Distance one — the trap. Under the fixture, within arm's reach. What you'll see: one fixture affected, everything else in the property draining normally. Water backs up in that basin, sink, or shower and nowhere else. Often a smell. What's in there: hair and soap in a bathroom, grease and food waste in a kitchen, or something dropped. How it's cleared: a plunger, or removing the trap directly. This is the distance where you can frequently fix it yourself, and it's the cheapest possible outcome. What it usually means: a habit, not a defect. Strainers and a grease routine prevent almost all of it.
Distance two — the branch. Inside the wall or floor of that room. What you'll see: one fixture blocked but plunging doesn't hold, or two nearby fixtures affected together — a basin and a bath, a kitchen sink and a dishwasher standpipe. Sometimes a fixture that backs up when a neighbouring one drains. What's in there: the same materials, accumulated further along where the pipe changes direction. How it's cleared: cabling through the fixture or a nearby access point, or jetting where grease is the cause. What it usually means: accumulation over time. This is the most common distance for a professional drain call.
Distance three — the stack. The vertical pipe serving multiple floors. What you'll see: fixtures on more than one floor behaving oddly, gurgling as other fixtures drain, or water appearing at the lowest fixture connected to that stack. What's in there: accumulation at the base of the vertical run, or a venting problem at the top of it. How it's cleared: access at a cleanout or the roof vent, cabling or jetting depending on what's found. What it usually means: in a house, a genuine restriction. In a multi-unit building, it's frequently shared and not caused by the unit reporting it.
Distance four — the building drain. Horizontal, under or beside the property. What you'll see: multiple fixtures across the property, and water rising in the lowest opening — a basement floor drain, a ground-floor shower — when a toilet upstairs is flushed. What's in there: substantial accumulation, or the beginnings of a structural problem. How it's cleared: through a cleanout, with cabling or jetting, and frequently with a camera afterward. What it usually means: stop using water in the property. Everything sent down returns to that lowest fixture.
Distance five — the lateral. Outside, between the building and the public main. What you'll see: the same symptoms as distance four, plus one or more of: recurrence on a roughly annual or seasonal cycle, backups that worsen after heavy rain, sewage smell in the yard, or an unusually soft or lush patch along the line's route. What's in there: roots, grease accumulation, a belly holding standing water, offset joints, scale, or a collapsed section. How it's cleared: cabling or jetting through a cleanout — and then a camera, because at this distance it usually isn't a blockage problem. What it usually means: a defect. Clearing manages it; finding the entry point or the deformation fixes it.