I just installed a new sink and vanity after having a plumber do the rough in... I did my test on my sink after by filling it full then draining it. Everything was good till the tidal wave from behind me hit. About a 1/4 of the water from the sink came up the open hole that I was going to put the toilet on next. Could it be a venting or slope issue with the rough in?

One of 2 things is happening:

1. New drains are NOT connected to the existing house drain system. Ask your plumber if he connected new and old.

2. Usually, during construction, there is debris that falls into the drains. You have to snake your drain before you set your toilet.

Let us know what was the problem... Back to you... Milo