Hello, I am installing a first floor bathroom and I need some help with connecting to the main drain. I am on a concrete slab and I already dug out the concrete and dirt. The opening is 2 feet wide by 6 feet long. The existing sewer line is 4 inch cast iron and it is 14 inches below the floor. The sewer line runs through the long end of my trench, that is, if you are looking down at the floor, you would see the exposed 6 feet of horizontal run sewer line heading to the street. The toilet will be a 12 inch rough in. The opening on the bottom of the toilet is 14 inches directly over and dead center of the underneath sewer pipe. Given that my trench is 2 by 6, what kind of run and fittings should I use to make the connection for the toilet and sink? Also where should the placement of vents go? This bathroom is on a sun porch with 9 foot high ceilings, so I have ample room to tie the vents together and run it out the roof. It would be easier than trying to tie into the main stack that goes out the second floor roof. Last item, I would like to have the toilet closer to the street than the sink. Not sure if that matters, but someone also mentioned something about a backflow preventer as well? I would appreciate any and all answers from anyone who is familiar with doing this type of work. Hopefully I described everything well enough. Sorry to not have pictures but any help is appreciated. Thanks.

Kind of close huh? OK! Here''s what I would do, Purchase a 4" PVC street combination wye and eight bend, two 4" shielded metal couplings a styliform closet wrapper and enough 4" PVC to get you above the slab plus 5 more inches. You will also need a PVC test cap to contain the sewer gas. Prime and glue the 5" piece on the hub end of the street combo. Measure end to end and add 3/8 ths. Of a inch. That will be your cut in the sewer line using a set of cast iron cutters.
Install the combo looking up and Prime and glue the other piece of PVC In the combo so it's above the slab. Wrap the closet wrapper around the stub up so you can prime and glue in a closet flange, Now patch the trench and stub up,
More questions? I'm as close as a click, Tom

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Thanks Speedball! Just wanted to ask you, can I tie the sink drain into that assembly somehow, or do I need to make another cut in the main drain?

The sink drain will connect to the toilet drain. The tub/shower drain will connect into the sink drain where it will be wet vented, Let me show you a typical drainage rough in. Good luck. Tom

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Thanks again Speedball! I spent about 4 hours making all of the connections. I tested and retested and thankfully no leaks and no nasty odors. Hopefully in a few days this bathroom will be kicking!!

Way to go! Thanks for the update. Let me know if we can be of any more help. Good luck, Tom