A U-bend is a curved section of pipe fitted below a sink, basin or appliance that retains a small volume of water, forming a seal against drain odours and insects. The two main forms are the P-trap, where the outlet runs horizontally into the wall, and the S-trap, where the outlet drops vertically through the floor. UK Building Regulations Part H1 requires every wastewater fitting to have a trap with a minimum water seal of 75mm for self-siphoning appliances. Traps can be made from PVC, polypropylene, chrome-plated brass or stainless steel, with serviceable joints for cleaning. Common problems include loss of seal through evaporation in unused fittings, siphonage caused by inadequate venting and blockage from fat or hair. Removing and cleaning the trap is one of the simplest drainage maintenance tasks.
U-bend (P-trap, S-trap)
Curved pipe section holding water to block drain odours rising into the room.
