Please explain this venting code

[Chris989]

2017 Idaho UPC

Pic 1 is a toilet with a 2" vent within 72",,,, pass
pic 2 toilet wet venting ~18" of a shared 2" drain from a tub,,,, Fail.
Pic 3 toilet, tub and lav wet venting with a 2",,,, Pass?

I was told to do it as picture 3 even though it looks like spaghetti, adds ~10' to my run, and makes a 180 degree turn just to pick up the tub and WC.

I grasp the concept of the lav/wet vent to flush possible blockage from the pipe. But if the toilet will siphon from the tub in pic 2, why won't it in pic 3? And if in pic 2 they is no drain to keep the vent clear, wouldn't that be the case in pic 1 also.

the layout is of a jack and jill bath and i have a few issues because the joist hange from a ledger on the foundation wall. I can not get up the exterior wall near the WC.

Anyway, Pic 2 makes life very easy for me, I just do not understand why it is not allow. All that said, after living overseas i can appreciated all of the building codes, Just can not understand how venting a bath group is OK but one other fixture is not.

Thanks
Chris

 

[Terry]

#2 would have passed in Washington, UPC code.
I'm assuming the vent on the tub would be 2" to take care of the toilet.
edited 3/23

 

[Sponsor]

 

[Chris989]

Terry,

Drawing #2 will pass? I have done it this way in the past but I was told to add the tub, I would have to add the entire bathroom group. (maybe I misunderstood my new inspector)

 

[Terry]

I would ask your inspector again. It doesn't pass with a 1.5" vent, but I would think it would with 2"

 

[Chris989]

I was planing on it being 2"