Upstairs shower drain slow

[Mike McKinney]

After a remodel of upstair I added a corner shower to the 1/2 bath after increasing bathroom size x3. I put a tee in the sink drain pipe to drain the shower. It has never flowed properly. No issues with sink. What could cause this. Thinking of replacing shower stall (currently have walls tiled but tiles keep cracking- too close to road and heavy equipment shakes the whole house). I don't want to put in a 1500 dollar stand alone shower and have same drain issues. Will raising the base up help? Someone mentioned vent but since goes into septic line don't want smell.

 

[Terry]

The vent on a shower drain prevents siphoning, which is the cause of smells from the septic. Vents are a good thing, and required.
If the lav had been run in 2" pipe, that would have served as a wet vent for the shower. If you pulled that off of a 1-1/2" line, you should add a vent for the shower.