1.5 vs 2" P-trap for tub


Old 10-24-16, 01:41 PM
Thread Starter
Join Date: Jan 2012
Location: USA
Posts: 263
Received 0 Votes on 0 Posts
1.5 vs 2" P-trap for tub

I'm curious what thoughts anyone has on making a standard bathtub p-trap 2" instead of 1.5". I've used 2" p-traps on tubs where the vent distance is near maximum, to make sure I kept 2" pipe all the way to the tub.

The tub drain assembly itself is pretty much always going to be 1.5" pipe and fittings so you'd have to reduce with a bushing as you connect to the 2" trap.

Assuming I have the space for a 2" trap is that a good idea to prevent clogs?

Or, is the larger trap just reducing the drain velocity thereby making it more likely that debris builds up in the trap?

I think there's little value to arguing that a 2" trap would let someone in the future replace the tub with a shower since likely the trap would have to be moved anyways.
Sponsored Links
Old 10-25-16, 09:03 PM
Thread Starter
Join Date: Jan 2012
Location: USA
Posts: 263
Received 0 Votes on 0 Posts
Well, I didn't get any replies.

On my latest project a 2" p-trap gave the perfect offset whereas a 1.5" p-trap required some extra fittings to get it located just right for the tub drain. I went with the 2".

Would still be interested in hearing thoughts on if it matters.
Old 10-25-16, 09:55 PM
Join Date: May 2015
Location: USA
Posts: 3,138
Received 2 Votes on 2 Posts
I doubt you'd see much difference in water velocity...the tub drain assembly being the limiting factor in every standard tub I've seen....

2" is a little easier to snake should that become necessary.
Old 10-25-16, 10:13 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
I used to have a large (about three feet by five feet) stall shower in my home. I removed that and installed a whirlpool tub. Since the existing drain was 2 inch I installed a 2 inch P-trap with the reducing fitting to the 1-1/2 inch tub drain. I am right at the limit of trap arm length without having a re-vent at the trap.

This was about ten years ago and I have had NO problems. The drain DOES gurgle when draining but I asked on a professional plumber's forum and the consensus was it was merely scouring the trap, a good sign. I have often gone several months without using the tub and have never "lost the seal" in the trap.

So, in my opinion, which will get you a cheap cup of coffee if you also slip the waitress a buck, you are good to go. Your LOCAL plumbing inspector may have a different opinion.
Old 10-26-16, 11:04 AM
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 5,417
Received 131 Votes on 121 Posts
I don't see any benefit (or detriment) to using a 2" trap. It's rare that a bath or shower trap gets clogged. It's usually the stopper or grate that gets all gunked up over time.

Installing a 2" pipe like you did is often a good idea depending on the length and also enables future changeover to a shower.
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
Ask a Question
Question Title:
Your question will be posted in: