What Blocks a Kitchen Drain (and the Fixes That Actually Work)

A kitchen drain carries a different load from any other pipe in the house. A basin sees hair and soap. A toilet is designed for one specific job. The kitchen sink takes hot greasy washing-up water, cold rinse water, food scraps, starchy cooking water, coffee dregs and, quite often, the waste from a dishwasher and a washing machine as well. Most of that goes down without trouble. A few things don't, and they tend to work together.

Knowing which ones cause the problem helps you stop it happening again, and prevention is worth more than any clearing method.

The main culprits

Fats, oils and grease

The trade calls this FOG. Hot fat from a roasting tin or frying pan is liquid when it goes down the plug hole. Somewhere along the pipe it cools, and as it cools it thickens and sticks to the pipe wall. The next lot sticks to the first. Over months, a pipe that started at full bore narrows to a fraction of its size, and the greasy lining catches every crumb that passes.

Oil that stays liquid at room temperature, such as most cooking oils, isn't safe either. It coats surfaces and mixes with other waste into a soft, sticky mass. Washing-up liquid only helps for a short distance: it breaks grease into small droplets, but once the water cools and slows they gather together again further along the pipe.

At city scale, this is how the huge fat masses found in sewers form. At household scale, it's the grey, cheesy deposit lining the inside of a kitchen trap.

Starch: rice, pasta and potato

Cooked rice and pasta keep absorbing water after they go down the drain. A handful of grains swells in the trap, and starchy water from draining pasta or potatoes leaves a glue-like film. On its own it often washes through. Combined with grease, it sets into a paste.

Coffee grounds

Coffee grounds don't dissolve and don't break down quickly. They're heavy and gritty, so they settle at low points: the trap, and any sag in the pipe. Mixed with grease they form something close to wet sand bound with wax. Loose tea leaves behave in much the same way.

Eggshells

Small shell fragments act like grit, and the thin membrane inside the shell is stringy enough to catch on joints and wrap around other debris. They belong in the compost or the bin.

Fibrous peelings

Onion skins, celery strings, sweetcorn husks and potato peelings tangle together in a trap and then catch everything else.

Food waste disposers

A disposer under the sink grinds scraps into small particles, but it doesn't change what those particles are. Ground grease is still grease. Ground rice is still starch. Coffee grounds come out much as they went in. Disposers need a good flow of cold water while running and for a short while afterwards to carry the waste through, and they cope badly with fibrous, starchy and greasy waste. In some areas their use is restricted or discouraged, so check local rules before fitting one.

Appliances that share the sink waste

In many kitchens the dishwasher and washing machine discharge into the sink waste, either through a spigot on the side of the trap or a branch on the pipe beneath it. That has two effects.

First, more material goes down the same pipe. A dishwasher pumps out hot water carrying the food and grease rinsed off your plates. A washing machine sends out lint, fibres and detergent residue at a fair rate.

Second, a partial blockage in the sink waste shows up in the appliances. A dishwasher that leaves dirty water in its base, or a sink that fills with grey water when the washing machine empties, often points to a blocked trap or pipe rather than a faulty appliance. Each connection should have a non-return valve, or the drain hose should loop up high before it drops to the spigot, so waste water can't run back into the machine.

When a new spigot trap is fitted, the small blanking disc inside the spigot sometimes isn't cut out. If a newly connected appliance won't drain at all, check that first.

The pipe itself: long runs and poor fall

Some kitchens simply block more often than others, because of how the waste pipe runs. A waste pipe needs a steady fall along its whole length so the water carries solids with it. Sinks moved to the other side of the room, islands with sinks, and extensions often end up with long horizontal runs. If the fall is too shallow, water slows, solids settle, and grease has longer to cool and stick. A pipe that sags between clips creates a low spot where sludge collects.

Minimum falls and maximum run lengths are set by local building rules, which vary between countries. If you're planning a new kitchen layout, ask whoever is designing the pipework how the waste will run and where the access points will go. A rodding eye or access fitting at each change of direction makes future clearing far easier.

Myth versus reality

Clearing a blocked kitchen sink

Work from the simplest method to the most involved, and wear rubber gloves throughout.

• Bail out first. Scoop standing water into a bucket so you can see what you're dealing with.
• Check the strainer and plug basket. Some sinks have a removable basket waste that clogs completely. Lift it out and clean it.
• Plunge. Use a cup plunger with water covering it. Block the overflow with a damp cloth, and if you have a double bowl, block the second plug hole too, or the pressure simply escapes into the other bowl.
• Remove the trap. Bucket and towel underneath, unscrew the trap and clean it out. Kitchen traps hold a surprising amount of grey grease. Check the appliance spigot while it's off.
• Rod beyond the trap. A hand drain snake fed into the waste pipe can break up a plug further along. Turn the handle as you push.
• Check the outside end. If your kitchen waste discharges into an outside gully, lift the grid and see whether the gully is full of fat. Scoop it out with a gloved hand or an old trowel.

The blocked drains guides gather these methods in one place, including when it's time to call someone in. The trap-removal technique is the same one described in how to unblock a bathroom sink, though kitchen traps are usually larger and a good deal greasier.

If you've already poured a chemical drain cleaner into the sink and it hasn't worked, don't plunge and don't open the trap. Splashback from a caustic product can burn skin and eyes. If the water will drain at all, run plenty of cold water through first, and tell anyone who comes to help exactly what you used.

When it isn't your blockage: shared stacks in flats

In a block of flats, each kitchen waste joins a shared vertical pipe, the stack, which serves several homes. If the blockage is in your own branch, only your sink is affected. If it's in the stack below your connection, the signs point beyond your flat:

• Your sink backs up or gurgles when you haven't used it, often when neighbours upstairs are running water.
• Dirty water appears in your sink or bath that you didn't put there.
• More than one flat reports the same problem at the same time.

Don't try to rod a shared stack yourself. Report it to whoever manages the building, whether that's a landlord, managing agent or residents' association. Who pays for shared drainage depends on your lease or tenancy and on local law, so check before arranging any work.

Habits that keep a kitchen drain clear

• Let fat cool in the pan, scrape it into a lidded jar or old tin, and put it in the bin when full. Some local authorities collect food waste that includes small amounts of fat, so check what yours accepts.
• Wipe greasy pans and plates with kitchen roll before washing them.
• Keep a mesh strainer in the plug hole and empty it into the bin or compost after every wash-up.
• Scrape plates into the food bin, not the sink.
• Put coffee grounds and tea leaves in the compost.
• Run the cold tap for a minute after using a disposer.
• Once a month, lift the plug basket and look into the trap with a torch. Catching a grey build-up early is a five-minute job; leaving it until the sink stops draining isn't.

A kitchen drain looked after this way can go years without a problem. One that takes the roasting-tin fat every Sunday will block sooner or later, whatever gets poured down after it.