I have a toilet in the basement with proper vent. I want to add a bathtub near the toilet. After I jackhammer up the concrete and install the tub trap and drainage into the toilet drain, how do I seal the floor under the tub? The outside part is open and it should be easy to pour concrete mix and seal the slab before tiling, but the inside part, directly underneath the tub would be difficult. If it is not sealed properly, water would seep into the basement. Did I miss anything obvious? If I did not explain this problem clearly, would you give me the correct sequence of this installation job?

Thanks in advance.

speedball1

You have a larger problem then cementing under the tub. You just can't tie the tub into the toilet drain without venting it.
Every code going dictates that you can not discharge a major fixture,(your toilet) past a unvented minor one,(your tub).
If there were a lavatory/sink down there you could tie the tub into the lavatory drain line and be vented and legal. Other wise you will have to run a separate vent for the tub.

Now, to answer your original question. Before you set the tub you will build a 14" X 14" box around the trap and stub up and fill it with dirt or cover it so no cement can enter. Then cement and patch under the tub. After the cement patch cures you may set the tub. You will also need a tub access hole installed in the wall,( 14 to 16 inches square) behind the tub so you can hook the tub waste to the trap unless you think you can do it from the basement. More questions? I'm as close as a click. Good luck on your remodel, Tom

slzuser

Thanks, Tom, for bringing me another important thing to consider.

The lavatory currently exists in the basement.
I was thinking of using a "T" in the vertical part (4" main) with a 2" arm to tie the tub drain into (with a slope). The whole thing would look like this: the tub drain starts with 2" PVC (would 1 1/2" be ok?) for about 4' and sloped by 2" and tied into the toilet drain; then 1' down the 4" drain is a 2" vent pipe to the vent stack (3'-4' away). Does this sound acceptable?

As for the cementing question, it sounds like a huge job if I do it properly. But what risk would I run into if I leave the hole under the tub open without cementing it? It is a dry basement.

speedball1

Stick with 2" PVC for the tub drain. You have the tub tieing in to the 4" toilet drain upstream from the 2" vent. This still has you discharging a major fixture past a unvented minor one. You MUST tie the tub in downstream from the vent to stay in code.
Most tubs, with the exception of cast iron use the floor as support. In fact when we set a plastic or fiberglass tub we "puddle" a mound of cement under the tub and set the tub on top so it "squooses" down and forms a base after it sets up. Patch the hole. Smooth it out, box a hole for the trap and do the job right. Cheers, Tom