Water runs down my vent stack only when the bathroom sink above it is used and then not all the time. I checked the roof, checked in the attic and all these areas are dry. Since it does not happen all the time I thought it has to do with a series of events before hand. I tried flushing the toilet then using the sink, ran the shower and then used the sink, but I can't seem to make it happen. I discovered this because the basement is finished and the drop ceiling tile was wet. I'm I correct to say the sink drain goes directly into the vent stack? All plumbing is PVC. Walls in bath are tiled and the other side of bath is the kitchen.