This problem has baffled us for years and stumped the one plumber we asked about it: during the winter months, particularly when the outside is very damp and heavy, the bathroom in the lower level of our hi-ranch bungalow in Manotick (ON) develops an unbearable odor of raw sewage. The odor comes on quickly and disappears as quickly, usually the next day or so. We stuff towels under the bathroom door to stop the odor from spreading throughout the house. I don't see any signs of anything actually backing up into the house. Just this odor. There is a pipe coming out of our roof above where the bathrooms are. Could it be that this is a release for the septic fumes and that in heavy damp cold weather, the fumes back up into the room, rather than go outside? This never occurs in the summer.
Answer:
You might pay attention to which direction the wind is coming from during these attacks. I think you'll find that it is always from the same direction - and it doesn't have to be a strong wind, a breeze will do it. There is a T you can install at the top of the vent stack on the roof and you might find that that redirects the exhaust so you won't smell it again. The gasses you smell are all lighter than air so there is no real way that they'd be prevented from going up the stack.
