Here is a confession. We've had a drain fly problem this summer. It was coupled with a nasty smell emanating from the bathroom attached to our office (the house's master bedroom but it works much better as an office - when it doesn't smell).
The bathroom seldom gets used, mainly because it was smelly. Dave never seemed to smell it, but to me it smelled musty, like the Pirates of the Carribean ride in Disneyland. I smelled it a few summers ago and it bothered me so much that I went in and proceeded to tear the whole bathroom apart. I made Dave install a new floor, new sink, and new toilet. Little did I know that the smell was coming from the shower drain.
Dave started smelling that mustiness over the last few weeks as the heat got bad. He was also annoyed by the drain flies we were seeing in the bathroom. Last Saturday he set about to clearing all the drains in an effort to get rid of the pests. It went really well in the main bathroom that we use daily. He got to the shower in the "spare" bathroom and while that seemed to be the source of the flies, it drained really well... too well.
He looked down the drain and thought he saw that it was broken. Later that evening, when the heat of the day wore off, he went under the house into the crawl space to check it out. While he was there he had me try out various taps, flush the toilets, and run the washing machine and dishwasher through to drain cycles (I forced them to the drain cycle so he didn't have to wait forever).
The news was not good.
It also looked and smelled really bad.
The main bathroom was fine, as was the toilet and the sink in the spare bathroom. The kitchen sink with garbage disposal, the dishwasher, and the washing machine, as well as the spare shower, all were draining into the crawl space. Dave thinks there were slow leaks but this summer a pipe ruptured and that was when it got pretty nasty.
Dave says it's actually not as bad as it sounds. Maybe.
So, we got the plumber in and he spent about 6 hours under the house fixing it. He's a great guy!!! Now we still have to clean up the mess and damage. Not good.