he and I can install the tub drain. I don't know anything about this type of plumbing, and neither does he. Does anyone who has done this think we should attempt it ourselves, or should we hire a plumber. I have done other plumbing work such as faucets, sink drains, etc, but never anything like this.
Until you gently excavate the gravel you will never know what is there.
Most likely you simply have a stub of pipe to which you must add a trap. The necessary venting should have taken place during the rough in in order to get approval.
There isn't a lot of difference between adding a sink drain and a tub drain. The worst part is working below your feet. Installing the drain is a 2 person job as one has to hold the drain from inside the tub while the other tightens the nut under the tub. An inexpensive custom wrench to fit inside the drain is the only secure way to hold it. The glue up kits are far easier to use than the old all compression nut style and should last longer.
I would use one of the better made pop up tub drain kits and regular glued PVC trap.
After testing I would replace the gravel and pour about 1" of concrete on top of it. Not high quality concrete as you may have to break it out again someday.
I guess the first step before calling a plumber would be to dig out the gravel and see what you have.
If the trap and vent connection are there - and where you want them - then it should be a fairly simple plumbing job. Put the tub in place, hook up the drain, tie into the vent, and wash up. Lots if pictures can be found on the web related to the proper connections.
The other question is whether or not you guys can frame the room for the tub. You didn't say anything about that, so I'm assuming that's not a issue.
I'll tell you what I did many years ago when I had to *move* a drain in my slab floor to upgrade the shower:
I borrowed a jack hammer, broke up the slab until I had exposed the drain, the trap, the connection to the main sewer pipe (which was right next to the shower), etc. All cast iron.
At the time, I had no idea what to do next, so I asked some friends for the name of an honest plumber. The guy came over on the way home from work, looked at the existing setup and told me what I had to do to install the new shower base. Well worth the $25 and cup of coffee.
An industrial size tube or two of silicone rubber will fill any voids that you are causing if the drain pipes don't end up matching exactly. But, as the gravel may be hiding almost anything, start digging before you do very much planning.
An expendable 5$ thrift store shop vac is great for cleaning out mystery pipes in the floor like that. If the pipes are iron, most rental places have cheap half-day rent on metal detectors good enough to see through the rebar and 6" fabric in the slab, to figure out where the heck the pipe goes, or if there even is one.
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