Trap connection (32mm, 40mm, 50mm)

A trap connection is the threaded or compression inlet on a waste trap that matches one of the standard UK waste pipe sizes, typically 32mm for basins, 40mm for sinks and showers and 50mm for some commercial appliances. UK traps and waste pipework follow BS EN 274 for performance and dimensions, and Building Regulations Part H1 sets minimum sizes for branch wastes. Trap connections often include a back nut and rubber washer to seal onto the waste pipe and a separate connector for the discharge side. Choosing the correct trap size during refurbishment is important to maintain self-cleansing velocity and required water seal depths. Where two pipes of different sizes must be joined, reducing connectors and bushes are widely available, although stepping up rather than stepping down is generally recommended.