Kitchen sink draining slowly: the five usual causes
A slow sink is already a partly blocked sink, and every wash adds to it. Here is what is actually in there and which habits keep it away.
1. Fat, oil and grease
The dominant cause, by a wide margin. Cooking fat goes down warm and liquid, cools on the pipe wall and sets there. Once a layer exists it catches coffee grounds, food particles and detergent solids, and the bore closes from the outside in.
Nothing about a chemical cleaner removes a hardened grease layer. Volume of hot water helps more, and keeping fat out of the sink entirely helps most: let it cool, scrape it into the bin, wipe pans with kitchen roll before washing.
2. Coffee grounds and food debris
Grounds do not dissolve. They are fine and dense, so they settle wherever flow slows and then bind into whatever grease is already there. A sink strainer costs a couple of pounds and removes most of this problem before it starts.
3. Detergent scum and hard water
London sits in a hard water area. Soaps and detergents react with the minerals and leave a soft film on the pipe wall, which is an ideal surface for everything else to cling to. Scale rarely blocks a drain by itself, but it gives fat and fibre something to hold on to.
4. A waste run with too little fall
This is the one people never suspect, and it explains sinks that block again every few months no matter how careful the household is.
Any kitchen moved during a refurbishment usually ended up with a longer waste run and less fall than the original layout. Water still gets away, but solids settle, and the deposit rebuilds within weeks of every clearance. If your sink blocks on a schedule, the run itself may be the fault rather than what goes into it.
5. A silted outside gully
A kitchen sink usually discharges to a gully outside. If that gully is full of leaves and silt, the sink backs up no matter what happens indoors. It is also the only part of a drain you can inspect yourself: lift the grate and look for standing water.
Clearing gullies twice in autumn prevents a surprising share of the call-outs we attend.
When to stop trying
If the bowl empties but slowly, if it gurgles after draining, or if both sides of a double sink are affected, the restriction is past the trap and worth clearing properly. See how we clear blocked sinks, or coverage in Clapham, Hackney and Fulham.