I've put a 2-story addition on my house, working on the plumbing now. There are 2 full baths in the addition, one on top of the other. To make things complicated, the upstairs toilet & sink are above the downstairs shower, and the upstairs shower are above the downstairs toilet & sink.

As the distance from each of these groupings (upstairs toilet/sink, downstairs shower) / (downstairs toilet/sink, upstairs shower) is more than 6 feet apart, and to minimize cutting into the framing, I was considering running 2 stacks, one for each of these groups, then tying them back together in the attic for the final vent out the roof.

My questions are as follows:
1. Can I have a toilet & sink drain above a shower drain on the same 3" stack, effectively wet-venting the downstairs shower, or do I need a separate dry vent for the downstairs shower?

2. On the other side of the room, where the upstairs shower is above the downstairs toilet & sink, can the downstairs toilet/sink be wet vented into the stack that drains the shower above?

The inspector told me that 2" venting is enough, but I'm getting confused on that whole "not to diminish in size" part because the main sewer line is 4", I want to run 3" for the toilets, then I'll be going down to 2" for the final venting. Is this allowed?

I've put a 2-story addition on my house, working on the plumbing now. There are 2 full baths in the addition, one on top of the other. To make things complicated, the upstairs toilet & sink are above the downstairs shower, and the upstairs shower are above the downstairs toilet & sink.

As the distance from each of these groupings (upstairs toilet/sink, downstairs shower) / (downstairs toilet/sink, upstairs shower) is more than 6 feet apart, and to minimize cutting into the framing, I was considering running 2 stacks, one for each of these groups, then tying them back together in the attic for the final vent out the roof.

My questions are as follows:
1. Can I have a toilet & sink drain above a shower drain on the same 3" stack, effectively wet-venting the downstairs shower, or do I need a separate dry vent for the downstairs shower?

It depends on what your local authority finds acceptable.

Most of the West Coast has adopted the Uniform Plumbing Code, which outlaws wet venting in all cases.

While other area's allow wet venting.

Personally, I would call the local plumbing inspector and ask if it is allowed in your area.

2. On the other side of the room, where the upstairs shower is above the downstairs toilet & sink, can the downstairs toilet/sink be wet vented into the stack that drains the shower above?

Again, this depends on your local authority.

The inspector told me that 2" venting is enough, but I'm getting confused on that whole "not to diminish in size" part because the main sewer line is 4", I want to run 3" for the toilets, then I'll be going down to 2" for the final venting. Is this allowed?

No.

Reducing the size of the venting is not allowed.

You will need to go all the way through the roof with a 3" vent.

Thanks. Will the lower fixtures on each stack need separate venting, or are they OK to wet vent? Since the inpector told me that 2" vents are OK, maybe he thought I was referring to only the sinks/showers?

Thanks. Will the lower fixtures on each stack need separate venting, or are they OK to wet vent? Since the inpector told me that 2" vents are OK, maybe he thought I was referring to only the sinks/showers?

Being in Seattle, I would have to supply a separate vent for each drain.

Technically, between the two baths you have fewer than 16 fixture units, which is the maximum fixture unit loading for a vertical 2" vent, but since you've already oversized the vents on the water closets with 3" vents, you will need to continue with the 3" venting all the way through the roof.

I would like to add that in my area, The Tampa Bay Area, that we are allowed to wet vent our fixtures. Check with your local Building Department.
Cheers, Tom