Step 5 of 6
Sink P-trap step 5 of 6: Check the washers
Each pipe going into the U-bend wears a slip nut and, under it, a tapered washer. Check both washers: the thin, tapered side faces down into the U-bend and the flat side up against the nut. A cracked or squashed washer leaks: replace it.
Tools: none, just your hands
Common mistake: putting a washer on upside down. A slip washer is tapered so that the nut pushes its thin edge down into the U-bend’s mouth and wedges the joint shut. Turned over, its flat side meets the mouth, the nut squashes it unevenly, and water seeps past it and drips from under the nut as soon as the tap runs. Take the nut off and turn the washer round: tapered side down into the bend. The switch in the player shows what goes wrong.
All 6 steps
What you need
- Bucket or bowl · to catch the water in the trap
- Bottle brush · long and thin
- Rubber gloves · and an old towel
- Slip-joint pliers · only if a nut is stuck
- New slip washers · only if one is cracked or flattened
Questions
Why does a sink have a P-trap?
The water that always stands in its U-bend stops sewer gas coming up the drain into the room. It is also where hair, soap and grease collect, so it is where most sinks clog.
Do I need tools to take a P-trap off?
Usually not: a plastic trap’s slip nuts undo by hand. Slip-joint pliers help if one is stuck; wrap a cloth round a metal nut so they do not scratch it.
Which way does the slip washer go?
Tapered side down, into the U-bend, with the flat side up against the slip nut. The trap maker’s sheet puts it as “cone side down, into the lower side of the trap”.
How tight should the slip nuts be?
Hand-tight, then a little more, about a quarter turn, to seal. Overtightening cracks plastic nuts. Check them again for drips after two or three days.
It still drips after putting it back. What now?
Undo the nut that drips and check its washer: it may be upside down, cracked or squashed. Put it the right way round, or fit a new one, and tighten again.
Safety
Wear gloves: the water in a trap is dirty. Do not pour chemical drain cleaner into a sink you are about to take apart, and if you already have, wear eye protection and let a plumber do it.
About this guide
This guide shows a common bathroom sink’s plastic P-trap, 1¼ inches (32 mm) across: a tailpiece from the basin, a U-bend held on by two slip nuts, each over a tapered washer, and a trap arm into the wall. A kitchen sink’s trap is usually 1½ inches and comes apart the same way.
The steps follow a P-trap maker’s instruction sheet: pull the bend straight down to take it off; the rubber washer goes cone side down into the trap; hand-tighten every nut, then a little more to seal, and test for leaks. The water-flow readings are illustrative.
Sources: P-trap installation instructions (sheet sold with a trap at Lowe’s), iFixit: How to unclog a sink.
The 3D models, drawings and text are our own.