Vent size at takeoff

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Dusty Fellow

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Working on my cast iron dwv 2-bath remodel and here’s a quick question, mostly because I’m without the ideal part and far from a supply place that carries it for at least a few days.

2 inch P trap and arm for the tub. Vent takes off around 12” after the P-trap and will be joining another 1.5” vent to head through roof. Can I use a 2x2 wye for the takeoff and reduce it to 1.5” pipe right after takeoff or must it be a 2x1.5 wye?

I’m of course asking because it will take me a few days to get a 2x1.5 wye but I do have 2x1.5 couplings on hand.
 

wwhitney

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Seems like a judgement call, and I have little experience with what inspectors might think , but seems fine by me. I would consider a 2" x 1.5" no hub coupling on a fitting to be the equivalent of a 2x1.5 reducing bushing in plastic, and I can't imaging anyone objecting to that.

Cheers, Wayne
 

Jeff H Young

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I like to avoid piping below 2 inch on cast. but each job is differant and depends on framing as well.
Whats the main reason to go all castiron on this Job Dusty? Just looking for robust and quality?
 

Dusty Fellow

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I like to avoid piping below 2 inch on cast. but each job is differant and depends on framing as well.
Whats the main reason to go all castiron on this Job Dusty? Just looking for robust and quality?

Ya. This one will be a rental, and I don’t like plastic in walls where all it takes is one screw...

Secondly, interior load bearing walls are 2x4 with let-in and would require a lot of work with the old notches and holes for 2” pipe horizontal runs. Easier to just re-route some pipes and do horizontals in attic. Plus I would have to do protection plates for plastic from the other side where I wasn’t planning on touching drywall.

Sticking with 1.5 on one of the vents because I was looking to preserve the galv part that wasn’t needing to be modified, and didn’t want to touch the roof penetrations if I didn’t need to. If I was replacing the whole thing I would do 2” of course.

And it’s also a labor of love because it’s my mother-in-law’s house who passed away unexpectedly in 2020. I like working with cast if I’m not crunched for time. This is a project I get a few hours here and there to work on so I’m not bothered by how fast it could go, but when I do have time for it I want it to be put to good use, hence my question.

Same reason I’m sticking with copper for the supply even though pex would be much faster for me. Crawl space is tiny but right now floor is mostly up so going “robust” isn’t as difficult as it could be. A lot of rodent activity in the area so all it takes is a vent screen to come loose and then they’re in the crawl. At my parents’ house not too far from here rats chewed through plastic pipe straps (thanks builder!) and collapsed a main dwv branch, causing it to snap at a joint. Sewage leaked for months causing foundation probs. Rats happily chewed through PVC pipe and strap but didn’t touch the ABS interestingly enough (who knows if that’s a thing or just the palate of one rat family). Nightmare scenario but I’ve lived it so cast gives me a little extra piece of mind for that kind of stuff.
 

Jeff H Young

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Thanks Dusty , Those are all real good reasons for cast and for wanting to run 1 1/2. I've had the rodents knaw thru ABS and PVC as well and an old structure plays part some even have true 3 inch walls.
Using a 2 inch wye and a 2 x 1 1/2 reducer fitting or band I've done without issue from authority , I failed to mention.
 
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