: Why is this necessarily true? Wouldn't it depend on the level of the sewer line in relation to the level of the basement, assuming a blockage did not force sewage from the upstairs drains into the lower drains (which could happen just as easily in a 2-story structure)?

: Anyway, I'd be more interested in thoughts on what to look out for when choosing where to place vents.

: : If you connect basement toilets or showers into an underground drain system, there is no way to avoid the risk of a backup. In fact you can almost be certain it will happen at some point.

: : : I want to redo all my basement sewer drains, and need some thoughts on vent design. Presently, the only vent for the whole house is on the main sewer line outside the house (no sewer vent in the basement).
: : : 1) I'd like to run 2 in. vents pretty close to the 4 in. toilet pipes, make a vent header and tie in all the other sink and tub drains, and run the 2 in. header outside to tie into the existing vent.
: : : 2) I'd like to add a toilet and shower in the basement without the risk of ever backing up the sewer into the basement, esp. since the city guy said there are sometimes problems with sewer flow in our neighborhood.

: : : Any thoughts on what I should look out for? I'm going to put a bunch of cleanouts in the system, incl. one a few feet from the floor of the basement. Any thoughts would be mucho appreciated. Thanks in advance!