Lav trap on larger trap arm: actual problem?

Users who are viewing this thread

dave148

New Member
Messages
7
Reaction score
0
Points
0
I have a 2" trap arm for a lav, and just realized a 1-1/4" lav drain can't
use a 2" trap. The UPC shows the trap arm size must match the trap
size, but does anyone know what problem a 1-1/4" trap on a 2" trap
arm would cause? Self siphoning should actually be less than a smaller
sized arm, so I'm not sure what the problem would be if I just go ahead
and put the 1-1/4" trap on.

Dave
 

hj

Master Plumber
Messages
33,600
Reaction score
1,037
Points
113
Location
Cave Creek, Arizona
Website
www.terrylove.com
trap

Forget what you think the UPC says about arm and trap sizes. Install the trap. If you have the proper venting it will not siphon regardless of the arm size, and if you screwed up the venting, then it WILL siphon regardless of the size.
 

dave148

New Member
Messages
7
Reaction score
0
Points
0
Re: Trap

So it sounds like all I have to worry about is inspector confusion
on the final.
 

dave148

New Member
Messages
7
Reaction score
0
Points
0
Re: Trap

Oh. So how am I confused?
And what convinces you the inspector will not be?
 

dave148

New Member
Messages
7
Reaction score
0
Points
0
Re: Trap

Perhaps you could be more specific how I am misinterpreting the code.

The rough inspection for this construction was done under the '97 UPC.

Paragraph 1003.3 states, The size (nominal diameter) of at trap for a
given fixture shall be sufficient to drain the fixture rapidly, but in no case
less than nor more than one (1) pipe size larger than given in Table 7-3.
The trap shall be the same size as the trap arm to which it is connected"

Table 7-3 shows 1-1/4" for a single lav, so it looks like the trap can be
no bigger than 1-1/2". But since the trap arm was roughed at 2", the
end of paragraph 1003.3 would seem to insist the trap needs to be 2".

Of course, the lav drain is only 1-1/4".

The trap arm was roughed to 2" per instructions of the inspector.

Dave
 

hj

Master Plumber
Messages
33,600
Reaction score
1,037
Points
113
Location
Cave Creek, Arizona
Website
www.terrylove.com
arm

The trap arm was roughed to 2" per instructions of the inspector

That is the ONLY way I have installed trap arms for 55+ years and most of the time the trap IS 1 1/4". I know it is OK, the inspector knows it is OK, only you seem to have a problem with it.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,771
Reaction score
1,191
Points
113
Location
New England
In plumbing, you can always go from smaller to larger drain pipes...just not the other way.
 

Winslow

Plumber
Messages
445
Reaction score
0
Points
0
Location
Hawaii
If the distance from the trap to vent is too far for 11/2 (31/2 feet) then it is common to run the trap arm 2" then reduce it at the fixture to 11/2. You can then use either a 11/2 or 11/4 trap.
 

dave148

New Member
Messages
7
Reaction score
0
Points
0
Re: Trap

>The trap arm was roughed to 2" per instructions of the inspector

>That is the ONLY way I have installed trap arms for 55+ years and most of the time >the trap IS 1 1/4". I know it is OK, the inspector knows it is OK, only you seem to have >a problem with it.

Not exactly. This particular inspector also told me I needed a 2" trap. After the
rough inspection, I noticed in the code that I couldn't go over a 1-1/2" trap
for the 1-1/4" lav drain, so he basically misdirected me. The reason I had to
change any DWV in this area on my rough was because I had put two lavs
on the same trap arm. As far as I could tell, this was allowed by the code,
but he had never seen it before.

Another one of the inspectors in my area told me to put in a horizontal wet vent
in a particular location. Since that was clearly against the code I was working
under at the time, I had to get around the problem by getting an approval by
the plumbing plan checker (he knows the code well) on my layout before
the next inspection. Inspectors are human, and some know more than others.

While I could think of no physical reason there would be a problem with a
1-1/4" trap on a 2" trap arm, it is against the letter of the code, so I wanted
to run it by some of you to make sure I wasn't missing something.

Thanks,

Dave
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks