# Slope for Trap Arm

Discussion in 'IPC Plumbing Code Questions' started by ghaun, Mar 25, 2021.

1. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
Hello,

I am a bit confused by the slopes provided in table 906.1.

It makes sense that a 2" pipe at 1/4" per foot would max out at 8 feet in length. That would put the level right at the top of the pipe meeting the vent.

...but what I don't understand is the 1/8" per foot requirement for the 3" pipe. That would put it at only half full at 12 feet.

Am I misinterpreting that chart?

Thank you!

2. ### wwhitneyWell-Known Member

Joined:
Mar 17, 2019
Location:
Berkeley, CA
I think you mean Table 909.1:

https://up.codes/viewer/pennsylvania/ipc-2015/chapter/9/vents#table_909.1

Looks like the IPC doesn't want to let you use the allowance for less than 1/4" / ft slope on 3" and up to extend the trap arm, so the length limit is still based on 1/4" /ft.

Do you have a fixture with a 3" trap where you'd want a trap arm longer than 12'? I guess that could happen with a floor drain. The exception to 909.1 tells you the length limits don't apply to WCs.

Cheers, Wayne

4. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
Wayne,,

Thank you for your reply. I have a free-standing bath-tub that I am struggling to vent. I can get a vent to it right at 12' and was using a 3"drain to get that distance. I am confused as to what slope I need to run that drain.

5. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
...and 909.2 says the following: The total fall in a fixture drain due to pipe slope shall not exceed the diameter of the fixture drain, nor shall the vent connection to a fixture drain, except for water closets, be below the weir of the trap.

So which would take precedence. The chart is implying a greater restriction than 909.2.

6. ### wwhitneyWell-Known Member

Joined:
Mar 17, 2019
Location:
Berkeley, CA
The chart does. You could run the trap arm at anywhere from 1/8" to 1/4" slope (per ft), but you'd still need to meet the 12' limit.

Care to share a floor plan? No good wet venting options? [Edit: looks like you did in the other thread, I'll check that out now.]

Cheers, Wayne

7. ### James HenryIn the Trades

Joined:
Jul 23, 2019
Occupation:
Plumber
Location:
Billings, Montana.
A 3" drain is allowed 1/8" per foot because usually any drain 3" and larger will be installed to accommodate a toilet and if you have to much grade on the pipe the water will drain to fast and not carry the effluent with it. So, you are OK to have 1/8" per foot slope on your 3" pipe.

8. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
So, by installing at 1/8" per foot, I gain even more headspace in the pipe, better guaranteeing my vent, as opposed to a 2" at 8' which is really pushing the limit. Good to know.

9. ### James HenryIn the Trades

Joined:
Jul 23, 2019
Occupation:
Plumber
Location:
Billings, Montana.
By the way, by using a 3" drain you practically have a combination waste and vent , you will have no problems with the drain.

10. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
Wayne, thank you. The only way that I could wet vent is with a vanity sink, but that puts the trap arm there at a maximum, unless there is a concept of relief venting that trap arm. My other thread asked about the concept of a wet vent, being flat, and why a fixture was required at the end. I figured that it was for washing, so that was confirmed. Thank you!

11. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
Great, so are you suggesting that I do 1/8" slope? It is just a tub, so the water will have no problem draining.

12. ### Jeff H YoungIn the Trades

Joined:
Feb 27, 2020
Location:
92346
I wouldn't assume that IPC allowing 3 inch pipe to be sloped at a minimum 1/8inch per foot is better than running at 1/4 inch per foot causing the water to run away from the solids. If that were the case wouldn't code set the max fall allowed at 1/8th per foot? My code ca 708.1 won't allow 1/8 slope it requires a minimum of 1/4 " per foot presumably because they believe 1/4 inch per foot MINIMUM is a proper grade. Maybe my code is inferior and completely wrong. only on a hardship do they allow such flat piping. been hearing my whole career about having too much fall but I don't believe that having over an 1/8th or a 1/4 is going to hurt a thing I believe it is when slope is 3 times greater or 10 times greater than recommended that problem occurs. So we run 1/4" on even 4 inch.
In the case of Ghaun I would recommend 1/8th per foot because his code allows it and if trap arm is 12 foot he has no choice but to run 1/8inch per foot. I think following those guidelines he should have a well working and system that meets his Code. Even though I wouldn't call it optimal but adequate yes no problem .

13. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
Jeff, I completely agree. It is just strange how the requirements for trap arm length are not consistent. For a 2 in pipe, they consider the arm at a length of being completely full; though, for the 3" and 4", they want half full. Is that chart saying that a trap arm for a 3" pipe must be 1/8". If I have a 3" trap arm at 1/4" what is the length limit?

14. ### ghaunMember

Joined:
Mar 12, 2021
Location:
West Chester, PA
In looking at the san-tee's, I believe that it has something to do with the greater curvature in 3" and 4" versions when entering the vertical, as compared to 2" and smaller, thus limiting the head-space in the pipe able to be used for venting.