Can vent plumbing be behind a double wye?

Question

I am trying to figure out how to lay out my basement plumbing, and I wanted to use a double wye to connect my shower, toilet, and sink to the existing waste system in the house.

My question is, am I allowed to have the vent plumbing be behind a double wye since all 3 fixtures combine at this one point? Or am I required to have each fixture individually vented and then join the vents later into one common vent?

All of this work will be under the concrete slab. I am connecting to the existing waste system that runs under the concrete slab.

The existing waste system is connected to all the fixtures on the 1st floor (toilet, tub, sink, kitchen sink) of the house and then travels down into the basement, under the slab, and out to the road. This system is vented through the ceiling.

I should probably also ask if I even need to vent this new basement bathroom separately, or if the current venting system would be sufficient.

Answer 1

Around here the answer would be no. Our plumbing inspectors won't pass drain installations where a connection to a common drain line comes before the vent. The one exception would be the toilet, which can be wet vented to another fixture. The problem is that the air admittance valve is too small to vent the toilet, and the height of the toilet trap is above the height of the shower drain.

Keep in mind that local mileage on this stuff varies wildly, so I'd run it by your local inspector. Also, don't know if this is just a mistake in the diagram, but the wye connection to the cast iron pipe is upside down.