Drain size


Old 04-18-07, 09:58 AM
Thread Starter
Join Date: Dec 2005
Location: USA
Posts: 3,188
Drain size

I have a question about drain size. I know plumbing codes require a 1 1/2" drain for a tub and a 2" drain for a shower. My question is why?

My house has a 3/4 bath with a shower. It has a 1 1/2" drain and it works fine. I have never had a drain problem. Another bath has a tub surround/shower combo with a 1 1/2" drain. These fixtures are plumbed as built. My master bath has a separate tub/shower that I installed. I put in a 2" drain for the shower per code.

I'm helping my son renovate a bath in a summer house he bought. We are installing a shower and in order to meet the 2" drain requirement we will have to do some serious demo to run the new drain line. He wants to use the existing 1 1/2" plumbing. We have done some experimenting and the 1 1/2" drain is more than capable of handling the 4-5 gpm that his supply can provide. Are we missing something or is the 2" requirement overkill?
Sponsored Links
Old 04-20-07, 12:47 PM
Join Date: Jan 2007
Location: Central Indiana
Posts: 389
I think the reason for the 2" requirement on showers is that the 2" is much less likely to clog over time; a clog in a shower is an immediate crisis whereas a tub will just hold the water.
Old 04-20-07, 03:28 PM
Join Date: Mar 2007
Posts: 230
I'd love to meet the guys that made this one up. I've been doing this for over thirty years and I can't figure it out either. The water comes out of the valve at the same rate whether it is a tub or a shower. It all has to get down the drain eventually. So after the tub fills up it all spills over the top at the same rate as it would in a shower. So apparently the code writers thought that it takes about the amount of time for a tub to fill up (as opposed to a shower pan) to figure out you have to turn off the faucet!
Old 04-21-07, 06:12 PM
notuboo's Avatar
Join Date: Sep 2001
Location: Kansas City MO
Posts: 1,780
I answered this one before.


You need to know what your local code is. Either UPC, IPC, or a variate of either of these at a state level.
Old 04-21-07, 08:03 PM
Thread Starter
Join Date: Dec 2005
Location: USA
Posts: 3,188
Notuboo- I appreciate that you answered this previously. I know the code in my area is for a 2" shower drain. At least it was 3 years ago. I got a little confused by your answer though because it sounded as if the 2" requirement was an old one and that it had recently been changed (at least in some areas) to allow for a 1 1/2" shower drain. My house is 24 years old. The original shower and tub/shower both have 1 1/2" drains. I have to assume that 1 1/2" was acceptable (at least here) 20 years ago.

This time i was trying to ask whether I should be concerned (code notwithstanding) if I go with the existing 1 1/2" line.

Now I'm in a position where my options are to open 2 walls, one of which was recently sheetrocked, to run a 2" drain or to cheat. I guess option 3 would be to make sure the requirement is still 2". :0 )
Old 04-13-08, 06:55 AM
Join Date: Apr 2008
Posts: 1
notch out wall

Is it ok to notch out a wall,2/6 studs for a 2in. drain?The wall is about 30 ft. wide,I'll be notching 20ft.but not the corners.The outside is supported with at least 1/2 in. plywood.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes