Confusion on Max drain fixture units

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Hi, would like to convert an upstairs tub to stall shower and need to be sure i have sufficient drain size to downstairs before demo. House is slab so really not practical to change to a 2" vertical drain if its currently a 1&1/2". I can verify by removing baseboard on downstairs closet as i have seen the drain before but forgot to measure - will do ASAP. The vertical drain also shares an upstairs double sink vanity and a downstairs vanity sink.

I have 2 plumbing books and also checked online source and it seems the allowable fixture units is different for every source - a 1&1/2" drain varies from 1 to 3 units for horizontal and 2 to 4 for vertical. Is this because of local differences?

To summarize, my drain units will be stall shower (currently a tub), 2 vanity sinks and an additional vanity sink downstairs. Based on measurements and relative locations, it seems that the tub drain goes directly to the vertical drain pipe, i.e. the tub horizontal is likely not shared with the vanities but of course the vertical will carry all 3. The house has truss between first and second floor. I removed some drywall on garage ceiling but unfortunately cannot see any drain because the tub drain is over a downstairs closet wall.

Also of course will have to redo tub drain to 2" for stall shower.

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In the Trades
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Berkeley, CA
You want Table 703.2 from the UPC:

If the drain entering your slab is carrying a tub and 3 lavatories, that's 5 DFUs per this table:

So the drain at that point should already be 2".

Both a tub and a shower are 2 DFUs, so the tub drain already has to be 2" where it is horizontal and has been vented (a 1-1/2" trap arm is allowed on a tub, and a vertical 1-1/2" drain may carry 2 DFUs). So you should be able to change to a shower and only have to replace 1-1/2" piping up to the vent, and possibly any vertical drain immediately thereafter (if the vent takeoff is via a san-tee). None of the branch drainage (carrying more than one fixture) should require upsizing, as there is no increase in DFUs.

This assumes your existing DWV complies with the UPC.

Cheers, Wayne
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