Sink drain runs into sanitary tee for toilet. How do you prevent backflow from flush? Could I install a trap before the tee?
What kind of trap --- a " P " trap ?
There is already one under the sink and you shouldn't insert another after that one in series.
Besides ---- the sink trap is generally higher than the flood level for the toilet which is the bottom of the bowl. Any blockage preventing the toilet from draining will result in the toilet overflowing the bowl.