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I'm a plumbing newbie and I'm adding a bathtub where one didn't exist before and I want to make sure that I am plumbing the drain properly.

I have a 2" drain available to me directly below the tub and a 2" vent available on the back wall behind where the faucet will be.

Is it okay for the tub drain and trap to be 1-1/2" and then go into a 2" vent/drain?

I've attached a picture of my mockup.

1.5 to 2" drain/vent

Further down the line I'm fairly certain the 2" line goes into a 3" toilet which is separately vented. It's under concrete but the 2" line goes in that direction.

Thanks!

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yes that will be fine. vent size increase as they go UP and more lines get tied into it. the size of drain lines increase as they go DOWN and more lines get tied together. so yes a 1½ drain and vent can tie into a 2" vent and drain, as long as it's an individually vented fixture

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    True but you would improve the quality of your answer by providing some source links. – The Evil Greebo Jul 18 '18 at 12:11

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