What's the max permissible distance for shower drain to main drain/vent stack?

[240sxguy]

I have been reading a lot of posts on this and would like confirmation as I have my basement ceiling open working on my lower level bathroom and realized that they put an AIV onto the upstairs shower. The distance from the shower to the main stack is maybe 3'. Would this require an AIV? The prior owners had a home warranty and the plumbing company didn't do stellar work (as evidenced by when I had them out) and I suspect they may have put an AIV in instead of actually clearing the clogged vent stack.

I can take photos if needed. Thank you!

 

[wwhitney]

First, Wisconsin has its own plumbing code that deviates from the IPC and UPC, so the answer may be slightly different in Wisconsin.

But the basic answer is that the shower trap arm is limited to one pipe diameter in fall, while it must fall at least 1/4" per foot. Note here that the trap arm is defined as the drain from the trap outlet to the vent connection. And that the shower fixture drain (drain carrying only the shower) must be vented before or as it joins another drain (with one exception in the IPC). Note also that the UPC imposes a further length limit on trap arms: 42" for a 1-1/2" trap arm, and 60" for a 2" trap arm. I suspect that Wisconsin has similar limits.

So if your shower trap is just 3' from the main stack, there are various reasons that it may have an AAV on it rather than relying on the main stack for a vent. Here's a non-exhaustive list:

- The shower fixture drain has to drop more than one pipe diameter to make its connection to the stack.
- Some non-bathroom fixture joins the shower fixture drain before it joins the stack, which precludes wet venting.
- A WC or some other disqualifying fixture drains into the stack above the shower drain connection, so the stack can not function as a vent for the shower.

Cheers, Wayne

 

[Sponsor]

 

[240sxguy]

First, Wisconsin has its own plumbing code that deviates from the IPC and UPC, so the answer may be slightly different in Wisconsin.

But the basic answer is that the shower trap arm is limited to one pipe diameter in fall, while it must fall at least 1/4" per foot. Note here that the trap arm is defined as the drain from the trap outlet to the vent connection. And that the shower fixture drain (drain carrying only the shower) must be vented before or as it joins another drain (with one exception in the IPC). Note also that the UPC imposes a further length limit on trap arms: 42" for a 1-1/2" trap arm, and 60" for a 2" trap arm. I suspect that Wisconsin has similar limits.

So if your shower trap is just 3' from the main stack, there are various reasons that it may have an AAV on it rather than relying on the main stack for a vent. Here's a non-exhaustive list:

- The shower fixture drain has to drop more than one pipe diameter to make its connection to the stack.
- Some non-bathroom fixture joins the shower fixture drain before it joins the stack, which precludes wet venting.
- A WC or some other disqualifying fixture drains into the stack above the shower drain connection, so the stack can not function as a vent for the shower.

Cheers, Wayne

Of course they do. lol. typical.

The shower drain joins right where the WC joins. I will attach a photo but this could definitely be why. I'm tempted to have this portion of the stack replaced as I just don't care for how all this is done compared to what's right now. The toilet is also in a terrible spot up there and needs to be moved over like 6"