Horizontally laid wye to provide vent path from trap arm allowed?

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Mathew Johnston

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Hi all,

I'm refinishing a small main washroom in and old house. Second floor corner room. Joists are 2x8. I'm having trouble reconciling all of the code requirements given constraints imposed by the room.

Referring to the below diagram (top down view of DWV)... The red section has too significant a slope at length to meet the trap arm requirements (I will explain why), and I'm considering the addition of the green section for venting. I don't know whether this horizontal section would violate the rule against horizontal vent runs below the floodline+6" limit.

The reason I have this problem is as follows:

The line from main stack out to the "sink and up to vent" runs perpendicular to the floor joists. To penetrate as close to code (middle of joist) as possible (6' = 1.5" slope for 1.5" pipe), the sanitary tee coming off of the stack is positioned somewhat low relative to the 8" joist space.

On the other hand, the shower's p-trap outlet must be positioned near the top of the joist space in order for it to fit in there. I've tried to maintain 3" pipe for as long as possible towards the shower drain to maximize the trap arm distance allowance, but just can't reach it without flooding the vent.

I have to split off horizontally to reach an exterior wall cavity where I can run the vent up to join back to the stack. Is this allowed? I hope so because I don't see any other option.

Thanks!


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Mathew Johnston

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Thinking about this further - is this not just a "horizontal wet vent" that happens never to be used as a drain? Why are horizontal vents allowed (it is, right?) if wet but not dry until you pass 6" above flood line?
 

Terry

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A wet vent is washed, and because it servers a picture, it can also be snaked to clear the vent if needed. A dry horizontal vent has no way to clear it.
You can wet vent a bathroom group with the lav using 2" pipe assuming all of the grades are correct for that. With 2x8 joists, it can cause issues. Sometimes it's easier to run below and fir down the ceiling to cover the pipes. I don't try to do much with 2x8's for that reason.
 

Mathew Johnston

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Thanks Terry! So, best bet might be to upgrade the planned 1.5" to 2", throw another layer of ply on the joists, and use the lav vent ("... sink and up to vent" label) as a wet vent for the shower. Right?
 
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