How do I fit a kitchen sink waste and strainer?

A kitchen sink waste is a short threaded tube through the bottom of the bowl with a washer stack either side of it, and every leak from one comes down to a washer in the wrong place, the wrong way up, or a backnut that has been either underdone or crushed.

Safety

What you'll need

• Waste kit, with its washers
• Silicone or plumber's sealing compound
• Adjustable spanner or waste key
• Bowl and towels
• Torch
• PTFE tape (optional)

Step by step

1. Work out what type of waste the sink needs.A kitchen sink waste is normally 1.5 inch and comes as a basket strainer, a plain strainer, or a strainer with a basket waste plug. A sink with an overflow needs a slotted flange so the overflow can discharge into the waste. A sink without one takes an unslotted flange.
2. Check the sink's overflow arrangement first.Some sinks have a moulded overflow channel; some have a separate hose from an overflow fitting to the waste body; some have none at all, particularly Belfast sinks. Getting a slotted flange for a sink with no overflow, or vice versa, is the usual buying mistake.
3. Get the old one out.Bowl and towels underneath, disconnect the trap, then undo the backnut. Hold the flange from above with a screwdriver across the strainer bars or a waste key while you turn the nut, or the whole thing spins.
4. Clean both faces properly.Old sealant and washer residue off the sink around the hole and off the underside. A new washer sitting on a lump of old sealant leaks immediately.
5. Bed the flange on sealant or its own washer.A ring of silicone or plumber's sealing compound under the flange, or the flat washer supplied for the purpose. Push it down and let the excess squeeze out — that squeeze-out is the evidence it has sealed.
6. Get the washer stack right underneath.From the sink downwards: the sink, then the sealing washer with its flat face against the sink, then any friction or spacer washer, then the backnut. Getting a tapered washer the wrong way up is the single commonest cause of a weep.
7. Tighten firmly, not brutally.Hand tight, then a firm nip with a spanner while holding the flange from above. A plastic backnut cracks if it is overtightened, and a composite or ceramic sink can crack around the hole.
8. Connect the overflow before the trap.Push-fit or screw the overflow hose onto the waste body's spigot and check it is fully home. An overflow that is not connected does nothing at all until the day it is needed.
9. Reconnect the trap and check the fall.The trap and the run to the waste pipe need a continuous fall away from the sink. A trap hanging low with a dip in the pipe collects water and blocks.
10. Test with a full bowl, not a running tap.Fill the bowl, let it drain, and watch every joint underneath while it does. A full bowl draining puts far more pressure on the seal than a running tap and is the only test that means anything.

Tips

• Wipe the squeeze-out from around the flange inside the bowl while it is wet, with a wet finger. Cured silicone in a strainer is unpleasant to remove.
• A basket strainer waste with a proper plug is worth the small extra over a plain grid, because it lets you fill the bowl to soak things and it catches far more before it reaches the trap.
• If you are fitting a waste disposal unit instead, it replaces the whole waste assembly and has its own mounting system and its own sink aperture requirement. Check the sink's waste hole diameter first.
• Do the waste with the sink out of the worktop if you are fitting a new sink anyway. It is a bench job made difficult only by being done inside a cupboard.

If it doesn't work

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