I built my house a little over a year ago, and did everything myself to save money, however I am not a professional plumber. I am located in the country, so I did not pull any permits. My plumbing has worked fine up until about a month ago. My drain starts at the kitchen sink, and runs 20' in 2" PVC where it ties into the tub, from there it is 4" PVC 2' to the toilet, 6' to lavatory which has the only roof vent, another 6' to the washer, and into 1 500 gal septic tank, with 200' field line. The problem is when the tub, or washer is draining the toilet has began bubbling, logic tells me that my roof vent is clogged, so I get on the roof and blow through it, the toilet bubbles. To check it I disconnect the trap under the lavatory and blow again, air flows freely. Confused I disconnect my kitchen trap as well, and begin draining a full tub, the toilet still bubbles. How can it still bubble with free flowing vents up and down stream?