Which way round do the washers go in a plastic waste trap?

Every basin, bath, sink and shower trap in the country is joined by the same plastic compression fitting, and it seals through a shaped rubber washer rather than through force. Almost every persistent drip under a basin is that washer being upside down or the pipe not being fully home, and neither responds to tightening. The cross-section drawing shows both cases side by side.

What you'll need

• A bowl and a cloth
• A replacement washer set, if the old ones have hardened
• Washing-up liquid, as a lubricant

Step by step

1. Take the joint apart with a bowl underneath.Plastic waste nuts undo by hand. Slide the nut back along the pipe and separate the joint. Water will come out of the trap and out of the pipe — the bowl is not optional.
2. Look at the washer's shape.It is not a flat ring. One face is flat and one is tapered or has a moulded lip. Hold it up and look at the profile from the side. This shape is the whole mechanism, and once you have seen it the rule is obvious.
3. Understand what the taper is doing.The inside of the fitting has a matching tapered seat. When the nut is tightened, it pushes the washer's taper into the fitting's taper, and that wedging action squeezes the rubber inwards onto the pipe all the way round its circumference. The seal is against the pipe, not against the fitting face.
4. Fit it flat face towards the nut.Flat face against the shoulder of the nut, tapered face pointing into the fitting. That is the correct orientation on every standard plastic waste compression joint. Reversed, the taper meets the flat of the nut and the flat meets the taper of the seat, so they touch only on a narrow ring and the rubber deforms sideways instead of onto the pipe.
5. Push the pipe fully into the fitting before tightening.A pipe that stops short leaves the washer sitting over a gap with nothing to compress it against, and it rolls as the nut tightens. Push the pipe all the way to the internal shoulder. On a trap, that means the trap's spigot fully home in the connector.
6. Check the pipe end is clean and square.A cut pipe end with a burr on it will cut or roll the washer as it goes through. Take the burr off with a knife or a file and chamfer the edge slightly.
7. Tighten by hand plus a small amount.These fittings are designed for hand tightening. Hand tight and then a little more with fingers is right. Grips crack the plastic thread, and a cracked thread never seals again at any torque.
8. Never use sealant on the joint.Silicone or PTFE smeared into a compression waste joint does not fix a reversed washer, does not fix a pipe that is short, and welds the fitting so it cannot be taken apart for cleaning. It is the classic sign of a joint that was never diagnosed.
9. Judge whether the washer can be reused.A washer that has been compressed takes a set. If it is still soft and flat with no cracking or hardening, it will reseal. If it is hard, glazed or has a groove worn in it, replace it — washers cost pennies and are sold in mixed sets.
10. Run water and check with dry paper.Dry the joint completely, wrap a strip of kitchen roll round it, and run the tap for a couple of minutes. Paper shows a slow weep that a visual check does not, and a slow weep under a basin is what rots a cabinet floor.

If it doesn't work