I found signs that it backed up again this morning, and proof this afternoon as a pool rose out of it when the washing machine drained. I called the Drain Doctor, and when he arrived I talked him through what I knew about the plumbing. He snaked back from the French drain, determined that the blockage wasn’t there, then went for the inside soil stack cleanout.
An aside: I don’t know how many other houses in our neighborhood have the peculiarities of our plumbing. The French drain is there because the driveway extends all the way to the back of our house, and is located right in front of our garage, which is in the basement. There’s a downward slope that used to pick up storm water from the street and take it down to the drain, in really heavy rain; fortunately, our driveway paving fixed that, but still any rain runoff from the driveway itself goes into the drain.
The drain, it appears, connects directly into our sewer pipe, with only a back pitch (it slopes up into the house) keeping nasty stuff from flowing up. Which means that when the main sewage line gets blocked, as it did tonight, the overflow went out back and came up the drain.
So the Drain Doctor guy snaked the line until we realized that he was hitting a blockage somewhere in the front of the house. We found another cleanout in the sump pump pit, which he opened up, and as it was somewhat, er, full, he realized he needed to snake both forward and back of the cleanout.
Which he did, over the course of about forty minutes. I take back everything I’ve ever said about plumbers: they earn their money. At the end, he sucked up all the stuff that came out of the cleanout into his own shopvac, poured his own Clorox into the sump pit to clear the smell, and left once he confirmed that we had clear running water going out the pipe.
The sad thing is, I’m pretty sure the blockage was caused by the previous owner, because tonight’s plumber pulled out some of the same things that our plumber in 2004 found—it just had never snarled up enough since then to be a problem, apparently. But it’s a good thing our pipes are clear, because I really feel like I need a shower.