Blocked Drains in Flats: How to Find the Blockage and Clear It Safely

How drainage works in a flat and in a tall building

Every sink, basin, shower floor and toilet has a trap: a bend or chamber that holds a little water to keep sewer air out of the room. A short branch pipe runs from the trap, under the floor or inside a wall, to a vertical stack that collects waste water from every floor. A vent pipe continues the stack up through the roof to let air in, and at ground level the stacks join a building drain with inspection chambers (manholes) that leads to the public sewer or a septic tank.

Singapore's Housing & Development Board (HDB) puts the usual split plainly: residents maintain the sanitary branch pipes in their flat, and the Town Council maintains the main sanitary pipes.^[3] Check your own building's rules, but the logic usually holds: what serves only your flat is yours, and the shared stack and building drain belong to everyone.

This gives the most useful rule for diagnosis: a blockage in a branch affects only that branch's fittings, while one low in a stack affects everyone above it, and the overflow appears at the lowest open fitting above the blockage, often a floor trap in a ground- or first-floor flat.

Warning signs and common causes

Drains rarely block overnight. Watch for these early signs:

• Water draining more slowly than usual from a sink, basin or shower floor.
• A glugging or gurgling sound as water goes down, a sign that air is struggling to get through the pipe.^[1]
• An unpleasant smell from a plughole or floor trap; if smell is the main problem, see how to find and fix drain odours.
• Water rising in the floor trap or shower area when a nearby basin, washing machine or toilet is used.
• The toilet bowl filling higher than normal after a flush and emptying slowly.
• Dirty water appearing in a floor trap or around a toilet when nobody in your flat is using water, which points to a shared pipe.

Kitchen sinks: grease and food

Fats, oils and grease (often shortened to FOG) from cooking are liquid when hot but harden as they cool inside the pipe, building up until it blocks. Northumbrian Water, a UK water company, warns that boiling water poured after them does not help: the fat simply sets further down the pipe, where it can affect neighbours too.^[5] Which?, the UK consumer organisation, adds that coffee grounds clump into hard blockages.^[1]

Basins, showers and floor traps: hair and soap

Long hair is a common cause of blocked bathroom drains, according to the bathroom manufacturer Geberit, and soap and dirt residue add to it.^[4] HDB says solid matter and grease are what choke floor traps, and advises keeping the grating in place to keep hair and other solids out.^[2] Where the whole bathroom floor drains to one trap, as in many Bangladeshi flats, bucket water, mop water and sand all reach it; Hong Kong's Building Rehab Platform names rubbish and sand at bends and traps as a cause of blocked building drains.^[10]

Toilets: anything other than toilet paper

Wet wipes, sanitary pads, nappies, cotton buds and kitchen roll do not break down like toilet paper. Which? passes on the advice of Severn Trent, a UK water company, to flush only pee, poo and toilet paper and put everything else in the bin.^[1] A weak or incomplete flush can look like a blockage; if the bowl empties normally but refills slowly or leaves waste behind, read toilet flushing problems instead.

One fitting or the whole stack? Find where the blockage is

Before you pick up a tool, spend two minutes working out where the blockage probably is. The answer decides whether this is your job, a plumber's or the building committee's.

Tools and materials

A small kit kept in one bag handles most blockages in a flat:

• Rubber gloves (not cotton, which soaks up chemicals^[8]), safety glasses and old towels.
• Two plungers: a cup plunger for sinks, basins and floor traps, and a flanged (bell-shaped) toilet plunger, kept separate for hygiene.^[1]
• A bucket and an old toothbrush or bottle brush for cleaning traps.
• An adjustable spanner, used gently, for stiff trap nuts.
• Long tweezers or a plastic hair-removal strip for floor traps and shower drains.^[4]
• A hand drain auger (snake) for sinks and floor traps, and a separate toilet auger for the WC.^[6]

Step by step: clearing a blocked fitting

Work from the gentlest method to the strongest, and test with a little water after each step. If water starts coming up somewhere else, stop.

Kitchen sink or wash basin

1. Stop running water into it. If the sink is full, bail the water into a bucket.^[1]
2. Lift out the plug strainer and remove any food or hair you can reach.
3. If the basin has an overflow hole, press a damp cloth over it so the plunger's pressure goes down the drain instead of out of the hole.
4. Leave enough water in the bowl to cover the plunger cup, press the cup over the plughole to make a seal and plunge firmly several times.^[1]
5. If the water still will not go down, put a bucket under the trap, unscrew the trap's nuts by hand or gently with a spanner, empty it and scrub out the grease and debris.^[1]
6. Refit the trap firmly, without forcing plastic threads, then run water and check the joints for drips.^[1]
7. If the trap was clean, the blockage is further along the branch: feed a hand auger into the pipe, turning it as you go, then withdraw it slowly. If that fails, call a plumber.^[9]

Bathroom floor trap or shower drain

1. Lift off the grating and any hair catcher or basket under it, and clean them over a bin, not back into the drain.
2. Pull hair and sludge out of the trap with gloved fingers or long tweezers; Geberit recommends tweezers so you do not damage the trap or pipe.^[4]
3. Pour in about half a litre of hot tap water, not boiling. If it does not drain, plunge it, leaving a little water to make a seal.^[1]
4. If it is still slow, the sediment may be below the trap's water seal. A hand auger can reach it; for serious chokes HDB advises engaging a licensed plumber.^[2]
5. Refit the grating and hair catcher, and keep the grating closed from now on.^[2]

Toilet

1. Do not flush again to test it; each flush adds water that may overflow. If the bowl is rising while the cistern refills, close the cistern's angle valve.
2. Put on gloves, lay old towels round the base and, if the water is high, wait for it to drop a little before you start.^[6]
3. Fit the flanged plunger into the outlet, fully under water, press down and give several firm pumps; if the bowl starts to drain slowly, keep going until it clears.^[6]
4. If plunging fails, push a toilet auger into the outlet until it meets resistance, then turn the handle to break up or pull back the blockage.^[6]
5. Flush once to test. If it still drains slowly, there may be a partial blockage further down the pipe; call a plumber.^[6]

Chemical drain cleaners: a last resort, used carefully

Most shop-bought drain cleaners are strong alkalis such as sodium hydroxide (caustic soda), oxidisers, or strong acids such as sulphuric acid; enzyme products use bacteria and enzymes instead.^[7] Testing 17 products, Hong Kong's Consumer Council found strong alkalis worked best on kitchen grease and oxidisers on hair, the strong-acid product was for professional use only, enzyme products were far slower, and several oxidisers were unsuitable for toilets.^[8]

The risks are real. The Swedish Chemicals Agency (KEMI) says caustic drain cleaners can quickly cause serious, permanent burns to eyes and skin; it advises trying by hand, with a plunger or with a drain cable first, choosing a non-corrosive product if you use one at all, and avoiding corrosive products in homes with young children.^[9] The Consumer Council adds that the heat of the reaction can deform plastic pipes and joints and cause leaks.^[8] Baking soda and vinegar are unlikely to clear a real clog; US Poison Control says the gas they make is unlikely to build enough pressure.^[7]

If you decide to use one:

• Read the label and follow it exactly, including whether the product suits toilets and your pipe material.^[8]
• Wear rubber gloves and eye protection, open a window, and remove standing water first unless the label says otherwise.^[8]
• Pour slowly and close to the plughole. Splash-back is most likely when the product is poured from a height or into a fully blocked drain.^[7]
• Use one product at a time, and rinse the pipe thoroughly with water before trying anything else.^[8]
• Store it in its original container, away from food and out of children's reach; some sachets look like sugar or salt.^[8]

When the shared stack or building drain is blocked

A blocked stack is not a job for one flat owner with a plunger. The waste comes from many flats, the blockage is often several floors down, and clearing it may mean opening access points on the stack or the inspection chambers at ground level. It is work for a plumber or drainage contractor appointed by the committee or management.

• Caretaker: note which flats and floors are affected and since when, to narrow down the blocked section.
• Committee: ask everyone on the affected line to stop using water, then call the building's plumber or drainage contractor.
• Contractor: expect them to clear the line from an access point or inspection chamber with drain rods or a powered machine; drain specialists may also use a CCTV camera or high-pressure water jetting.^[1]
• Afterwards: ask for a written note of where the blockage was and what caused it.
• Old buildings with repeated problems: Hong Kong's Building Rehab Platform suggests having a building professional assess whether replacing the common drains would be cheaper in the long run.^[10]

Repeated blockages on one stack usually have a fixable cause: kitchen grease, flushed wipes, renovation debris, or extra fittings connected without regard to the pipe's capacity; the Building Rehab Platform lists unauthorised additions that overload the system among drain defects.^[10] A notice by the lift about what not to put down drains costs nothing. After a renovation, have the contractor flush and check the drains; see how to inspect plumbing after a flat renovation.

Prevention: habits and a simple routine

Most blockages build up slowly from everyday habits, so prevention costs little. For routine care of the whole bathroom, see maintaining washroom fittings.

Common mistakes to avoid

• Pouring boiling water after grease to wash it away. It only moves the fat further down the pipe.^[5]
• Flushing a blocked toilet again and again, which can overflow the bowl.
• Mixing two drain cleaners, or following a drain cleaner with bleach.^[8]
• Plunging or opening a trap after pouring in a chemical cleaner.^[7]
• Leaving the floor-trap grating off, so that hair and sand go straight into the trap.^[2]
• Trusting 'flushable' wipes. Put every kind of wipe in the bin.^[1]
• Clearing your own floor trap again and again when the real problem is a blocked stack.

When to call a plumber, and repair versus replacement

Call a plumber when plunging and cleaning the trap have not worked, when a toilet stays blocked after plunging and augering, when two or more fittings back up together, when the same fitting blocks again within weeks, or whenever sewage is involved. KEMI gives the same advice: if simple mechanical methods fail, hire a professional.^[9]

Most blockages need clearing, not replacement. Replace a plastic trap that is cracked or has stripped threads, an old pipe that is badly corroded or broken, or a section that a camera survey shows is sagging or collapsed. An old floor trap that cannot take a removable hair catcher can be swapped for one that can during the next bathroom renovation; see how to maintain floor tiles and grout.

What drives the cost of clearing a drain

We could not find reliable published prices for drain clearing in Bangladesh, so this guide does not give figures. What you pay depends mainly on:

• Location: a sink trap is quick; a blockage deep in a branch under the floor or in a stack takes longer.
• Method: hand rodding costs less than a powered machine, jetting or a camera survey.
• Access: whether there are clean-out points or inspection chambers, or tiles must be broken to reach the pipe.
• Damage: broken traps or pipes, and cleaning up after an overflow into a lower flat.
• Shared costs: work on a stack or building drain is usually a shared building cost; agree in advance how the committee will pay for it.

For anything more than a quick visit, get written quotes that say what will be done and whether the work is guaranteed. See how to get and compare repair quotes.

Next steps

1. Right now: stop using the blocked fitting and check whether other fittings are affected.
2. If dirty water is coming up from below, warn your neighbours and the caretaker and stop using water.
3. For a single fitting: clean the strainer or grating, plunge, then clean the trap.
4. If that fails, use an auger or call a plumber; keep chemical cleaners as a last resort.
5. Fit sink strainers and floor-trap hair catchers, and put a bin in every bathroom.
6. Add drain checks to your monthly routine and ask the committee to schedule stack checks.