I am a newbee to both your site and the plumbing I am about to try to do. I am building a new garage. By the way nice site you have and alot of help. I am wanting to install a 3/4 bath and tie into my existing sewage tank. I am just going to stub in the water, electric and sewage for now. As for the sewage, I was told I needed a 3 foot piece of 4" to stub the toilet, 2" for the shower/sink and 3 inch for the sewage line to the septic tank and 3" for the vent. I am going to put the bathroom in the south east cornor, with the layout being: Shower, toilet, and sink. Do I put the 4" for the toilet, then a 90reducer for the 3" main and then the "Y" for the shower and vent from the 3" or would the "Y" for the vent/shower go into the 4" straight piece. I hope I'm not confusing you guys. Appreciate your help guys.
A toilet can be on either a 3 or, 4" pipe. A drain can go to a larger dia. but never can go smaller. That said you cannot reduce 4" in the building to 3" outside.
Depending on your local code you may use 3" for both the toilet and the line to the tank. Check with your local building inspection authorities.
Each trap needs to be vented separately - they can be tied together somewhere at or above 42" from the floor. Your toilet can be 3 or 4", but you don't reduce it - choose one and keep it that diameter. You can go from smaller to bigger but not the other way around. For the toilet, if you use 3", you must jacket the part coming through the slab to allow for later installation of the flange on the outside of the pipe. If you use 4", you can use an inside mount flange, and then can just cut the pipe off when the finished floor is in. It is important to keep the shower and toilet drain lines vertical...you can't change it after the slab is poured without major pains. You may want to consider recessing the slab where the shower is going to go - that will allow you to put in the preslope, liner, and setting bed and not have it much higher than the floor outside of the shower. Just because it is on concrete, doesn't mean you can skip the liner and preslope. Check out www.johnbridge.com to get some help and ideas on setting this up.