Hi, I have a half bathroom (a bathroom with a toilet and a sink only) down in my family room.
I would like to install a prefabricated shower by breaking the wall where the toilet paper holder is now, see pictures, (there is a large enough closet behind it).
The problem is that there is no drainage there of course and the floor is concrete.
I would like to avoid having to break the concrete.
Here is my idea, please tell me where I go wrong:
I would build an 8" platform under which I would run a 2" drainage pipe
This pipe would run over the existing tiles, along the wall behind the toilet and go under the sink (I would in case the pipe in plywood and tile it for beauty).
The question is:
1. how can I connect a 2" shower drainage pipe to a 1 1/2" sink drainage pipe in the wall, I imagine there are reducers available but is it OK to do so?
2. would it be necessary to put a trap somewhere and is so where?
3. do you think that when I wash my hands in the sink, the water would end up in the shower base?
4 If my idea is ridiculuos, what would you guys suggest?
You seem very knowledgeable on the subject, I thank you in advance.
Dom

1. how can I connect a 2" shower drainage pipe to a 1 1/2" sink drainage pipe in the wall, If this is the lavatory raiser in the wall you will cut in a 1 1/2" sanitary tee in thewall and install a 2 X 1 1/2" reducing coupling on the shower drain. A short piece of1 1/2" will complete the hook up.

2. would it be necessary to put a trap somewhere and is so where?
Absolutely! That's why you're platforming the shower base. To accommodate the trap you're going to instrall directly under the shower drain.

3. do you think that when I wash my hands in the sink, the water would end up in the shower base? Doubtful, unless the lavatory drain clogs.

If my idea is ridiculuos, what would you guys suggest?
I think we all would agree it would be best to open the slab and install it in the usual way.
But your way would work also. Good luck, Tom

In addition to Tom's advice...

If one of the new shower walls faces exterior wall, you could run new pipe to the outside, down the wall and connect with a drain outside, in the ground...