[Shop-talk] Soil stack venting issue

[Jim Franklin]

Maybe some of you have an innovative solution...

My house has a single cast iron soil stack that does both drain and vent. When I flush the toilet, it sucks air from the tub drain. The toilet and tub enter the stack separately, but at the same height, using an oddly specific T with a 4" toilet and a 2" tub inlet about 45 degrees apart. The tub does not have its own vent. 2 years ago I had a plumber snake the vent from the roof, he found "stuff", and it was fixed. Last week it started again.

My kitchen sink T's into the stack at knee height, so it's the highest item on the stack. My thought is, because it's a straight T, not a sanitary T, stuff from the sink is solidifying inside the T when the flow hits the back of the T, and eventually forming a cap of "stuff" just above the T entry, sealing off any venting. I can't tell if the sink is also being used as the toilet's vent because there are too many walls in the way to hear what's happening. 

My other thought is that the "stuff" is growing like a mold, and sealing it off. Does stuff grow in drains? 

Any thoughts on what it might be, and how to prevent it? I rarely put oils down the drain, and when I do they are cold and mixed with Dawn to emulsify it. There isn't enough canopy for leaves or squirrels to get in the roof vent. 

thanks,
jim