Proper DWV size for tub/shower combo?

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Jeffpo

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Hi,

Renovation in progress for an en-suite bathroom. Currently have a shower with a 2” DWV and a lav with a 1.5” DWV.

I would like to reconfigure the room room by replacing the shower with a tub/shower combo BUT I want to swap the positions of the current shower and lav.

I know that the lav can use the 2” DWV but I’m not sure about the tub/shower on a 1.5” DWV. I believe that a stand-alone shower requires 2” but seem to recall that 1.5 is (was) permitted for tub or tub/shower with a single head.

This bathroom is on the second floor. Converting the existing 1.5” DWV is possible but would be quite an invasive project. If forced to, I might bite the bullet to change it out or leave the fixtures in the current locations (which is less desirable).

Is a 1.5” DWV permitted for a tub/shower in Washington?

Thanks in advance.

Jeff
 

wwhitney

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A 1.5" trap and trap arm is sufficient for a tub (with or without shower) under the UPC, but there's a wrinkle. A 1.5" horizontal drain, other than the trap arm, is only sufficient for 1 DFU, while a tub is 2 DFU.

That means you'd need to upsize any horizontal drain after the vent comes off to 2". And a 1.5" trap arm is limited to 42". So it really depends on how the existing 1.5" drain is run. Unless it goes down fairly quickly into a 1st floor wall, where it enlarges to 2" before turning horizontal , you'll have to upsize a lot of it 2". At which point it probably makes sense to upsize it all to 2".

Cheers, Wayne
 

Jeffpo

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A 1.5" trap and trap arm is sufficient for a tub (with or without shower) under the UPC, but there's a wrinkle. A 1.5" horizontal drain, other than the trap arm, is only sufficient for 1 DFU, while a tub is 2 DFU.

That means you'd need to upsize any horizontal drain after the vent comes off to 2". And a 1.5" trap arm is limited to 42". So it really depends on how the existing 1.5" drain is run. Unless it goes down fairly quickly into a 1st floor wall, where it enlarges to 2" before turning horizontal , you'll have to upsize a lot of it 2". At which point it probably makes sense to upsize it all to 2".

Cheers, Wayne
Hi Wayne,

Thanks for the response. The drain is 1.5” all the way to the crawl space where it joins the 3” (or maybe 4”) main horizontal branch.

The current 1.5 lav drain is in fact very close to the wall and it goes vertical very early.

If the trap arm is less than 42” (which it would be) and I could route it to drain/vent at the vertical junction would that be acceptable? If it must be increased from 1.5 to 2 then it’s the whole line that would need upgrading—no point to doing only a portion of it. I think the work involved might not be worth it.

Cheers
 

wwhitney

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I don't quite follow your narrative, but the following would be allowed for a bathtub: a <=42" horizontal 1-1/2" trap arm (which must not fall more than 1-1/2") to a 1-1/2" san-tee for the venting to a vertical (up to 45 degree off plumb / at least 45 degrees above level) 1-1/2" drain. But where that 1-1/2" vertical drain eventually turns horizontal, you must upsize to 2": e.g. it could hit an upright combo to join an existing 2" horizontal drain, or you could just use a 2" LT90 with a 2"x1-1/2" bushing in the inlet.

Cheers, Wayne
 

Jeff H Young

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Yep tub requires a 2 inch drain but only a 1 1/2" trap arm. that's why I see no reason to run 2 inch at all except for structural reasons I run 2 inch for lavs as well and see that mostly it done that way . perhaps a stand alone bidet can be 1 1/2 not sure .
when I work with cast iron I don't use 1 1/2 at all even vents on commercial
You could if easier abandon the 1 1/2 and run lav and tub shower off same 2 inch line
 
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