So I recently found out my basement bathroom is unvented. It was illegally built by the previous owner and he somehow got it past inspection. My toilet, sink and shower drain slowly and the toilet regularly backs up into my shower. I installed an Air admittance valve on the sink hoping to solve the problem to no avail. Aside from installing a vent to my roof, is there a workaround or some other solution to get my sewer line to drain properly? Thanks in advance and any feedback is appreciated.