Old cast-iron drainpipe can behave unpredictably. Sometimes a small section of a pipe starts to crumble or a joint begins to leak even though most of the pipe is sound. If this is the case, you can easily make a repair. But if a cast-iron pipe is failing at several points, the most economical solution is to replace it with PVC pipe. This is a job for a professional.
When working with cast-iron, understand that the pipe is very heavy and must be held in place with special clamps attached to framing members. Never disturb these clamps; as if one weakens, a long section of pipe could come crashing down. Also, some older cast-iron joints were sealed with molten lead, but don't worry. This is not a danger to your health because only wastewater passes through the pipes.
If you need to patch a joint, expect to spend an hour doing so. If you're just patching a hole, it'll take even less time. Before you begin make sure the pipe is well-supported.
If water or a bad smell comes from an old leaded joint, use a cold chisel and hammer to gently tap the lead back into the joint. Use a wire brush and rag to clean away the corrosion.
Fill the resulting void with cast-iron pipe repair paste. Use a putty knife to apply the paste.
If a pipe rusts through or is punctured, clean the opening with a wire brush and fill the hole with two-part plumber's epoxy. Check the manufacturer's instructions for drying time. Don't use the pipe until the patch has completely set.