This is related (but different) to some topics that I had started a couple of weeks ago. To give you all some background, my wife and I just purchased a home that was originally built in 1940. The original structure had a full basement. There is a "new" addition (built circa 1965) that has a crawl space beneath it.

We recently added a half-bathroom to the addition above the crawl space. We connected all of the waste plumbing (3-inch PVC) from this new bathroom to a sewer hub in the basement floor. This sewer hub is about 25 feet away from the main waste stack (4-inch cast iron).

After connecting everything, the plumbing seems to be working fine... except that I noticed that, if I flush the toilet twice, the floor drain backs up (this floor drain is about a foot away from the location where I connected the new bathroom plumbing).

I initially thought that I simply needed to snake out the below-ground waste line. I even checked out a local big box store about the cost of renting a power snake. The guy there thought that perhaps the problem was a lack of venting, not a clogged waste line.

Sure enough, upon inspection, the house only has a 1 1/4-inch vent out the side of the house for the main stack. Even so, everything else in the house seems to drain fine... there is no chugging while things are draining (or sucking from the bathroom sink plumbing) that would suggest insufficient venting.

Any thoughts? Any idea of anything else that I should be checking?

Thanks!
Tim