We bought an old house that originally did not have a bathroom in it. Sometime in the 50s they added a bathroom & kitchen. There is a very small vent pipe on the outside wall of the kitchen, no vent for the bathroom. After purchasing the house we had a 2nd bathroom installed. I never really paid attention to the fact that there was no vent put in for the 2nd bath. I started renovating the kitchen. Right now I have the drain for the old kitchen sink capped off until i get the new floor & cabinets installed in the kitchen. Recently when using the shower, the toilet "gurgles". Then over the weekend my girlfriend was doing a load of laundry & when the washer drained it backed up through the shower drain. I went outside to the drain clean out that is between the house & the city sewer, well when i unscrewed the cap I had a sewer eruption... Not pretty to say the least... I snaked out the drain heading towards the city sewer & after about 10-15 minutes all of my waste water had drained to the sewer & I had less than a 1/4 inch of waste water in the bottom of the pipe & it was flowing to the sewer. Now it is 2 days later & I took a shower & the toilet started gurgling again. This is when I finally thought about there not being a vent anywhere except for outside the kitchen wall. Am I correct in thinking that my problem may be that I do not have enough venting? And when I opened up the clean out it "vented" enough to allow the waste to drain out? And if so can I add a vent? My new bathroom has the toilet at the end of the drain line, then it goes to the shower, can I cut in a Y and put a vent in between the toilet & shower? Also, say i get another sewer back up, what keeps the waste water from backing up thru the vent? Is it just a case of the shower drain or whatever is lower than the top of the vent?