Upstairs bathroom addition in old house

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Hi, I’m trying to install a second story bathroom in an unfinished attic space in an old house. Currently downstairs has a tub, sink and toilet vented to a 3” cast iron stack. I want to run a separate 3” drain from upstairs and connect either in the crawlspace or in the utility closet space in the first floor. I’m wondering if I can have the second floor bathroom drain above the first floor creating a small section of a wet vent. Will the first floor toilet have enough air? The bathrooms are right above eachother.

Currently the downstairs bathroom has the tub, sink and toilet all vented to the main stack but aren’t vented individually. I may add some additional vents for these next year but for now the bathroom seems to drain fine.


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In the Trades
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Berkeley, CA
I would think it would be better to tie in the new upstairs drain downstream of the downstairs bathroom, so as not to disrupt whatever apparently inadequate venting the downstairs bathroom currently has.

Or you could fix the downstairs venting now. An AAV at the lav may be enough to wet vent the tub and WC, a more accurate diagram (e.g. floor plan showing horizontal drains) would be required to be sure. Then if the downstairs bathroom is properly vented, you could tie in the upstairs drain anywhere downstream of the downstairs bathroom branch (downstream of all the bathroom fixtures).

Your drawing doesn't show the kitchen sink or any laundry hookups. So the above all assumes that those are connected downstream of each bathroom group. E.g. you don't have a kitchen sink connected between the lav and the WC.

Cheers, Wayne
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