Where this starts
Waste pipework is a tree. Each fitting has its own trap and short waste, those join a branch, branches join a stack, and the stack goes to the drain. A blockage stops everything upstream of it and nothing downstream, so the set of affected fittings tells you where along that tree it is.
SO RUN THE OTHERS. Fill and release the bath, the other basin, the kitchen sink, and flush the WC, and watch what each one does. If only the original fitting is slow, the problem is within a metre or so of it. If its neighbour is slow too, the blockage is past the point where they meet. If something gurgles or bubbles when another fitting drains, air is being pushed or pulled through a trap, and that means the restriction is further downstream than either of them.
WHAT ACTUALLY BLOCKS EACH ONE differs by fitting and it is worth knowing before dismantling anything. A KITCHEN sink blocks with congealed fat and food solids, usually in the trap or the first bend, and it builds over months. A BASIN blocks with hair and soap scum, almost always in the trap or the pop-up waste immediately above it. A SHOWER or bath blocks with hair at the trap. A blockage further out on a shared branch is more often an accumulation of all of those plus scale.
THE ONE TO CHECK FIRST, because it takes seconds and is commonly the whole answer: the pop-up waste mechanism in a basin. The plug linkage sits in the flow and collects hair around itself, and lifting it out often removes the blockage in one piece with no tools at all.
WHAT NOT TO REACH FOR FIRST: caustic drain cleaner. It works on soft organic blockages and does nothing to a solid obstruction, so on a real blockage it leaves a pipe full of caustic liquid for whoever opens the trap next — and it attacks some plastics and some older metal wastes. Mechanical clearing is both more effective and safer.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Hair and soap in the trap or the pop-up waste
Most commonOne basin or shower only, building gradually over weeks, often with visible material when the plug or the waste is lifted.
How to check: Lift the plug mechanism out. In a basin it is where hair collects and it needs no tools.
Fat and food solids in a kitchen waste
Most commonThe kitchen sink alone, worsening over months, worse after washing up in cool water, often draining better with very hot water briefly.
How to check: Whether hot water temporarily improves it. Fat that softens and re-sets is the signature.
A blockage in a shared branch
CommonTwo or more fittings slow together, typically in the same room, with water sometimes appearing in one when another drains.
How to check: Run each fitting and watch the others. Two affected means the restriction is past where they join.
A waste run with too little fall, or a sagging pipe
CommonA fitting that has drained slowly since installation or since a unit was fitted, with no accumulation to find, often on a long run under a kitchen.
How to check: Whether it has ever drained fast. A run that was always slow is geometry, not debris.
A blocked or restricted stack or drain
CommonSeveral fittings slow, gurgling elsewhere when one drains, or a WC that empties slowly — and sometimes a smell.
How to check: Whether anything gurgles elsewhere. Air being drawn through another trap means the restriction is downstream of both.
A trap that has been replaced with the wrong one
Less commonSlow draining after work was done, often with a bottle trap fitted where a tubular one was, or a flexible pipe with sags in it.
How to check: Whether it changed after work. A bottle trap restricts flow more than a tubular one and a concertina pipe collects everything.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
One slow fitting, two sharing a branch, or everything, are three different positions along the pipework, and nothing else locates the blockage as cheaply.
- 2
Air being drawn through a neighbouring trap means the restriction is downstream of both, which takes it out of the room entirely.
- 3
It is where hair collects, it is above the trap, and it frequently removes the whole blockage in one piece with no tools.
- 4
A fitting that has been slow since it was installed has a fall or a pipework problem, and no amount of clearing will find something that is not there.
- 5
Fat softening and re-setting further along is the kitchen signature, and it tells you the blockage is soft and where it is likely to be.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where several fittings are affected or something gurgles elsewhere, since the blockage is in the stack or the drain rather than under the sink.
- Where a WC is also draining slowly, which points at the branch or the drain and is not a trap problem.
- Where waste water is backing up into a lower fitting — a shower tray or a ground-floor gully — which means the drain is surcharging.
- Where caustic cleaner has already been poured in, because the pipe now holds a chemical that will come out at whoever opens it.
- Where the run is concealed in a wall, a floor or behind fitted units and cannot be reached without removing them.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.