Vertical drop of a horizontal wet vent waste line

[Stereo]

Our code is 2018 IRC. I am remodeling to add two back-to-back bathrooms which will drain and vent through a straight horizontal wet vent/drain. I am clear about all the requirements/restrictions except one. Can I drop the horizontal vent/drain vertically?

The line will, of course, drop 1/4” per foot but, in order to clear an obstacle, I would like to drop the last 16” of the line no more than 8” before it dumps into the stack. I would use 45’s. I want to keep the rest of the line up closer to the joists for more headroom and to reduce the vertical drops of the drains from the toilets and shower outlets to their p traps (which will be well within the 24” maximum however I run this branch).

Thanks for your advice.

 

[Reach4]

I am clear about all the requirements/restrictions except one. Can I drop the horizontal vent/drain vertically?

Except for a toilet, after the vent for the fixture, only, whether a wet or dry vent.

 

[Sponsor]

 

[James Henry]

I would like to drop the last 16” of the line no more than 8” before it dumps into the stack

 

[Stereo]

Except for a toilet, after the vent for the fixture, only, whether a wet or dry vent.

I would appreciate clarification. The wording in the pdf code clarification posted by James Henry says "the horizontal pipe acting as the wet vent may not have vertical offsets." However, the accompanying diagram shows two prohibited vertical drop locations prior to the last fixture. Reach4's comment, above, suggests that I can put in a vertical offset after the last fixture. Which is it? Or does the horizontal drain cease to be a wet vent after the last fixture? If the latter is the case, then the two statements would not be contradictory.

Please see my drawing for my layout. The area in question is boxes in red. The drawing also refers to another vertical drop of the last shower drain, but that is a separate issue. That shower has to be dry-vented because it will be entering the wet vent vertically due to an HVAC obstruction. (Note: I know it is best practice to have a another fixture upstream of a WC for washout but I can't find the room to do that without adding angles to the WC drain. I would think it would be more important to keep the WC drain as straight as possible than to have washout from above. I also don't have the required 24" of room for a cleanout at the top of the run, but the inspector says the WC can serve as the cleanout.)

In a related question, do I need to dry-vent the lavs if the drains enter the wet vent horizontally and the running distance is less than 6' on a 1-1/2" drain? I know with a dry vent, the vent must be 6" above the flood line but is a wet vent different? I can only find diagrams of wet vents that use a lav dry vent as the required dry vent for the horizontal wet vent, but in my case, I have a separate dry vent further up the line.

Also, thank you for your quick responses to my initial post. I apologize for my delay in following up. I did not receive an email alerting my to your responses so thought there were none. I'll keep a sharp eye out this time!

 

[Reach4]

Addressing the part in red, I think yes. If the part in black is OK up to the stack, it is OK to drop to vertical then. I did not analyze the black.

IPC is more permissive than UPC or IRC. So many things in this IPC document will not apply to you: https://wabo.memberclicks.net/assets/pdfs/Plumbing_Venting_Brochure_2018.pdf

I did think that the concept in figure 1 of that document illustrates dry and wet vents. That may or may not be helpful to you.

 

[hj]

Neither the right hand shower or the vanity are vented according to your drawing.

 

[Stereo]

Neither the right hand shower or the vanity are vented according to your drawing.

Sorry, HJ. I only included the vents I thought were pertinent to my main question. I will be installing dry vents for the lavs but I thought the upper shower is vented via horizontal wet vent. It is within the distance limits and will enter the main horizontal drain horizontally. Perhaps my 2-d illustration makes it appear like it's entering vertically. Please correct me if I'm wrong.