That existing 4' high loop is really delving into a rather gray area of the code. Normally the trap is considered the official point of transition between an appliance drain line and the house DWV system and in that case the manufacturer of the appliance may determine the required diameter of the appliance drain line which is determined by the volume and velocity of the discharge from the appliance. Generally the manufacturer will print a table in the installation guide that defines the size, length and permissible vertical lift of the drain line with recommendations to increase the line size if you need to exceed the length or vertical lift. That table will also define a maximum length or lift which is determined by the output pressure of the pump in relation to pipe friction loss or maximum vertical static head. In most cases the code will require that the appliance drain must be in an accessible location to permit servicing in the event that line should become obstructed. Whenever a drain line is permanently installed in the wall it is then classified as a portion of the structure DWV system and it would require that there must be a trap at the beginning of the concealed section of the line and the minimum permissible diamter of the line would then be determined by the DFU (drainage fixture unit) load value of the appliance or fixture being served by that drain. In the case of a dishwasher the DFU load is 2DFU's which would require a minimum of an 1-1/2" drain line. The plumbing codes also require that a dishwasher, washing machine, HVAC unit or a home water treatment discharge line must also be connected by means of an "indirect waste receptor". Under some local codes you could achieve the indirect waste receptor by installing an "Air Gap" or they may require you to install a standpipe on the trap and connect the dishwasher drain to a standpipe in the same manner as a washing machine line is commonly connected to a standpipe. (Some codes require all dishwashers to have an air gap, regardless of how the drain is connected. Check your local code.) Under the UPC whenever we have two common vented gravity flow drains connecting to the same vertical riser we are required to use a "Back to Back Tee" (4-way) however code prohibits connecting two commonly vented drains by means of a Back-to-back if one of those drains is receiving the discharge from a powered drain I.E. a dishwasher or washing machine. The discharge from a powered drain is required to be connected by means of a directional fitting (baffled Tee) and generally the baffled tee should be placed lower than the gravity drain. In your case the simple solution would be to connect the dishwasher to the disposal input port, providing that is permitted by your local code as well. (For reasons known only to the AHJ some codes prohibit connecting a dishwasher to a disposal input port..One code that immediately comes to mind here is the Illinois State Code.) Your screen name is Tom-in-Seattle so I am assuming that you are actually in Seattle. I am not certain but i believe your code is modelled after the UPC (Uniform Plumbing Code). If so the defined trap and waste arm size for a kitchen sink is 1-1/2" but under the UPC all vertical risers and drain lines serving a kitchen drain must be a minimum of 2" pipe. You may use an 1-1/2" pipe for the vent. There is some question about whether the length of the horizontal offset on the vent in your attic would pass, but I doubt that it would even come into play unless you happen upon a hardnose inspector that is really intent on finding a problem.

Hi there, would anyone mind giving me a ballpark figure of what the slab-smashing, drain-dragging might cost? My husband and I totally remodeled our last kitchen, we had a crawlspace then, for $16k in 4 months. We had it down to the studs & subfloor and took out a wall. We bought a new house over the summer and it's on a slab. After living here for a couple of months we can see that the floor plan of the kitchen, living room & dining area doesn't really work for us. But since neither of us have ever lived on a slab we don't know what to expect if we did it ourselves, or if we would have to have someone come in and do the work. So, any info you have with respect to who did the work, what got moved and how far, and what that part of the remodel cost, would be very helpful. Thanks!

Thanks, Vith. The goal is to gain counter space to one side of the existing sink cabinet, and I thought we might gain enough by shifting the sink cabinet as far to one side as the existing plumbing (inflows) allows. We may end up moving plumbing anyway, but we're on a tight budget so I was looking at less costly options. But if I understand correctly, you're saying the drain could still be in the center of the sink; is that right? I like the fact that an offset drain allows you to empty the strainer without removing, say, a large baking sheet from the sink, so I may opt for the offset drain anyway. I thought it might also simplify the under-sink plumbing as well.

Did you buy a new sink and cabinet? If you have the cabinet out, it is usually not too difficult for a plumber (or maybe a good handyman) to open the wall and cut into the vertical drain pipe and move the sink drain connection to a lower point on the pipe. Assuming the vent pipe runs up, it can stay in its current location. The fitting mounted lower on the drain pipe would allow more space to fit the trap for a deeper sink. Wall repair is relatively simple because its all going to be hidden under the sink area where a wall patch won't even be visible. If you look at this picture you can see how it is typically connected and vented. https://inspectapedia.com/plumbing/Plumbing-Vent-Distance-Codes.php The plumber would just move the connection lower on the vertical pipe. If you do have issues with reconnecting the vent, you should be able to use a studor vent (air admittance valve) as a replacement. Bruce