Bathroom remodel venting help.

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TSplumbing

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I’m in the process of remodeling my (very old) houses current only bathroom, and from everything in reading here and elsewhere online it looks like my plumbing isn’t currently vented correctly.
The attached picture is my current set up, 4” CI coming in one corner of the laundry room, with a 2” CI vent immediately, then 6’ after the vent is where all my drain connections in the house are tied in, washer, kitchen and bath sinks, tub and finally toilet last in line. This leaves my trap arms a little or a lot longer than I think they should be, and the vent connection up to 18’ away from the furthest fixture.
My current plan is to cut everything out, tie into the 4” CI with PVC, and add a 3” vent straight through the roof. I also want to add a second bath stacked above the first. My question is does the order of fixtures matter, and in my new drawing will anything need an individual vent that I would later tie back together in the attic.

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TSplumbing

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I should have added that because I just removed the lath and plaster I have every wall open and part of the subfloor, but am very limited in space below the floor, 2x8 construction and a tight crawl space below that, no basement. I have lots of plumbing repair experience but not rough in.
 

Terry

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The kitchen and the washer, laundry tray need their own venting and can't be "wet vented" with bathroom fixtures.
So if there is any plan to wet vent a bathroom, the kitchen and the washer would come in downstream of that.
If you're not wet venting, then every fixture trap gets vented within the code distances, and can be re-vented at six inches above the highest flood lever of fixtures served. Normally that's six inches above a 36" kitchen countertop making it 42"

Kitchen sink, 2" drain and cleanout with 1.5" p-trap and arm.
Washer, 2" standpipe, cleanout with 1.5" vent.
Toilet, 3" drain or 4", 2" vent
Tub, 2" drain, 1.5" p-trap, trap arm, and venting
Lav, can be 1.25 p-trap and arm, but PVC comes in 1.5" so that's what we use. We're not in the 60's anymore.
Running a vent through the roof in 3" works.

dwv_b2.jpg


An upstairs bath drops down and connects below the downstairs plumbing.
The downstairs venting can connect on the upper floor at 42"
 

TSplumbing

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Thanks Terry, so ideally I would wet vent the bathroom to save on total materials. Am I correct in understanding that I could use a 2” vent behind the shower or bath sink, thus venting the bathroom group? Provided I reorder my drain connections so kitchen, laundry room, and upstairs bath all come in downstream of my toilet.
 

Reach4

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Sink yes. Venting a shower will not wet-vent a lavatory (bathroom sink) due to the lavatory trap being higher than the shower trap.
 

Terry

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Thanks Terry, so ideally I would wet vent the bathroom to save on total materials. Am I correct in understanding that I could use a 2” vent behind the shower or bath sink, thus venting the bathroom group?
Yes, a 2" can vent the bathroom group.
If a home has a 3" drain, they like to see the same area for venting through the roof, which can be made up by two 2" vents and a 1.5" vent. When I was plumbing a lot of new construction, the 1.5" handled the kitchen sink, and between the two bathrooms, well three really, I would use two more 2" vents. Or I could have used a single 3" but that never worked on the layouts I had.
 
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