Why do pipes get blocked? What you can do to prevent it

In waste water pipes, gravity is what keeps the flow moving, so every horizontal pipe must slope gently downwards in the direction of flow. If during a renovation the pipe is left too flat inside the screed to save space, or one end lifts as it is fixed to the wall, the water slows down. Slow water drops the sand, soap residue and hair it carries, and a thin ridge forms on the bottom of the pipe.

The typical sign of a fall problem is a drain that never blocks completely but always runs slowly. After a shower, water stays in the shower tray for a while and then drains away slowly.

A pipe that was laid at the right fall to begin with can still give way over time. PVC pipes in a suspended ceiling with widely spaced support clips soften as hot water passes through and droop between two clips. In underground lines, the same result comes from the fill beneath the pipe settling.

Some water always remains in the sagging section. This small pool turns into a reservoir where grease and sediment collect, and the blockage forms at almost exactly the same spot every time.

Every point where a drain line changes direction slows the flow a little. Using two gentle bends instead of a single right-angle elbow reduces the chance of a foreign object catching. In renovations, common mistakes include connecting a wide pipe to a narrow fitting or adding two bends one after the other.

Places where different materials meet are also risky. If a PVC drain pipe has been forced onto an old cast iron stack with a gasket instead of a proper transition fitting, a step is left on the inner surface at the joint. Hair and fibres catch on this step, and soap and grease stick to them.

Cast iron and galvanised pipes, common in older buildings, rust and scale on the inside over the years. A rough surface holds far more material than a smooth plastic pipe. Limescale deposits from hard water also build up layer by layer on these rough spots and narrow the inner diameter of the pipe.

On lines like this, a blockage comes back quickly even after it is cleared, because the surface that material sticks to is still there. Seeing the condition of the pipe also matters when choosing the clearing method, since a badly thinned cast iron pipe may not withstand rough treatment.

If the pipe leaving the building runs under a garden or pavement on its way to the street line, tree roots are a risk. Roots follow moisture and enter through the smallest gap at the joints, grow inside the pipe like a fringe and catch everything that passes, like a strainer. In old clay or concrete pipes, ground settlement, heavy vehicle loads or underground works can crack the pipe and partly collapse it.

The signs of these problems usually show up outside the building rather than inside: the water level in the manhole stays high, one spot in the garden stays constantly wet, or the ground sinks.

Even a correctly laid line blocks when it is misused, while on a faulty line good habits noticeably lengthen the time between blockages. In apartment buildings the shared stack means one flat's habits affect the flats below, so a short reminder from management to residents is also useful.