How to unclog a blocked bathtub

Bath plugs look alike from the outside, but they work differently inside and each one comes out in its own way. If the plug goes up and down when you press it, if the overflow cover has a lever or a turning knob, or if the stopper is attached by a chain, you already know your type.

Set the click-clack plug to the open position, hold the cover and turn it anticlockwise, and it lifts out. On some models the cover will not turn by hand; in that case, gripping the edge of the body under the cover with a dry cloth stops your hand from slipping. The first thing to come out from underneath is often a grey, slimy bundle of hair, longer than you expected.

With a lever plug, you first undo the screws on the overflow cover. Together with the cover, a bent rod comes up out of the channel, with a weight or a spring on its end. That rod and spring are very good at catching hair, and if you refit them without cleaning them the plug will not open fully and the water will still drain slowly.

A chain stopper is easier, but if the grate under the stopper is screwed down you need to remove that too. You can pull out the hair left behind the grate with a plastic toothed hair strip.

The overflow channel is a separate path that carries water to the drain when the bath is overfilled, and it joins the main drain pipe under the bath. Because water does not pass through it all the time, soap and mould build up there unnoticed, and over time they cause a smell and narrow the flow at the junction.

As soon as you have removed a lever plug, you can reach into the channel with an old toothbrush. The cover seal is another part to check; a hardened seal can let water leak behind the bath when it is full.

While you clean the channel, do not rush when lowering the lever mechanism's rod back in. If the rod goes in at the wrong angle, the weight on its end will not seat properly in the plug body, and the bath will either never fill or will slowly leak water even in the closed position.

On a bathtub a plunger is almost useless unless the overflow hole is blocked. Each time you press, the plunger pushes water into the pipe, but if the overflow channel is open the pressure escapes up through it and never reaches the blockage. Pressing a wet cloth over the hole, or asking someone to cover it with their hand, changes the result.

In many bathrooms the bath, the sink and the floor drain join in a single pipe under the floor and run to the shared stack. If the blockage is past the point where they join, then as you empty the bath you will see gurgling, bubbles and dirty water rising from the sink's drain hole or the floor drain.

With this symptom the flow will not improve however thoroughly you clean the bath's own plug, because the restriction is in the shared pipe. If you live on the lower floors and the water in the bath rises when a toilet is flushed, the problem is most likely in the building's shared stack, and it is right to inform the building management.

If you have cleaned the plug, brushed the overflow channel and used the plunger correctly, but the water still stands in the bath, the blockage is somewhere hand tools cannot reach. At this stage do not try to open up the area under the bath or break the bath panel; most of the pipework under a bath can be cleaned by reaching it through the drain opening and, where there is one, the side access hatch.

The Cantley team sends a camera in through the drain opening to show whether the blockage is in the trap, the junction elbow or the shared stack connection. Hair and soap build-up is cleared with a motorised drain snake, hardened deposits with high-pressure water jetting, and the bath is not moved from its place.