Peter Langevin wrote:
I live in a 1 1/2 story home, one bathroom on the first floor. I had
a remodeling contractor out last week to give me an estimate to add a
second bathroom to the upstairs bedroom. His quote came back
significantly ($5000) higher than the other 2 I got, and when I asked
him what the difference was he said it was due to a "wet venting
issue." I mentioned that 2 other contractors hadn't mentioned it,
and he essentially told me that they didn't know what they were doing.
SNIP

The last guy is 100% correct.

Here is why:
When you flush the upstairs toilet, the slug of water rushing down
the stack will create a vacuum as it passes the lower toilet and
will try (might succeed) to siphon the water out of the bowl.

In addition, the sinks/tubs on the 1st floor have vent connections
into the stack using TEE fittings. That slug of water from the 2nd
floor will easily branch off into those vent lines, disrupting trap
seals on the sinks.

In h-rise bldgs, there is not one single stack. There will be
at least one soil stack and a separate vent stack. All waste
will drop into the soil stack and all traps will be connected
to the vent stack.

Go to the BigBox store and get a basic plumbing book or go the
local library. Good books will have pics showing how it is
correctly done.

Or, call the town inspector and ask. After all, the 2 guys with
the low bids *will* be getting permits. Right??

It's a big job in a residence to add another stack thru existing walls.

Jim