My house was built in 1949 and renovated in 2001 (just before I moved in), and is built on a slab. The only original bathroom starting having a sewage smell every few months several years ago. This bathroom was not used regularly, so I would just run water in the sink and bathroom, and the smell would go away. Over the years, the smell has become more frequent and stronger. Now it's daily. I have had the wax seal replaced, and the toilet. And the smell remains. The plumber stated that the flange was tilted (that's why he used 2 wax seals). The smell still remains, but not as strong. There is no backup or gurgling when the toilet is flushed. The city did find roots in the portion of the line that they are reponsible for and cleaned it out, but the smell still comes back.