A bottle trap that weeps is usually the washer
I put a bowl under the bathroom basin in May and found a wet patch on the cupboard floor. I thought the waste pipe had split. The drip was coming from the bottle trap, a slow bead at the cup. I undid the bowl. The washer was twisted and sitting on the thread, not in the seat. I had done that trap up last winter after I cleared a clog, and I had cross-threaded the cup. The plastic was fine. The washer was doing nothing.
A bottle trap is a short U you can unscrew. Water sits in the cup and stops smell coming back. Washers seal the cup to the body, and the nuts to the tail and the wall pipe. If a washer is missing, pinched, or on the thread, the joint weeps. People wrap the cup in tape. Tape is not a seat. Bowl off, washers in the grooves, back on. If the plastic is cracked, I stop. A cracked cup will not hold a washer.
A tap that drips is usually the washer in the head is the spout above this. I isolate that tap if I am taking the trap right off, and I check the isolation first. An isolation valve that will not close is usually the slot. If I cannot shut the tap, I still have a plug in the basin. I do not re-pipe a waste for a weep. I do not start on a live supply I cannot isolate.
When I check I put a dry tissue on each joint. The wet one is the job. I undo that nut or the cup, not every joint because I am under there anyway. I look at the washer. Flat, split, or a permanent kink means a new ring the same size. I clean the seat. Grit on the face is a drip you will put back. I seat the washer in the groove, start the thread by hand, and tighten until it meets, not until the plastic creaks. I run the tap and watch. A bead at the cup is still the cup washer. A bead at the wall nut is that washer.
Skip PTFE on a trap washer. The washer has to sit square. Skip a metal jubilee clip on a plastic cup. You will crush it. Skip doing the cup up with grips from the first turn. Hand-tight, then a little. Skip leaving the old washer because it looks almost round. Almost is the bead on the floor. Skip a new trap because one ring has gone, unless the body is cracked. If it is cracked, I stop and I replace the trap, not the whole waste run.
Hair and soap sit in the cup. That is the point of a bottle. I empty it over a bucket, not over the cupboard. I do not poke the wall pipe with a hanger until I have the cup off and I can see. If the blockage is in the wall, that is further than this job. I put the washers back and I leave the pipe.
This shows as a smell first, or as a stain on the shelf under the basin. Winter, the cupboard is shut and you notice late. Summer you have a bowl in there for cleaning and you see the drip. I now feel the cup after I have done the trap up, and I look again in the morning.
I do not need a new basin because the trap wept. I need the washer in the seat, the cup square, and a stop if the plastic has split.
If I later add a buy link here, it will be a paid promo.