Venting Suggestions for bathroom addition

[Mark1400]

I am in the process of adding a bathroom above a garage addition and am looking for ideas on how best to vent the fixtures. The attached diagram shows how the drains are currently plumbed (nothing is sheetrocked yet). The shower, toilet and sinks all connect in to a 3" pipe that ultimately connects to the house main drain line right before my septic tank. Whatever vents are added, my plan is to add a stack and go through the roof.

 

[wwhitney]

You've got an issue on your shower, as a 1-1/2" trap arm is limited to 6' in length before the vent, if you use a perfect slope of 1/4" per foot. That's because the trap needs to be vented before the trap arm falls more than one pipe diameter, and 1-1/2" / (1/4" per foot) = 6'. If you plumb the shower drain with 2", then you can go up to 8'.

So given your current arrangement, you'll need to pull a dry vent off the shower before it joins the WC. That would be with an upright combo, (or the IPC allows a san-tee on its back if space is really tight), and the dry vent has to stay vertical until 6" above the shower flood rim. That would be easy to do if the shower trap arm passes under a partition wall you can run a vent in; hard to do otherwise, and a reason to redo/reroute the shower trap arm.
[Or under the IPC, you could put an AAV directly on the shower trap arm, it would need to be accessible, open to the air, and at least 4" above the trap arm.]

Once the shower is vented, that will wet vent the WC. The sinks are at a higher elevation, so they will need a separate vent. Best place would be between the two sink traps; that will dry vent the upstream sink, which will wet vent the downstream sink. The IPC does allow common venting, so you would be allowed to add the dry vent downstream of the downstream sink, as long as the vent takeoff is again before the upstream sink's trap arm has fallen more than one pipe diameter.

Lastly, once the lavs are properly vented, they could wet vent the WC and shower, eliminating the need for a shower dry vent. However, that would require a few changes: the drain that carries two lavs and is a wet vent needs to be 2" in diameter, not 1-1/2". Also the lav needs to join either the WC or the shower before the other joins in; as drawn, with the WC and shower joining upstream of the lav coming in, the lav can't wet vent anything. And as mentioned, the shower would have to join the wet venting drain (either the lav, or the combined WC/lav) within the 6' / 8' limit for 1-1/2" / 2" trap arms.

Cheers, Wayne

 

[Sponsor]

 

[Mark1400]

Thanks Wayne. I could add the dry vent between the shower and toilet as you suggest - as there is a wall that I could run it up into. It would be about 4' from the shower trap puts me within the 6' limit. I could terminate this with an AAV, as it would be easily accessible. I could possibly continue it to tie it into the vertical stack that would be be hind the sink area, but it would be a lot more work (although I know this is preferred).

For the lavs, it would be a lot more work to increase them to 2"and reroute where the enter the main drain, so the idea of a venting either between the two lavs or just downstream of the downstream lav would work.

So one more followup question. If I go with a dry vent for the tub/toilet with an AAV and a vent between the lavs that goes up to fresh air through the roof, can that vent stay at 1-1/2" all the way through the roof?

 

[wwhitney]

Yes, under the IPC, 1-1/2" is enough for the vent for the bathroom. Although in cold climates, it's a good idea or required to increase the vent size to at least 3" for the roof penetration, to reduce the chance of the vent opening frosting over. That would start just inside the thermal envelope (so just above the wall top plate if your ceiling below the attic is insulated) up through the roof.

Cheers, Wayne