1 1/2" tub to 2" shower?

[Mikey]

the national standard plumbing code allows a 1 1/2" drain on a shower

Well, thank you. so does the 2015 IRC. The old NPC (2009) allowed 1 1/2" for a single shower head, but required 2" for multiple shower heads (but didn't got into the situation where I have multiple shower heads, but only one can run at a time). I don't have access to most of the new codes, so I can't build a definitive list, but I think I've got a good case to present to my AHJ. And it may be moot, because after some demolition, it looks like running a 2" drain isn't impossible, just more work than using the existing 1 1/2". But it will give me an excuse to buy a new tool!

 

[frodo]

the national standard plumbing code allows a 1 1/2" drain on a shower

op is from washington
a UPC state
and....the code changed 2015, it allows a 1 1/2'' trap on shower now

 

[Mikey]

Well, that clears my conscience and is worth a donation. Thanks.

btw, your link to gift-giving fails.

 

[frodo]

Well, that clears my conscience and is worth a donation. Thanks.

btw, your link to gift-giving fails.

wat?????
i must fix the damn thing, thanks for the heads up

 

[Mikey]

I finally got an explanation I'm satisfied with. A shower releases water at 2.5gpm or so, whereas a tub can dump a tubful in a few minutes. So I thought the tub needed a bigger pipe. But, as crud builds up in the drain, the large release from the tub will scour the drain better than the shower would at its constant low flow rate. The shower needs the larger pipe because a) it doesn't scour as well, and thus b) may need to be snaked, and the larger pipe accommodates the snake better. Also, fwiw, in our area the tub can get along with a 1 1/2" trap, but requires a 2" drain downstream of the trap. Seems like a conflict, but wth.