Trap Arm Length Question

[Mike Brubaker]

I have a question that I am a little confused by. When looking at the maximum trap arm length, based on pipe size, does the trap itself have to be the size of the trap arm? For example, you can go 8 feet with a 2 inch pipe, but does the trap its self have to be increased to 2 inches? The code specifies trap size but not trap arm size. I see a lot of Plumber’s using the normal trap size of 1 inch or 2 inch and then going into aI have a question that I am a little confused by. When looking at the maximum trap arm length, based on pipe size, does the trap itself have to be the size of the trap arm? For example, you can go 8 feet with a 2 inch pipe, but does the trap itself have to be increased to 2 inches? The code specifies trap size but not trap arm size. I see a lot of plumbers using the normal trap size of 1-1/2 inch or 2 inch and then going into a larger pipe to get the distance. is that to code, or does the trap size need to be increased as well?

 

[wwhitney]

For the IPC, in use in Florida, Table 909.1 "MAXIMUM DISTANCE OF FIXTURE TRAP FROM VENT" is based on "SIZE OF TRAP," not size of the trap arm. So upsizing the trap arm relative to the trap, which is allowed I believe, does not give you any extra allowable length of the trap arm.

Chapter 9 Vents: Vents, 2020 FBC - Plumbing, 7th edition | UpCodes

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Cheers, Wayne

 

[Sponsor]

 

[Mike Brubaker]

For the IPC, in use in Florida, Table 909.1 "MAXIMUM DISTANCE OF FIXTURE TRAP FROM VENT" is based on "SIZE OF TRAP," not size of the trap arm. So upsizing the trap arm relative to the trap, which is allowed I believe, does not give you any extra allowable length of the trap arm.

Chapter 9 Vents: Vents, 2020 FBC - Plumbing, 7th edition | UpCodes

UpCodes offers a consolidated resource of construction and building code grouped by jurisdiction

up.codes

Cheers, Wayne

That is the way I read it as well. I know there are a bunch of municipalities, allowing the other way, but it does not seem right to me. I assume you would have to have the upsized trap and then you could go to a smaller fixture drain into the trap.

 

[Elton Noway]

2" trap arm with 2" trap

 

[hj]

The trap ARM length is based on the physical properties of the pipe, NOT the trap, but if unsure, just use the larger trap and reduce its connection to the fixture.

 

[Elton Noway]

True enough. The trap size doesn't dictate the size/diameter of the pipe nor extend its allowable length. The trap is sized per IPC based on the fixture (e.g. tub 1 1/2"- Shower 2"). The 2" trap arm and trap in my earlier photo would be to code for a shower but using an adapter could easily be stepped down and use a 1 1/2" trap and still be to code for a tub.