Vent size for kitchen sink

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Greetings,
This is probably very simple but just want to confirm if I can use an 1-1/4" dry vent for a kitchen sink that will have a dishwasher and disposal. The trap is 1-1/2" and the drain is 2". The vent will go through the roof and is about a 20' run. There was an existing 1-1/4" galvanized that I am replacing (100 year old house) and would rather go back with the same size rather than drilling larger holes through the studs.

Many thanks-
 

Jeff H Young

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sounds like a weired code so a 2 inch drain line needs only a 1 inch vent? An 1 1/4 should vent ok but im not quoting your code mine requires 1 1/2" .
 

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Jeff H Young

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Jeff, it's 1/2 the diameter of the drain with a min of 1-1/4" is how the code reads.
got it 1 1/4" minimum! I would certainly use that minimum if I could especialy when structural conditions apply , king studs and such
 

wwhitney

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Thanks Wayne. It's actually 25' of developed length. Would you do 1-1/2" or stick with 1-1/4"?
If it was easy to enlarge holes and didn't cause any structural concerns, I'd go with 1-1/2". If it was problematic, I'd go with 1-1/4". For the roof penetration, I like to use a short section of copper, and 1-1/2" copper DWV should fit through the same roof boot as 1-1/4" Schedule 40.

Cheers, Wayne
 

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If it was easy to enlarge holes and didn't cause any structural concerns, I'd go with 1-1/2". If it was problematic, I'd go with 1-1/4". For the roof penetration, I like to use a short section of copper, and 1-1/2" copper DWV should fit through the same roof boot as 1-1/4" Schedule 40.

Cheers, Wayne
There are actually notches in the studs from the existing galvanized which are not allowed so I have doubled up the studs and plan to use 4" stud shoes for structural suppor (passing through more than 2 studs). The new 1-3/4" hole for a 1-1/4" vent or new 2-1/8" hole for the 1-1/2" vent will also be going through the window king stud which my structural engineer said I would need to add a 16 gauge Simpson plate at the new hole and at the existing notch at the 4x4 king stud. Love the idea of using copper at the roof! I still feel uneasy about the new hole and existing notch at the king stud. Another thought is to ditch the whole vent and add an AAV. Not a huge fan of those, especially since I've gone through the trouble of exposing the wall and cutting out the old galvanized.
 

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got it 1 1/4" minimum! I would certainly use that minimum if I could especialy when structural conditions apply , king studs and such
That's what I'm thinking. I thought the min would be the best although concerned about performance of the drain with a 1-1/4" vent. Perhaps a 20 DFU AAV is a better solution???
 

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id rather have a real vent andwouldnt hesitate at using the 1 1/4 just unusual to have any pipe that size in a house . but youll never have a problem with it. copper 1 1/2 would fit as wayne said bigger is better but frankly I dont think it will perform any better and would concentrate on other issues , interesting topic But I think your good whichever choice you make , I would care more about the structure
 

wwhitney

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Since you have to route through load bearing studs, definitely go with 1-1/4" for the vent. If there was a particularly tricky part where 1-1/4" copper would fit easier, I'd even do that for some or all of the run.

Cheers, Wayne
 

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Since you have to route through load bearing studs, definitely go with 1-1/4" for the vent. If there was a particularly tricky part where 1-1/4" copper would fit easier, I'd even do that for some or all of the run.

Cheers, Wayne
Thank you all for the guidance. Plumbing today with the 1-1/4"
 
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