Toilet won't flush properly? 3 things to check before calling a plumber

A toilet that flushes weakly, takes ages to refill, or runs constantly is one of the most common call-outs I get. Most of these you can fix yourself in fifteen minutes with parts from any DIY shop. Here is what to check before you reach for the phone.

The three most common faults

In order of frequency:

1. Cistern not refilling fast enough (or refilling forever)
2. Flush valve failing (water leaks from cistern to bowl constantly)
3. Partial blockage in the trap or pan (slow drain, weak flush, sometimes both)

Each has a different fix.

1. Cistern not refilling

Symptoms: you flush, the cistern slowly fills back up over several minutes, or never quite gets to full level.

Lift the cistern lid. (Most lids just sit on top, lift straight up. A few are screwed in place with one or two visible screws). Look at the inlet valve, usually on the left side. There are two common types:

Old brass float valve

A long arm with a plastic or copper ball at the end. The ball sits on the water surface. As water rises, the ball lifts the arm, the arm closes a valve. When the float gets old, the seal hardens and the valve drips, or the float fills with water and sinks (you can shake it; if you hear water inside, it is failed).

Fix: replace the whole valve. Around £15 to £25 in parts at any DIY shop. 30 minutes if you have a spanner and a willingness to turn off the water and take your time.

Modern plastic side-entry or bottom-entry valve

A vertical plastic stem with a sliding float collar. These valves are reliable but the diaphragm inside them clogs with hard water debris. Symptoms: water trickles in slowly. The valve itself usually still works, just slowly.

Fix: turn off the isolator (small screw on the pipe feeding the cistern, turn 90 degrees), unscrew the valve top, lift out the diaphragm, rinse it in vinegar to dissolve any limescale, refit. Free fix, 15 minutes. If that does not work, the whole valve costs about £15 to £25 to replace.

2. Flush valve failing

Symptoms: cistern keeps refilling on and off because water is escaping from cistern to bowl. You may hear water trickling into the bowl all the time. Drop a few drops of food colouring into the cistern (do not flush). If the colour appears in the bowl within 15 minutes without flushing, the flush valve is leaking.

Modern toilets have a "flush valve" or "flapper" at the bottom of the cistern. This is a rubber or plastic seal that lifts when you flush, then drops back down to seal the cistern. After a few years the rubber hardens, the seal fails, and water leaks slowly.

Fix: turn off the isolator, flush to empty the cistern, remove the flush mechanism (usually unclips or unscrews), take it to the DIY shop and buy a matching one. £15 to £40 depending on whether you replace just the seal or the whole mechanism. 30 to 60 minutes.

This is the fault I see most often. Saves about a litre of water a minute, which on the metered supply that more London homes are now on, costs real money.

3. Slow drain or weak flush

If the cistern fills properly but the flush itself is weak or the bowl drains slowly, the problem is downstream of the cistern.

Possibilities, easiest to hardest:

Holes around the rim are blocked

The bowl has small holes around the inside of the rim. Hard water and limescale block these over years. Symptom: weak flush, water comes out in dribbles instead of a sweep around the bowl.

Fix: pour a litre of warm white vinegar into the cistern, leave overnight, flush in the morning. Repeat for a few days if needed. If that does not clear it, a wire (a straightened coat hanger works) gently pushed into each hole from inside the bowl can clear them. Slow but free.

Partial blockage in the trap

The trap is the U-bend immediately under the toilet. A slow drain (water level drops slowly after flushing) usually means a partial blockage there. Common culprits: too much paper, "flushable" wipes (not actually flushable), child's toy, sanitary products, dental floss tangles.

Fix: a toilet plunger (the bell-shaped one, not the cup-shaped sink one) used firmly five or six times often clears it. If that fails, an auger or "toilet snake" (£15 from any DIY shop) reaches further into the trap.

Blocked soil pipe

If the toilet flushes weakly but drains, then later you notice the drain outside the house is slow or backing up, the blockage is in the soil pipe. This is a plumber call. Could be a build-up over years (wipes, fat, debris) or a structural issue (a collapsed pipe, tree roots in old clay soil pipes, common in Victorian terraces).

Soil pipe clearance in London: typically £150 to £300 for accessible blockages, more if rodding or jetting from outside is needed.

Quick checks for each scenario

Before you do anything else, eliminate these:

1. Isolator valve. The screw on the pipe feeding the cistern. If it has been knocked closed (or partly closed), water flow drops. Flick it open with a coin in the slot.

2. Recent water company work. Pressure to your house can drop after street works. Cistern fills slowly because mains pressure is reduced.

3. Faulty flush handle. The lever or button is just not lifting the flush mechanism enough. Check by lifting the flush valve directly with your hand from inside the cistern. If a manual lift gives a strong flush, the handle linkage needs adjusting or replacing.

When to call a plumber

• The blockage will not clear with a plunger or auger
• The soil pipe outside is backing up or smells
• Water is leaking from the base of the toilet onto the floor (different problem, the wax seal under the toilet has failed)
• You have replaced parts twice and the problem keeps returning
• A toilet in a flat is leaking through to the flat below

What I bring on a toilet call

Common parts in the van:

• Float valves and inlet diaphragms (most popular brands)
• Flush valves and seals
• Wax seals for toilet bases
• Augers and short rods for soil pipe access
• Replacement isolator valves

Most toilet calls I can finish in one visit. Cost in London 2026: £80 to £180 for the call plus repair, £200 to £400 for soil pipe blockages depending on access.

Save on water bills

If you are on a water meter (most newer London homes are, and Thames Water is rolling out meters across the rest), a leaking toilet can add £10 to £30 a month to your bill. The food colouring test takes 30 seconds and costs nothing. Worth doing once a year on every toilet in the house.

If you would rather have someone diagnose the issue and quote both the repair and any wider plumbing improvements at the same time, send a WhatsApp with a photo of the inside of the cistern and a description of what is happening.