Grease, and why hot water makes it worse
Cooking fat goes down as a liquid, which is what makes it feel harmless. Twenty feet along a pipe under a slab, at ground temperature, it is a solid again — and it has coated the pipe wall on the way rather than passing through.
The common advice is to run hot water after it. That does not solve it; it moves the problem. The grease stays liquid a little longer and solidifies further down the line, where it is harder to reach and where it starts catching everything else that passes.
Detergent has the same flaw. It emulsifies the fat temporarily, and the emulsion breaks once it cools and dilutes.
The only thing that actually works is not putting it down there. Pour cooled grease into a jar or an empty tin, let it set, and bin it. Wipe greasy pans with kitchen paper before they go in the sink.
Worth knowing
This applies to bacon fat and roasting-tin drippings, but also to salad oil, mayonnaise, cream, butter and gravy. If it is solid in the fridge, it will be solid in the pipe.
Wipes, and the word “flushable”
“Flushable” means it will leave the toilet bowl. It does not mean it breaks down.
Toilet paper is designed to disintegrate in seconds when agitated in water. Wipes — baby wipes, disinfectant wipes, makeup wipes, and the ones explicitly sold as flushable — are made from bonded fibres so they can be scrubbed with while wet. That is the whole point of them, and it is exactly why they survive the trip.
They then catch on any imperfection in the line: a joint, a root, a slight sag. Once one catches, the rest accumulate on it, and they bind together into a mass that a plunger will not shift.
Nothing goes down a toilet but human waste and toilet paper. That includes cotton buds, dental floss, tampons and sanitary products, cat litter of any description, and paper towel — which is also engineered not to fall apart when wet.
Coffee grounds, eggshells and the settling problem
These fail differently. They do not coat the pipe and they do not bind together — they simply do not float.
Coffee grounds are dense and fine. They pass through the trap, then settle wherever the flow slows: in a low spot, a sag, or against an existing accumulation. Over time they pack into a dense plug that behaves like wet sand.
Eggshells break into small sharp fragments that do the same, with the added property of catching stringy material and holding it in place. Ground eggshell also mixes readily with grease, and the combination sets considerably harder than either alone.
Neither is helped by a disposal. A disposal makes the pieces smaller, which makes them settle more evenly and pack more densely. Both belong in the bin or the compost.
Food that swells, and food that wraps
Two separate categories, both routinely put down a disposal on the assumption that a disposal handles food.
Starchy things swell. Rice, pasta and oats keep absorbing water after they have gone down. A small amount becomes a paste that expands to fill the pipe, and it is remarkably effective at it. Potato peel does something similar, turning into a starchy sludge that coats.
Fibrous things wrap. Celery, onion skins, corn husks, artichoke leaves and asparagus ends are held together by long tough fibres. A disposal does not cut those; it spins them, and they wind around the impeller in exactly the way a rope winds around a propeller.
A garbage disposal is for incidental scraps rinsed off a plate. It is not a bin, and the things it handles worst are the things people assume it exists for.
Hair, and the bathroom equivalents
Hair is the bathroom's version of the grease problem: it does not dissolve, and it catches on everything.
On its own it would often pass. Combined with soap scum — which is what soap becomes when it meets our hard water — it forms a mat that grips the pipe wall and grows steadily.
That combination is why hard water areas see more bathroom drain calls. The soap is contributing an adhesive that softer water would not produce.
A cheap hair catcher over the shower and basin outlets prevents more call-outs than any other single thing on this page, and it costs almost nothing.
One thing to stop reaching for
Chemical drain cleaner is worth its own warning. It works by generating heat and reacting with the blockage, and on a partial clog it sometimes appears to succeed.
What it does not do is clear a root mass, a sag or a wipe accumulation. On those it sits in standing water in the line, still caustic, doing nothing.
It is then a hazard for whoever opens the pipe — and repeated use damages older pipe and rubber seals. On a main line backup it is wasted money at best.
If a drain will not clear with a plunger and a hair catcher, it needs looking at rather than dosing.