The short answer
It depends how bad the build-up is. For a fresh, mild grease slowdown, hot water with a little washing-up liquid is the simplest fix — the heat softens the fat and the detergent emulsifies it so it flushes away, with no chemicals and no pipe risk. For a more established grease layer, an enzyme (biological) cleaner digests fat steadily and is kind to pipes and septic tanks, though it works over hours. For a stubborn grease plug that water and enzymes will not shift, a caustic-soda-based unblocker dissolves it quickly but must be handled with care. The most reliable long-term answer, though, is prevention: never pour fat, oil or grease down the sink — let it cool and bin it, and flush regularly with hot water.
Fat, oil and grease are the most common cause of kitchen drain problems, coating the pipe until it slows or blocks. Several methods clear it, each suited to a different severity. Here is how they compare, and how to stop it recurring.
Clearing grease
- Mild/freshHot water + detergent
- EstablishedEnzyme cleaner
- Stubborn plugCaustic unblocker (with care)
- Never pourFat, oil, grease down the sink
- PreventionCool and bin, flush hot water
Why grease blocks drains
Cooking fats, oils and grease pour easily when hot but cool quickly in the pipe, where they congeal and stick to the walls. Over time the coating thickens, narrowing the bore until the drain slows and finally blocks. Grease also traps food particles and binds with flushed wipes in the wider sewer to form fatbergs. Because the problem is a film coating the whole pipe rather than a single object, the aim is to soften and emulsify the fat so it washes away, not just to punch a hole through it.
That is why method matters. Hot water re-melts fresh fat; detergent breaks it into a form that flushes; enzymes digest it biologically; and caustic chemistry dissolves it fast. The right choice depends on how thick and set the build-up has become, and on what your pipes and drainage system can tolerate.
| Method | Effectiveness on grease | Pipe-safe? | Best use |
|---|---|---|---|
| Hot water + detergent | Good on fresh, mild | Yes (hot, not boiling on plastic) | Early slowdown, prevention |
| Enzyme cleaner | Steady, over hours | Yes, gentle | Established build-up, septic tanks |
| Caustic unblocker | Fast, strong | Harder on pipes/seals | Stubborn grease plug |
| Plunger / snake | Mechanical only | Yes | Soft clog near the trap |
Indicative comparison for guidance. Match the method to the severity.
Choosing the right method
For a kitchen sink that has just started to drain slowly, begin with hot water and a squirt of washing-up liquid. Pour a kettle of hot — not boiling, if your pipes are plastic — water with detergent down the plughole and let it work; the heat softens the fat and the detergent emulsifies it. This is free, safe, and often enough for a fresh build-up, and a weekly repeat keeps grease from establishing in the first place.
If the slowdown is more set in, an enzyme cleaner is the gentle next step. Its bacteria and enzymes digest grease steadily, usually left overnight, and it is kind to pipework and safe for septic tanks and soakaways, where caustic products are best avoided. For a genuinely stubborn grease plug that hot water and enzymes will not clear, a caustic-soda-based unblocker dissolves fat quickly — but treat it with respect: chemical-resistant gloves and eye protection, good ventilation, follow the dose, and never mix it with bleach, acid or another product. A plunger or hand snake is a useful mechanical alternative if a soft clog has formed near the trap.
Prevention beats clearing
The most effective approach to grease is to keep it out of the drain entirely. Never pour fat, oil or grease down the sink, even when hot and runny, because it cools and congeals downstream. Let pan fat cool, then scrape it into the bin or a container; wipe greasy pans with kitchen roll before washing; and use a sink strainer to catch food scraps. Running hot water with detergent after washing up helps move any residual fat through while it is still warm.
If grease has already caused a blockage that hot water, enzymes and a careful dose of unblocker will not clear, the build-up is likely thick and compacted along the pipe, and mechanical clearing is needed — rodding or, more thoroughly, high-pressure jetting, which scours grease off the full pipe wall and restores the bore. A drain that keeps blocking with grease despite good habits may have a fall problem or rough surface trapping fat, which a CCTV survey can reveal. But for most households, simple prevention plus the occasional hot-water flush keeps kitchen drains clear without any chemicals at all.
Where grease collects and how to spot it early
Grease rarely blocks a drain in one place at random; it tends to accumulate where the flow slows or changes direction. The trap under the sink is the first collection point, followed by bends, junctions and any low spot or slight sag in the underground run where water lingers. Knowing this helps you target the problem: cleaning out the trap by hand clears the most common grease catchment in minutes, and a recurring blockage further down the same kitchen line points to build-up at a bend or a fall problem in the pipe that a survey can confirm.
Catching grease early is far easier than clearing an established plug. The first signs are a sink that empties more slowly than usual, a faint gurgle as it drains, or a lingering smell from the plughole as decomposing residue collects. Acting at that stage — a hot-water-and-detergent flush, or an overnight enzyme treatment — usually restores normal flow before a full blockage forms. Left unattended, the slowdown progresses to standing water and a clog that needs stronger measures, so a slow kitchen sink is worth treating promptly rather than tolerating.
For homes that cook a lot of fatty food, building a routine into kitchen habits pays off. Scrape and wipe greasy pans into the bin before washing, pour cooled fat into a container for disposal rather than down the sink, fit and regularly empty a sink strainer, and run hot water with detergent after washing up so any residual fat moves through while still warm. A monthly enzyme treatment adds a biological safety net that digests the thin film that inevitably forms. Combined, these habits mean grease almost never reaches the point of blocking the drain, which is comfortably the most effective and lowest-cost approach of all.
Frequently asked questions
Can I pour cooking oil down the sink if I run hot water after?
No. Even with hot water, the oil cools and congeals further down the pipe, coating the walls and building up over time. Always let fat and oil cool and put it in the bin or a container, and wipe greasy pans with kitchen roll before washing them.
Does hot water dissolve grease in drains?
Hot water softens and helps flush fresh grease, especially with washing-up liquid to emulsify it, but it does not truly dissolve a set, thick build-up. For an established grease layer an enzyme cleaner or, if needed, a caustic unblocker is more effective, and for a compacted plug jetting clears it best.
What clears a really stubborn grease blockage?
A caustic-soda-based unblocker dissolves stubborn grease quickly if used carefully, but for a thick, compacted layer coating the pipe, high-pressure jetting by a drainage company is the most thorough fix because it scours the full pipe wall and restores the original bore rather than just boring a channel through the grease.
Sources & further reading
Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.