Sometimes I smell sewer and it seems that the drains are slower. What might be causing this?

It is not often that you will smell sewer in your home in the city. I have however come across a couple situations where conventional thinking did not find the source of the smell. My normal procedure is to check the traps in the basement floors as it is likely they may have dried out. Next I would look for venting issues. If a vent is not properly designed it can cause a trap to syphon a trap dry and the sewer gases will travel up through the open trap. Finally I check the actual sewer line for breaks. If none of these things look to be the source I check the vent on the roof. In the winter season the vent stack can frost over and plug the vent off. If the pressure in the sewer system is great enough the sewer gases can apparently travel through the water in the traps causing the smell.