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ADU Sewer System

Robert_AUT Member Posts: 3
Hi all!
New to the forum here - quick intro. My name is Robert and I was born and raised in Austria/Europe. I built my own house and other buildings there, construction is very different though. My biggest challenge here is to make sure I read the code right.
And with that, here's my question. I'm building an ADU on top of my garage at my house in Colorado. Since it's a small unit I'm trying to put all the plumbing in one 2x6 non load bearing wall. There is an 11-7/8 BCI underneath the subfloor that is limiting me somewhat (only want to cross once). Below is a schematic of what I'm trying to do. From what I read in the IRC it should be ok but don't want to overlook anything. Where I live the IRC was amended and doesn't allow the hot water heater pan or pan under the washing machine drain to the outside (freezing conditions). Following questions:

Do you guys think routing everything as shown below including vents is correct? What about pipe sizing (note: green should be 1-1/2in)? It will tie in with the existing sewer in the house (not shown here)

Do I need additional venting for the shower tub drain line or can I tie it in as shown?

What type of p trap is best used for the drain of the (electric) hot water heater pan? I understand I need an air gap there but also would like some type of p trap that can't dry out.

Thanks everyone!!


  • Intplm.
    Intplm. Member Posts: 1,960
    edited March 20
    May I ask why you are going to all of this trouble?
    There are other less intrusive piping practices available .
    Have you considered the use of a sewage ejector?

    Is the drainage above or below the main drain?

    Your drawing seems fine but it does not show pipe lengths.
    Another vent installed serving the shower will help and is probably required by your local code.
    That vent is required at least five feet or eight feet from a main vent pending on what plumbing code you are following.
    I would add a relief vent there.

    The water heater pan does not drain to a sanitary drainage system. It is an indirect waste and can go directly to the floor.
    Do not add it to the sanitary drainage at all. It will smell of sewage. Unless you are planning to install a trap primer, but I would not recommend that either.
  • Robert_AUT
    Robert_AUT Member Posts: 3
    edited March 20
    Hello and thank you for your comments! I'm always open to suggestions for redesigning the system. The way as outlined seemed to be the simplest way for me. I'm trying to only have one vent stack go through the roof (less possibilities for water issues). The ADU sits on the second level on top of my detached garage. I have enough grade to work with and won't need a sewage ejector. The 3in sewer pipe shown will be routed along the ceiling of the garage (boxed in an insulated) and through the garage floor then underground to the crawlspace and connect there.
    Wall to wall spacing in the sketch up is 8ft 3in. I need to measure pipe length but most likely will be right on the edge of the max. distance of 8ft from the shower trap to the main vent stack (2018 IRC).
    The problem I have with the water heater pan is that it is sitting in an insulated compartment in an attic above an unheated garage. Our county amended section P2801.6.2 in the 2018 IRC with "The pan drain shall extend full size and terminate over a suitably located indirect waste receptor or floor drain". I could just add a floor drain in the compartment for the heater. Just don't see another way than having that floor drain go into the sanitary drainage system. Installing a trap primer is a great idea, didn't hear of that type valve before.

    Thank for all the input!
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,297
    Hi, Might your inspector allow an air admittance valve? That would probably simplify the shower venting.

    Yours, Larry
  • Robert_AUT
    Robert_AUT Member Posts: 3
    Hello Larry,
    I thought about that too but am not a big fan of those. Plus I'm not sure where I would put that in this case. The trap will sit in an insulated space under the subfloor. I would still have to route it to somewhere different. Ideally I can stay below the 8ft max distance to the vent stack but just not sure if I'm creating a problem with that. Could it such the shower trap dry when flushing the toilet? That's my main concern.

  • Mustangman
    Mustangman Member Posts: 92
    You had better check with codes on the 8' from the vent stack being okay. I live in a class 3 city which requires plumbers to be licensed as a journeymen and masters. Right across the river, codes are different. In the bigger city, they push that ALL fixtures must be vented. The inspector gets that it isn't always possible to vent right off the trap. Typically, if you explain the situation to the inspector before you install it, he will strech that distance from a vent stack to a few feet or come up with another path. In the little city... They hire out inspections to a inspection firm. What a mess. They hire young kids who took an online course to become an inspector. Gosh the stories I could tell. The point is, check with the inspectors before you just override or misinterpret the written code. Trust me, asking first boosts the inspectors ego too. I have learned that pissing off any inspector is going to haunt you.... even if he is wrong.