[firewalker]

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?

[rsimplicio]

First off, congratulations on building your own house! That must be one heck of a sense of accomplishment. I'm envious!

If I'm reading this correctly:

1. Kitchen sink to tub is 2", bath to toilet is 4" (how big is the tub trap & drain?), toilet to lav is 4" (I'm assuming the lav trap, drain, and vent are less than 4", probably 2"), 4" to laundry (is this a standpipe or sink?), and 4" out to the septic tank.
2. What size is the vent pipe out of the roof?
3. I have a feeling the problem may be in that there is only 2' between the tub and toilet, and that you have it arranged tub-->toilet-->lav instead of tub-->lav-->toilet.
4. Is there a reason you didn't vent the Kitchen sink and laundry separately from the bath? I'm just curious, not being critical. Every code I have read allows sharing the vent in the bath group. I'm also curious how you managed to build a house and get an occupancy certificate without any permits or inspections, but again, this is pure curiosity.
5. Did anything happen within the last two months, e.g. change in plumbing, other issue with drain (slow, clog, septic tank issue, etc.)?