Under the basement concrete floor is a 4 Inch diameter ABS sewer pipe. I am renovating and would like to add a 1 1/4 inch ABS sink drain to it. I can only access the top of the pipe. Is there any way to do this?

Hi Bettyy...

There is no way to legally tap into a horizontal main drain line without installing a WYE fitting. Here, you'd just need to chip out a couple feet of concrete and install the wye fitting. You would install a 4"x2" wye and then install a 2" test tee cleanout just out of the floor to keep to code. You could reduce the pipe to 1.5" drain and then run that to the sink where you would then reduce to 1.25" at the PTRAP or at the transition fitting from the PTRAP to the sink drain tailpiece. You would also need to install an 1.5" vent for the sink to drain properly and to keep sewer gasses from entering the house, OK?

With all that being said, they do sell a SADDLE VALVE that can be used to connect in from the top of the ABS pipe but you would still need to access the bottom of the pipe to tighten the clamps and saddle valves are definitely illegal and will be flagged as such by any home inspector should you sell the place down the road... ;)

Questions? Let me know...

Mark