I want to install a grease interceptor in my house. The problem is that it will not fit under the sink, so I plan to put it in the basement.

The problem is that the sink, laundry and dishwasher all drain into a 4" drain that goes into the basement. I want to put the interceptor in line with this drain, which would mean I would have a 4" pipe connected to the 1.5" interceptor inlet, and then the 1.5" interceptor outlet would connect again to the 4" drain and go on to the rest of the sewer.

Will I have a code problem doing this?

  • You may be able to reconfigure your sink plumbing to drop a 1.5" line down through the floor so you can install the grease trap below. You just need to make sure it's vented properly so as to not create a S-trap under the sink. Maybe post a photo.
    – isherwood
    Commented Feb 20, 2019 at 17:50

2 Answers 2


It's a problem, with drains you can never reduce down you must always increase. The the 4"pipe going into the 1 1/2" is a problem.

  • Well the items (sink laundry dishwasher) only require a 1.5" drain, so is my only way around this to rip out the 4" drain and install a 1.5" drain. Seems kind of crazy. Commented Feb 20, 2019 at 15:44
  • When wastewater is funneled into a reducer it can back up quite a bit. It also builds up debris.
    – Joe Fala
    Commented Feb 20, 2019 at 15:50
  • I understand that, but the lines feeding the 4" are 1.5" so in this case the interceptor will not be restricting anything. Commented Feb 20, 2019 at 16:44
  • From a code perspective (at least in my area) it's not allowed and the worst thing that can happen is an inspector can make you change it. In practice it's generally a bad idea but I see your point. I would advise changing the pipe. I've seen some awful plumbing atrocities that have worked extremely well for years and years,but they still weren't to code. If it's your house, use your judgement. Worst case, it doesn't drain well or backs up down the road.
    – Joe Fala
    Commented Feb 20, 2019 at 16:59
  • 2
    @TylerDurden, you have to consider the combined flow of all conjoined pipes. Your statements above about the 1.5" lines are misguided. A 1.5" stack wouldn't meet code for that reason and isn't a solution here.
    – isherwood
    Commented Feb 20, 2019 at 17:47

drains must not reduce its the law

  • 1
    One line answers are discouraged, could you provide the code reference (although I do agree).
    – Ed Beal
    Commented Feb 20, 2019 at 19:32

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