Warning one — it's slower than it used to be. Nothing dramatic. The basin empties a little less quickly than it did last year, the shower has a shallow film of water around your feet by the end, the kitchen sink takes a beat longer. Most people never register this as information because the change is gradual and each week resembles the last.
What it means: something is accumulating. Grease coating a kitchen line, hair binding at a bathroom trap, or scale narrowing an older pipe. The pipe has begun to lose diameter.
What it costs to act: nothing. Strainers, a change of habit, and hot water flushing. This is the stage where the whole problem is preventable.
Warning two — water pools, then clears. You run the tap and water gathers in the bowl before draining away. The shower drains, but only after you've finished standing in it. This is the stage people usually notice and then tolerate, often for months.
What it means: the accumulation has narrowed the pipe enough to restrict normal flow. It's no longer early.
What it costs to act: a plunger, a trap clean, or a scheduled drain call at standard rates. Still cheap, and still on your terms.
Warning three — gurgling, or a smell. The drain makes noise as it empties, or a smell arrives that comes and goes. Sometimes another fixture gurgles when this one drains.
What it means: air is struggling to move through the system, which happens when a line is substantially restricted — or, in some cases, when the vent side of your drainage isn't working. A smell from a drain that still flows can also be a trap that has dried out in a fixture nobody uses, which is free to fix.
What it costs to act: a scheduled service call, and it's the last comfortable moment.
And then it blocks. Usually at the least convenient time, because that's when the most water is going down it.
What it costs now: a callout, at whatever rate the hour carries — plus whatever's on the floor if it overflowed.