How to Fit a Toilet: Complete Guide

Fitting a toilet is well within reach for a confident DIYer, and the job itself is not difficult. What catches people out is buying first and measuring second — because the position of the soil pipe dictates which pan will actually fit.

Get the measuring right and the rest is a couple of hours.

Measure Before You Buy

Three measurements decide everything.

1. Soil pipe centre height from the finished floor. This determines whether you need a horizontal, angled or offset pan connector.
2. Distance from the wall to the centre of the soil pipe.
3. Available width, particularly if the toilet sits between a wall and a basin or bath.

Also identify the outlet type on your existing pan: horizontal (straight back into the wall), P-trap (out the back), or S-trap (down through the floor). Buying a pan with the wrong outlet is the single most common mistake.

Choosing the Pan Connector

The connector is the flexible or rigid fitting joining the pan outlet to the soil pipe. Options:

• Straight — when the pan outlet lines up directly with the soil pipe.
• Angled (usually 14 or 90 degrees) — for a modest change of direction.
• Offset — shifts the outlet up, down or sideways by a few centimetres. Useful when a new pan does not quite match the old position.
• Flexible — accommodates awkward alignments. Convenient, but the concertina inner surface catches waste more readily than a smooth one, so use a rigid connector where you can.

Removing the Old Toilet

1. Isolate the water at the valve on the supply pipe, or the main stopcock.
2. Flush and hold the handle down, then sponge the cistern dry.
3. Disconnect the supply pipe. Keep a bowl underneath.
4. Undo the cistern fixings — to the wall, and the bolts through into the pan — and lift the cistern off.
5. Undo the screws holding the pan to the floor. They are often corroded; penetrating oil helps.
6. Rock the pan gently side to side to break it free of the connector and lift it clear.
7. Stuff a cloth into the open soil pipe immediately. It stops smells coming back into the room and stops anything falling in.

Fitting the New Toilet

Assemble the cistern first

It is much easier to fit the flush valve, fill valve and doughnut washer with the cistern on the floor than in position.

1. Fit the flush valve through the base with its rubber washer, and tighten the back nut hand-tight plus a small nip.
2. Fit the fill valve, checking the washers are the right way round.
3. Sit the large doughnut washer over the flush valve outlet.
4. Bolt the cistern to the pan, tightening the two bolts evenly and alternately. Even pressure is what stops ceramic cracking.

Position and connect

1. Push the pan connector onto the pan outlet.
2. Offer the whole toilet up and slide the connector into the soil pipe. It should go in firmly with hand pressure. Do not use sealant — these joints seal on rubber lips, and sealant prevents them seating properly.
3. Check the pan sits flat. If it rocks, use plastic shims rather than tightening the fixings down onto an uneven floor.
4. Mark the fixing holes, move the toilet aside, and drill.

Fix down and test — in that order

1. Reconnect the water supply and turn it on.
2. Let the cistern fill and flush two or three times before fixing the pan down. Watch the connector joint and the base for any leak.
3. Only once it is dry, screw the pan to the floor — firm, not tight. Use the plastic washers supplied; screwing hard into ceramic cracks it.
4. Silicone around the base afterwards, leaving a small gap at the back so any future leak shows rather than hiding under the pan.

That last detail matters. A fully sealed base traps a slow leak against the floor, and the first sign is a rotten floorboard rather than a puddle.

The Leaks That Appear a Week Later

• Doughnut washer not seated evenly — leaks only when flushed. See our guide to a leaking toilet cistern.
• Pan connector not pushed fully home — smells first, then leaks at floor level.
• Supply connection over-tightened — the washer distorts and weeps.
• Pan rocking — movement gradually breaks the connector seal. Shim it properly at the outset.

When to Call Someone

Get help if the soil pipe needs moving, if the floor under the old toilet is soft or damp, if the pipework is lead or cast iron, or if the toilet is on a macerator system. Each of those turns a straightforward swap into a different job.

Drainage requirements for domestic soil pipework are set out in Approved Document H, and the Chartered Institute of Plumbing and Heating Engineering has guidance on when work becomes notifiable.

Frequently Asked Questions

Do I need building regulations approval to replace a toilet?

Not for a like-for-like replacement in the same position. Moving it, or creating a new WC where there was none, generally does require approval.

Should I use sealant on the pan connector?

No. Pan connectors seal on rubber lips and sealant stops them seating correctly. If a connector needs sealant to hold, it is the wrong connector.

Why does my new toilet smell?

Nearly always the pan connector not fully home, letting drain air past. Check it is pushed properly into the soil pipe before looking anywhere else.

How tight should the floor screws be?

Firm enough to stop movement, no tighter. Ceramic has almost no give, and the crack usually appears days later rather than immediately.

If the Soil Pipe Needs Moving

Repositioning a toilet means moving the soil pipe with the correct fall, which is a bigger job than it looks and unforgiving if it is wrong. We cover Milton Keynes and the surrounding area for bathroom plumbing. Get in touch with your measurements and we will tell you what is involved.