Blocked Kitchen Sink: Trap, Waste Pipe or the Shared Stack?

Where is my kitchen sink actually blocked?

Kitchen waste takes a short, predictable route, and the blockage is somewhere along it. From the plughole the water drops into the trap, the curved or pot-shaped fitting under the bowl that holds a little water to keep drain smells out. From there a 40mm plastic waste pipe runs along the back of the units and through the wall. In a house it usually ends over or into a gully outside, a grated pot at ground level that also takes rainwater. In a flat it normally joins a vertical soil and waste stack that the whole building shares.

What you see at the sink tells you which part to look at first.

• Water drains slowly from one bowl only, or stops dead as soon as you pull the plug: the trap or the first few centimetres of pipe
• Both bowls of a double sink are slow, or the dishwasher leaves dirty water in the sink when it pumps out: the shared section of waste after the two traps join
• The sink empties at first and then backs up, with gurgling from the plughole: a partial blockage further along the waste run, often where it turns a corner or goes through the wall
• Water comes up in the gully outside, or the downstairs toilet or shower is slow as well: the underground drain, not the sink
• In a flat, the sink fills when a neighbour above runs water: the shared stack

How to unblock a kitchen sink safely, in order

Start with the gentlest step and only move on when it has failed. Put a bucket and an old towel under the trap before you begin, and wear rubber gloves: kitchen waste water is greasy and full of bacteria.

• Scoop out any food you can see in the plughole and take out the basket strainer if there is one
• Plunge. Use a cup plunger, not the flanged toilet type. Leave a few centimetres of water in the bowl so the cup is covered, then block the overflow slot with a wet cloth held down firmly. On a double sink, put the plug in the other bowl. If a dishwasher or washing machine drains into the trap, pinch its drain hose shut with a clamp or a clothes peg so you do not push dirty water back into the machine. Work the plunger in ten or so firm strokes and pull it off sharply at the end
• If plunging has not moved it, empty the trap. Bail out the bowl, then unscrew the large plastic nuts by hand, keeping the bucket underneath. A bottle trap has a base that unscrews; a tubular trap comes apart at its two nuts. Clear the fat and food, rinse the parts in a washing-up bowl, not the blocked sink, and refit with the rubber washers seated square
• Run the hot tap for a minute. If the water now flows freely, you have found it
• If the trap was clean, the blockage is in the pipe beyond it. A small hand-held drain snake fed in from the trap outlet will reach a couple of metres along a straight run

Never mix drain cleaning products, and never plunge, snake or open the trap after using one. Caustic and acidic unblockers react violently together, and a chemical still sitting in the pipe will splash out of the plunger or the trap onto your hands and face. If someone has already poured one in, stop and tell whoever comes out exactly what was used.

What not to pour down a blocked sink

The usual advice is to pour boiling water down the sink. With a modern plastic waste system, a kettle of boiling water poured into a sink that will not drain sits against the trap and the joints and can soften or distort them, and it sends near-boiling water back at you if you then plunge. Hot water from the tap is enough to help loosen fat once the water is actually moving.

Chemical unblockers are a poor fit for kitchen blockages in particular. A solid plug of fat and coffee grounds does not dissolve quickly, so the product sits in the standing water doing little except making it hazardous. If the pipe does clear, it often only opens a narrow channel through the fat, and the sink blocks again within weeks.

Bicarbonate of soda and vinegar is harmless but does not have the force or the chemistry to shift a real blockage. It can freshen a slow sink, but it will not clear one that has stopped. If the sink smells more than it blocks, see a smelly sink, which is often a dry or badly fitted trap rather than a blockage.

When the blockage is in the drain outside

In a house, go outside and find where the kitchen waste pipe comes through the wall. If it ends over an open gully and the gully is full of standing water, grease and leaves, the sink is backing up because the gully cannot empty. Lift the grid, wear long gloves, and scoop out the sludge into a bin bag. Fat from years of washing up often sets in the gully as a grey crust. Once it is clear, run the sink and watch the gully drain.

If the gully is empty and dry while the sink will not drain, the blockage is in the waste pipe between the wall and the gully, which is often a joint, a sagging section or a bend packed with fat. That section can usually be rodded or replaced without disturbing the kitchen. A pipe that also drips at its joints is a separate fault; see a leaking waste pipe.

If the gully is full and will not empty even after clearing the grid, the blockage is further down in the underground drain. Lift the nearest inspection chamber cover, bending at the knees because they are heavy, and keep children and pets away while it is open. If that is full too and a neighbour has the same problem, it may not be yours to clear at all. What to do next is in an outside drain blocked or a manhole overflowing.

Is it a shared drain, and who clears it?

In England and Wales, most private sewers and lateral drains were transferred to the water companies on 1 October 2011. Since then, a drain that serves your home and a neighbour’s, or one that runs outside your property boundary, is the water company’s to clear, at no cost to you. A drain that serves only your home and sits inside your boundary is yours. How to check which you have, and how to report it, is covered in whose job a blocked drain is.

Flats are different inside the building. The kitchen waste from your sink to where it joins the shared stack is normally yours. The stack itself, and the drains under the building before they leave your boundary, are usually the freeholder’s, which in practice means the managing agent. Your lease will say. If your sink fills with dirty water when a flat above uses theirs, that is a stack blockage: report it to the managing agent straight away, and do not keep using the sink, because the water has nowhere to go but out of your plughole.

If water from the sink has reached electrics, such as a socket behind the units or a dishwasher, switch those circuits off at the consumer unit before you touch anything, provided the unit is dry and you can reach it without crossing the wet floor.

What a plumber does, and when to call one

Call someone when you have plunged and cleaned the trap and the sink still will not drain, when the sink is overflowing and cannot be emptied, or when dirty water is coming up in more than one fitting. On arrival the engineer works along the same route you did, but with longer reach: a powered drain snake or rods from the trap outlet or a rodding eye, and a look at the gully and nearest chamber to confirm whether the blockage is on your side at all. If fat has lined the whole pipe, a short section may be cut out and replaced.

The £60 call-out covers the visit at any hour, and the repair price is agreed with you on site before any work starts. If the toilet is also backing up, it is almost certainly the drain rather than the sink, and our blocked toilet unblocking visit covers the whole run. When a toilet is the main problem, read a blocked toilet with the water rising first. And if the problem is a drip under the sink rather than slow draining, start with a pipe leak under the sink.