Trying to move bulkhead - issue with structure in the way of pipes.

[pulper1]

I have a question here regarding my vent. In the pictures, there is the main vent (on the right) which ties into the toilet and vanity vent (on the left). This is currently installed with 45 degree adapters at the top, presumably (I thought) so that they would not cut too much out of the top plate. Then a bulkhead was constructed to hide the pipe. I got rid of the bulkhead and now have been playing PVC lego to see how I can get this to work.

A few specifics: This is a non-load bearing wall. The top plate consists of a laminated 2×6 with another 2×6. There would be approximately 2 inches left in the top plate at the site of the vents after notching out the 2x6s and the 2×8. In other words, there is 2 inches of space between the pipes and the wall. I would use a specific type of galvanized metal strap between the two sides of the top plate once cut. The roof vent is about 20″ above the top plate.

When getting ready to cut more out of the the top plate, I noticed that the structure (joist/rafter tie) directly attached to the top plate only allows me approx 2.5 inches of room. Since I'm using 3" pipe (which I believe is 3.5" diameter - schedule 40), 2.5 inches won't work without notching the structure. It would appear that's why the previous workers did the bulkhead.

I have a couple questions here.

-I'm wondering what kind of structure this is. I first thought that these were joists but now believe these are rafter ties with vertical braces. They are 2x6 and are 16" OC.
-Just a shot in the dark, and I'm assuming no, but if they are rafter ties, does this change the notching rules (d/6 and d/3) for this structure? There is no load directly on top of these joists/rafter ties.
-Another option for me is to cut holes in the wall instead in order to join the pipe on the right with the pipe on the left. The wall is 2x6. However, even for non-load bearing walls, I believe that 3.5" in a 5.5" wall won't work (60% I believe is the max).
-Could I reduce to 2.5" PVC for the vent?

Any other suggestions, other than rebuilding the bulk head or going through the other side of the wall and building a bulk head there?
I've attached a few more pics.
as an FYI, this was a normal roof before (same angle on each side) that was renovated previously (not by me) into making the roof more shallow on one side to allow more headroom (and a bathroom) on this side of the roof.

Thanks!

 

[JG plumbing]

Where are you going to find 2.5 inch pipe and fittings?

 

[pulper1]

Sorry - I'm assuming by your post that 2.5" isn't available or readily available. If that's the case, then references to 2.5" in my post should be changed to 2".
thanks for the info.

 

[JG plumbing]

What fixtures do you have and how many?

 

[pulper1]

i hope i'm answering this question correctly - excuse my ignorance if not:
Top Floor (where this wall is): shower, toilet, vanity.
Main Floor: shower, toilet, vanity, kitchen sink, dishwasher
Basement: washing machine, sink by the washing machine
outside has a faucet for the hose.

 

[pulper1]

I think I see where you're going with this. I found this in the Michigan plumbing codes, section 906:
https://up.codes/viewer/michigan/mi-plumbing-code-2015/chapter/9/vents#906So it would appear with anything between 10 and 21 fixtures, i can convert to 2" from 3" if my "MAXIMUM DEVELOPED LENGTH OF VENT (feet)" is less than 150 (which I'm having a bit of trouble understanding).
Assuming the above is correct, I have a quick question. If I switch from 3" to 2", can I later switch back to 3" again so that I can rejoin the pipe that is venting out the roof (to save me from having to reflash and re-do that pipe above the roof)?

 

[JG plumbing]

Yes. That's what I was going to recommend. Keep them as two separate 2" pipes. Then come back together as a 3".

 

[JG plumbing]

You have 19 fixture units. 2" will do. But I'd still come back together with 3".

 

[pulper1]

Thanks again for your reply! I am hoping this will work. However, now that I look into it more, I may have used what I thought is a plumbing fixture and not what it actually is.
For example, I have a garbage disposal, which I'm not sure if that counts. Also, on the main floor, it is a bathtub (not a shower - sorry) - not sure if that changes anything. Also, for the shower on the top floor, it will have a diverter that uses a shower head as well as a handheld. I also have a furnace humidifier - didn't even think of that before.
I originally thought my number of fixtures is low but based on the count you gave, it appears to me that anything with hot and cold water counts as two fixtures. Hopefully none of the above adds to it so the plan will still work.
Please let me know. Thank you.

 

[pulper1]

Also - one more thing. I've read a few forums where people have indicated something similar to "Once a minimum pipe size has been introduced in to the venting system, you never decrease it all the way to the point of termination [ i.e. - open air ].".
Does this apply to what I'm looking at doing here (decreasing from 3 to 2)? Thanks again.

 

[JG plumbing]

I'm not really sure what that means. Once a minimum pipe size is introduced, you can't then decrease it, your at the minimum. So I don't really know what that person is getting at.

Unless they are saying don't use the minimum pipe size for the entire developed length. Which can be true. It just depends.

 

[JG plumbing]

I didn't count your dishwaser as a seperate unit. Or your hose bibb, although the additional shower head does make a difference

 

[frodo]

vents are different than drains. you can reduce the vent that has been oversized to its minimum
then it is acceptable to revent into a larger pipe

 

[pulper1]

Thanks for your help! I appreciate it.