Before building code changes, sewer lines in older homes were made of clay, cast iron or lead. New homes generally have sewer lines made of either PVC or ABS hard plastic. When connecting a PVC toilet flange to an existing lead sewer pipe, the connection between the two materials is made with a flexible rubber coupling. Toilet flanges look like a short section of plastic pipe with a lip attached at one end, which has screw holes used to attach the flange to the bathroom floor.
Measure between the top of the lead pipe and one inch below the surface of the bathroom floor. Use a handsaw to cut a section of 4-inch PVC sewer pipe to that length. Make the cut straight, and scrape away all burrs or plastic fragments from the cut with a workman's knife.
Slip a rubber flange halfway onto the end of the lead pipe. Push the newly-cut section of PVC pipe into the other half of the rubber flange (the flange will new be half on the lead pipe, and half on the PVC pipe). Tighten the metal clamps around each end of the flange with a screwdriver.
Apply PVC primer around the top outside end of the section of PVC pipe. Also prime the inside bottom of the PVC toilet flange. Apply PVC cement to both primed areas and push the flange down onto the PVC pipe, so that the flange lip rests on the floor. Install 1 5/8-inch galvanised screws through the holes in the flange's lip and into the floor. The flange is now secured to the led pipe.
If the top of the lead pipe is level with the bathroom floor, cut the pipe so that it is one inch below the floor surface (use a hacksaw). Push an instant set closet flange down onto the lead pipe, and secure the flange to the floor with screws. This type of flange is usually made of cast iron and has a rubber gasket on its inside which creates a watertight seal between the lead pipe and flange.