Finally figured out why my sink was backing up

Same thing happened to me in my old condo, except it was the kitchen sink and it only acted up on weekends when we were out all day. Water would come back up through the drain with little bits of food in it, which made no sense since the sink sat unused for hours. My plumber buddy came over and pulled the trap apart under the sink. It was packed solid with grease that had hardened into a waxy lump, plus some coffee grounds stuck in there. He said those quick liquid drain openers I'd been dumping down there every week were doing basically nothing on grease like that. He scooped it out by hand and we ran water for a solid ten minutes to test it.

The part that surprised me was the vent. He asked if the building had ever cleaned the vent stack, and apparantly in a lot of condo buildings those get neglected for years. A partly blocked vent can make a slow drain act worse than it is, especally when other units are running water at the same time. I brought that up with our management company and they actually scheduled a vent check a few months later.

Worth asking them what their records show for the last time the building lines were properly cleaned, not just snaked. Snaking pokes a hole through a clog but doesn't scrape the pipe walls, so grease and sludge start catching again pretty fast. That's been my experience anyway.

Our bathroom sink in a second floor unit used to do the same thing, water sitting fine for days then coming back up with gunk while we were at work. I chased it the same way you did, cleaning out the stopper and the first few inches of pipe, pouring boiling water and dish soap down every week. Helped for a while but it always came back. What finally cracked it was noticing the timing. It only backed up on weekday mornings, right when the units above us were all showering. Turns out the water coming down from upstairs was hitting a partial blockage further down the shared run and pushing back into our sink, the lowest easy outlet. I asked the building to run a camera through the common line and they found a belly in the pipe where the old cast iron had sagged, catching everything that flushed past my unit. That was on them to fix, not me, and once they jetted it out the problem stopped for good.

So if your backups only happen when you're out during peak hours, it might be worth keeping a simple log of the days and times. If there's a pattern tied to when other units are using water, that's your leverage to push management to scope the shared line instead of shrugging at you.

I had this kind of trouble with my laundry tub drain in a previous place. It didn't back up all at once, but the water would just go down relly slowly, like it was struggling through thick sludge. I thought maybe the thing right at the drain hole was blocked up, so I tried using a stiff brush on the outside of the drain opening. That totally didn't change anything. I guess I just assumed the blockage had to be far down in the pipe. Does the stuff that gums up the pipes change a lot depending on if it is a sink or a washing machine drain?

You know, when I dealt with a run-off issue in my old place, it wasn't always the soap scum. Sometimes it was just mineral scale buildup from hard water, especially around the flange connections. Modern traps are good, but the joints themselves can calcify over the years. Instead of just pouring stuff down, I had a mason friend come with a small pressure washer and blasted those connection points. A heavy snake just pushes through, but blasting the area can really scrape the scale off the pipe walls where the backup starts. That's how you stop it from coming back!

Update: clearing out the trap fixed most of it, water goes down like normal now. But it still burps up a little gunk when we're gone all day, which sounds like the guy whose kitchen sink only acted up on weekends while they were out. Makes me wonder if the drain line is fine and it's actually a venting problem. Has anyone had a blocked vent cause backups only when a fixture sits unused?