Cast iron pipes are strong and durable, and that's why they were popular for decades as plumbing pipes. Cities used cast iron pipes to route water to homes, and homes and businesses used cast iron pipes for fresh water and waste lines. When other pipe materials became more popular, cast iron pipes were used less. The practice of using them for plumbing hasn't been standard practice for decades now. That means if you have cast iron piping in your home, the pipes are pretty old and near the end of their lives.