Just had a bathroom renovation with new sink and toilet installed. When water goes down sink drain, the toilet and shower drain back up. When toilet is flushed water backs up and takes a while to go down. What could the cause(s) be?
You do not say if the plumbing was already installed and you only installed
the sink and the toilet. Let us assume the plumbing was there.
The cause is that right after the sink or the toilet or the shower, there is
a stoppage and it needs to be cleaned. If other places are plugged-up too, then the main line is plugged-up. The other reason is the vent pipes
going to the roof.May be they are plugged-up too.