Why is it that I sometimes smell sewage in my house?

The most common reason for this could be a ruptured sewer line under or near your house. The reason you may not smell it all the time is that the wind may take it away or there are may be intermittently ventilated. Sometimes, if bathrooms or sinks are not used for some times, the traps will dry up and allow gases to migrate back into the house. Drain traps are the “P” shaped pipes that are generally found under the sink. These traps hold a small amount of water in them that can evaporate over time. If you notice an odor coming from one drain, try running a little water through it and see if the smell goes away. If the smell is throughout the whole house, chances are you have a sewage leak.