How to Connect a Black Plastic Pipe to a Cast Iron Drain


Building codes used to allow for lead, clay and cast iron drain/sewer pipes to be installed in homes. Today, however, though cast iron can still be used in many areas, most drain/sewer pipes are made of either PVC or black plastic known as ABS (acrylonitrile butadiene styrene). When joining a new ABS sewer pipe to an existing cast iron pipe, a flexible rubber coupling joins the two together. This process is fairly easy and takes less than an hour to complete.

Things You'll Need

  • Tape measure
  • Pencil
  • Handsaw
  • Utility knife
  • Soapy water
  • Brush
  • Flexible rubber coupling
  • Screwdriver
  • Eye goggles
  • Measure and mark the section of ABS sewer pipe to the correct length (mark it with a pencil). Make a straight perpendicular cut through the pipe at the mark using a handsaw. Scrape away all plastic fragments or burrs from the cut pipe end using a utility knife.

  • Mix a solution of soap and water. Brush the solution onto the outside end of the cast iron drainpipe. Push the flexible rubber coupling halfway onto the end of the drainpipe.

  • Brush the solution onto one outside end of the ABS pipe and push the pipe into the other end of the coupling (the coupling will now be half on the cast iron pipe and half on the ABS pipe). Tighten the metal hose clamps around each end of the coupling, using a screwdriver. The coupling now has a water-tight seal to both pipes.

Tips & Warnings

  • A hacksaw, circular saw or powered miter saw can also be used to cut through the ABS pipe (the miter saw will make the straightest and cleanest cut).
  • If joining two pipes of different diameters, flexible rubber reducing couplings can be used. However, make sure local building codes allow for two different size pipes to be joined in one sewer line before installation.
  • Wear eye protection when sawing through the ABS pipe.

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  • Photo Credit handsaw image by AGphotographer from
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