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Double p-trap?

frenchlanny Member Posts: 4
My washing machine standpipe in my 1965 house is in the outside wall with the p-trap just up from the vent to roof. The floor is concrete slab, no basement. I am moving the machine about 11 feet away, around a corner. I need to tie into that existing drain line. If I install my new standpipe with a p-trap prior to running the new drain line to the existing drain pipe, is that creating a double p-trap?

I have other questions. I think I understand that I can install a studor vent near my new machine location rather than joining to the existing roof vent. Is that correct?

Last question: The wall studs are 2X4. I believe building code allows for 1 1/2 inch drain line. Is there a structural problem with running the drain line through the 4 or 5 studs in the load bearing wall?


  • PC7060
    PC7060 Member Posts: 1,102
    edited November 2021
    Washers require a 2” drain line with a minimum of a 18” leg above the trap.  

    I recommend you completely rerun the line and remove the old trap.  You will not be able to run the pipe laterally through the studs. Can you run the new pipe on the surface of the wall and  run the horizontal portion over the connect to original drain? 

    Regarding drilling laterally through studs, this link will take you to a IBC site which describes the limits on notching and boring. 

  • Jamie Hall
    Jamie Hall Member Posts: 22,175
    Don't have to go to the link if you don't want. Just think... you need a hole a bit over 2 inches in diameter. a 2 x 4 stud is 3 and a half inches. That leaves you slightly less than 3/4 inch on both sides, assuming you manage to drill the hole straight. This is going to hold something up? I don't think so.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 10,914
    IMO, 1 1/2" for a washer is border line. Mainly because of suds that form when pumping out.

    I would surface mount the pipe, use 2" with a new trap down low at the washer.
    Add your Studor vent after the trap as high as possible.
    Boxing it in would be less work than drilling studs down to nothing being left.
  • Intplm.
    Intplm. Member Posts: 1,618
    Your idea of using a "studor' or a sure vent is a good idea.
    I cannot recommend drilling through the studs of a load-bearing wall.
    Try to run your drainpipe into your basement separately if you can, to then connect to the main drain there. Possibly in an inside corner of the rooms or closets, vertically.
    Also, run your horizontal line exposed too, box them in, or...frame them in, as not to see the pipes if seeing the exposed drain pipes is a concern.
    What you are creating is a dedicated vertical drain serving your washing machine in an updated 2-inch drain pipe.
  • mattmia2
    mattmia2 Member Posts: 8,888
    You could run the line in front of the wall and fir it out to the depth of the pipe if you are down to the studs, would have to protect the pipe with steel plates.
  • PC7060
    PC7060 Member Posts: 1,102
    Too funny. Four of us (so far) have said the same thing to the OP. Sure hope they come back to read ‘em
  • frenchlanny
    frenchlanny Member Posts: 4
    I agree with all of you. The new location for the washer and dryer is in the garage and it would definitely be less work to surface mount the 2" drain line. That is what I had expected to be the best option. I will be installing kitchen cabinets where the washer currently is so I will need to notch and work through the cabinet backs that will be in the way. I will be running the dryer vent along the same wall as the drain line will be but will not reenter the main part of the house like I will need to do with the drain line. My only concern is the necessity to remove the existing p-trap. It looks like it is built into the outer foundation footer. I hoped to avoid tearing into the concrete. With it being in excess of ten feet away from where my new standpipe will be will there really be issues?
  • frenchlanny
    frenchlanny Member Posts: 4
    P.S. If only I had a basement, I could easily remove the existing p-trap. It's a concrete slab floor. Basements are pretty rare in Oklahoma.
  • PC7060
    PC7060 Member Posts: 1,102
    Traps are to be immediately below drain inputs.
    Code does not allow two traps in series, potential for creating an air locked section.

    Cutting a standard slab is pretty simple, pre-tensioned slabs are a much more complicated.