What not to pour down a drain?

Fats, Oils and Grease

This is the single largest cause of kitchen drain failure and the most misunderstood, because grease goes down as a liquid and gives no immediate sign of trouble. It sets on the pipe wall as it cools, usually some distance from the sink, and the deposit is sticky, so every subsequent coffee ground and food particle adheres to it. What starts as a film becomes a lining, and the pipe's working diameter shrinks year on year until a normal evening's washing up is enough to stop it.

Bacon fat and frying oil are obvious. Less obvious are the ones people pour without thinking: the fat poured off a roasting tin, butter and cream rinsed from a pan, salad dressing, mayonnaise, dairy, and the grease dissolved in genuinely hot dishwater, which simply re-sets further down. Collect fat in a tin, let it solidify, and bin it. Wipe greasy pans with kitchen paper before they reach the sink.

Things That Behave Like Cement

• Coffee grounds. They do not dissolve, they are dense, and they settle in the bottom of a trap and compact there.
• Flour, rice, pasta and oats. All of them absorb water and swell. Flour plus water in a trap is paste.
• Eggshells. A disposal breaks them into sharp fragments that bind readily with grease.
• Fibrous and stringy produce. Celery, corn husks, onion skins and potato peel wrap around disposal blades rather than being cut by them.
• Cat litter. Designed to clump on contact with liquid, which is exactly the wrong property.

A garbage disposal changes none of this. It is a grinder, not a solvent, and everything it grinds still has to travel the full length of the drain line. In a house with hard-water scale or an older rough-walled pipe, finely ground food is arguably worse than large pieces because it packs into the irregular surface. If a disposal is backing the sink up repeatedly, the issue is usually the branch line rather than the unit, and that falls under kitchen drain cleaning.

Wipes, Fibres and the Main Line

Wipes labelled flushable pass through the toilet and then stop being anyone's idea of flushable. They do not disintegrate the way toilet tissue does, they catch on any irregularity in the line, and they bind with grease into the dense mass that causes a large share of main line blockages. The same applies to paper towel, cotton pads, dental floss, hair collected from a brush, and period products.

The reason this group matters more than the kitchen list is location. A branch line blockage affects one fixture and is a contained job. A main line blockage affects the whole house and ends with water coming up through the lowest drain, usually a basement floor drain. If you are seeing that, or several fixtures backing up together, it has stopped being a maintenance question and become an emergency plumbing one. Where roots or a damaged lateral are involved, sewer line repair rather than cleaning is the likely outcome.

Chemicals, and What Changes on Septic

Paint, paint thinner, solvents, motor oil, antifreeze, concrete and grout slurry and unused medication all belong at a household hazardous waste facility rather than in a drain, both because of what they do to the pipe and because municipal treatment is not designed to remove them. Grout and concrete rinse water deserves a particular mention: it sets, and it sets hard, in whichever trap it stops in.

If your property is on a private system, add caustic drain cleaner and bleach in quantity to the list. A septic tank works through a bacterial colony, and both kill bacteria without discriminating. The consequence is not an immediate backup but a tank that stops processing solids properly and a drain field that fails years early, which is a far larger bill than any drain job. Septic services and our private water and waste guide go into how the two systems interact.