Plumbing the depths

Been quiet for a few days, I know, but not because nothing is going on. The shed is almost finished out back (some rain and snow (none that stuck) delayed things, but the builder is working hard at this moment in some near-freezing temperatures, and the thing should be finished, including roof shingles, tomorrow) just in time for it to be too cold for me to want to haul anything out there.

Meanwhile, I came home Saturday to find what looked like a minor mud explosion in my downstairs bathroom. Something had forced a lot of the gunk that built up in the pipes up out of the drain and splattered it around a bit. The cleanup was quick and easy but clearly there was a pipe problem. Still is, actually. As I write this, I've located several small leaks in the drainpipes (shower, sink, toilet), none of which should be too hard to fix - that's another post. The bigger problem -and what likely caused the eruption - is a severe clog somewhere in the line.

The water in the pipes is clear - run the shower and it runs down past the trap no problem, for example. But run any of the three long enough and you can hear burbling from at least one of the others; when I used a plunger to test the drains I'd often hear it in both the others. All three - plus the outflow from the washing machine upstairs, and an uncovered(!) vent outside which I've recently covered - feed into a main standpipe and I'm guessing that's where the problem lies:

Trying to figure out what to apply to the clog, though, and how is the trick. With the traps on all three bathroom fixtures - and the clog looking like it's past the traps - it seems like pouring something down the laundry pipe would be best. There's a short run of PVC at the top, though, and the best cleanser I have is lye - I'm reading that lye is not good for PVC (and absolutely not good for aluminum, galvanized steel, and other specific metals with which it can react; fortunately I have none of that - just copper, cast iron, and pvc). I've ordered some enzyme cleaners which will be good for keeping the pipes clear in the long run, but I need to get this clog out soon if I want to actually use my bathroom again - 45 second showers don't really work.

The sink is mostly PVC to the stand pipe. The shower looks like all copper from the drain down. But again both, and the toilet, have a full trap somewhere on the line so I think I'll try the laundry pipe first - if a dose or two does damage the PVC it should be a lot easier to replace that where it connects to the cast iron on the second floor than any of the other lengths.