clogged drain suddenly drains on its own
I'm a plumbing novice so maybe this isn't uncommon, but it was weird to us and would like to find out what may have happened. Our main line backed up into our shower after my wife flushed the toilet. She was mortified and used a plunger for a good 20 minutes to clear the shower drain, but not surprisingly the plunger had no effect. Than we tried a bladder and garden hose, but it didnít help either. She then gave up and started searching for another solution. (Like I said, she was mortified and didnít want to call a plumber just yet. Based on previous experiences, I was ready to call a plumber as soon as it happened). After roughly 20 minutes of just sitting there without us doing anything, the drain suddenly cleared on its own and drained quickly. Things now drain better than in ever in the 5 years weíve lived in the house. For example, whenever the dishwasher would drain, there was always some backup into the kitchen sink, but now that's gone.
So, my questions: What the heck happened? Is this common? Normal? An indication of anything?
Some background that may or may not be relevant: The house was built in 1952. Itís on a slab. The main line backed up in 12/29 and 4/11. Each time a plumber cleared the line and things were better. The dishwasher still backed up in the kitchen sink, but it was much improved. But each time, the things slowly got worse. Four years ago, we had to replace the p-trap in the shower.
Thanks in advance.