Trying to build a house and decided to do my own rough in plumbing in the slab. I know the slope is 1/4" per foot but not sure how to run lines to the main plumbing drain. A 5 ft run from a toilet to the main line which runs down the middle of the house has a debt of 1 and 1/4". A 20' run from a toilet or shower would have a 5" drop. Obviously these two lines would splice into the main line at different debts. Can someone enlighten me on this? And, its a 2 and 1/2 bath house. 3" or 4" main septic line? Thanks very much.
Your stacks will usually be 3" in a residence, as will your underslab "mains". But everything is sized based on the DFU's (drainage fixture units) of each individual run of pipe. A 4" drain is not uncommon, but once again it is based on DFU's...Code driven.
3" pipe has to have 1/8" slope (min) per the IPC. 2" and under requires 1/4". Too much slope and your liquids will leave your solids behind.
At a basement bath group for instance, you'll only hit the main drain once from all the fixtures in that group...Not separately.
Don't forget the backflow requirements. BIGTIME important, and it is easy to do wrong. This usually will apply to basement bath groups but occasionally upper floor(s) as well depending on the lay of the land. There's a lot to know about backflow device placement.
I also presume you understand venting and know the requirements???
My suggestion would be to ask for assistance from the local building department. If they're nice, perhaps you could sit down with them and show them a riser diagram of the proposed DWV configuration to make sure that it will work. It would be a shame to have to do it twice and it would be a bigger shame for it not to work.