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cshaffer 04-18-2008 01:24 PM

Tapping into 3"DWV line under basement slab
I am in the process of adding a bar sink in my finished basement and I am going to run a 2" drain line under the floor. I have a 3" DWV line under the floor about 6' from where the sink will go. The 3" line runs directly into the house main 4" DWV out to the sewer about 4' from where I need to tap into it.

Forgive my ignorance here, but my question is:

After I jackhammer the floor open what is the easiest way to connect the 3" > 2" wye? There won't be much give/flex in the 3" pipe to slip the wye on the line without hammering up an extra 8 ' of flooring (not preferred). Is there a technique or fitting I need to use in order to install the wye in the middle of an existing line that is still mostly under the slab?

Also, the bar has no access to install a vent, would an AAV under the sink be acceptable in this case?


Boston Plumber 04-18-2008 04:25 PM

2 Attachment(s)
Hello cshaffer:

The answer to your question lies in using shielded couplings...see picture.

Here, depending on TYPE of underground pipe, these shielded couplings allow legal (plumbing code legal) transition from say 3" cast iron pipe to 3" pvc pipe (these pipes are different thicknesses and these clamps make up for difference by making rubber sleeve thicker on one side).

So, if adding 3" x 2" wye to pick up sink and drain underground is 3" cast iron you would need (2) 3" cast iron x 3" pvc mission clamps (shielded couplings/same thing), (1) 3"x 2" pvc wye, 3" pvc pipe, cement/primer and all the other fittings you will need.

You will want to add say a 4" piece of PVC pipe into each end of the 3"x 2" pvc wye. Then measure the length of the fitting with pipe installed at both ends....Say you measure 12" overall -->> That means you want to measure and mark 12.75 inches on the pipe underground and then make STRAIGHT sawzall cuts on these marks.

Why the extra 3/4"?? So you can install the clamps, bend rubber sleeve back, slip wye in, and FLIP rubbers back so cover pipe ends. Now install the clamps over the rubber , tighten appropriately and the WYE is installed!!

I recommend using the new LENOX DIAMOND sawzall blade to make theses cuts. The new diamond saw should cut through 3" cast iron in 10 minutes or less each side.

I have assumed cast iron underground pipe. If PVC underground, then same steps as above, only will just need 3" pvc mission clamp or similar...;)

When come out of the floor with new 2" pipe, use a 2" long sweep elbow and add a 2" Dandy cleanout (see pic.) before going off to pick up bar sink.

Hope that helped some. Good luck.

In terms of an AAV...if they are allowed by code in your area, they work fine. Just make sure there is a readily accessible access panel so can replace it in future if needed.

cshaffer 04-22-2008 08:07 AM

Thank you for the guidance, it was a big help knowing exactly what to buy and how to do it. It was much easier than I had imagined. A trip to HD and 20 minutes of work and done. Everything was PVC so cutting was no problem at all.:thumbsup:

Boston Plumber 04-22-2008 04:48 PM

Glad we could help...;) Anytime. Thanks for the update.

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