Bought a house built in the 70’s & is on a slab. On a septic and has an old system (remnants) and a newer system. In the master bedroom was a closet that had bi fold doors and then a dry wall and then a regular door to access the same closet. I tore the wall between the doors to open it up and the is a black pipe that’s capped off and extend about 5/6 inches. How do I tell where this goes, what it is, is it even used? Have pic but unsure how to load

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What size is the pipe? Is it steel or plastic?

Plastic and the cap measures about 2 1/4 inches

Since it's plastic it's probably part of the drain/vent system. It's not in use but is probably still attached to the drain. It would probably be a lot of work to get rid of it completely but it could be done. It would probably be pretty easy to cut it down and cap it so it only stuck up a couple inches

it no doubt an unused drain pipe.
i would cut it flush with the slab floor, pack some newspaper in the bottom and fill the pipe with hydraulic cement.

cut it off flush with the floor and glue a test plug/cap into it.

use an internal cutter, cut 1/4" below finished floor - glue on cap - done

cutter:




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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638