Toilet sucks air when washer drains
My house is approx 5 years old. Had basement (walkout) plumbing roughed in. Was carefully inspected for design, but somebody could have dumped concrete down a vent pipe for all I know. There are two vent pipes for the downstairs bath - one between the toilet and shower, one between toilet and sink (both run up the interior walls and tie in together in the attic). We just finished all drywall and fixtures. Everything works fine: toilet and sink drain and flush OK.
Noticed that when upstairs washer empties, which dumps a big flow down the main vertical stand pipe, the toilet sucks air and water level lowers. The toilet is upstream of the stand pipe (in other words, the "emptying order" in the basement is toilet first (in the front of the house) to main floor drain pipe, then verticcal standpipe from upstairs to floor level main drain (about 1/2 way between the front and back of the house) in basement, then out via gravity to septic field behind walkout. It is like the washer dump creates a vacuum in the main drain that affects the toilet.
From what I have read on your most helpful site, this is probably a vent problem of some sort upstream of the toilet. We have two primary vent stacks on the roof. The one servicing the toilet in question supports the kitchen, laundry room, and basement bath (only). This recent installation is the first time the roughed in basment vents would be used.
Looking from the attic, the vents "spider out" to the various areas from roof vent in question. So I don't see how we can snake it from the roof or maybe even the attic. How would you know which branch you are snaking into? What are your suggestions for diagnosing and correcting this problem, preferably without having to tear out any of the new bathroom's drywall?
Are we seeing a vent problem with the toilet or the washer? And finally, how serious of a problem is this? Does it make sense to hire someone with a "scope" snake (still not sure how you would direct it to the correct vent, though?)
My husband is a resourceful do-it-yourselfer who can tinker with this on weekends if he has some suggestions on what to do. The wife (me) just wants everything to work correctly with minimal fuss and destruction.