UK Trap Sizes — Waste Pipe Traps

The trap size must match the appliance waste outlet: 32 mm (1.25 inch) for wash basins and bidets, 40 mm (1.5 inch) for baths, showers, sinks and washing machines, and 100 mm for a WC pan with an outlet over 80 mm, the standard UK pan, which merchants supply as 110 mm outside diameter soil pipe. Using an undersized trap will restrict flow and cause slow drainage. The trap connects to waste pipe of the same diameter.

Depth of seal is the vertical depth of water held in the trap, measured from the overflow level to the bottom of the dip. This water seal prevents sewer gases entering the building. BS EN 12056-2 requires a minimum 75 mm depth of seal for most traps (50 mm for waste appliances connected to a ventilated branch). A deeper seal provides better protection against siphonage.

Bottle traps are compact and neat, making them ideal for exposed pipework under pedestal basins or wall-hung basins where appearance matters. However, they have smaller water capacity and are more prone to blocking than P-traps. P-traps and S-traps have better flow characteristics and are preferred wherever the pipework is concealed (in boxing, behind pedestals, or in floor voids).

A running trap (also called an inline trap or resealing trap) is fitted in a horizontal waste run rather than directly under an appliance. They are used when the appliance has no integral trap, for condensate drainage from boilers, and in ground-floor waste runs. Some self-resealing designs can recover their seal after siphonage. They are available in 32 mm, 40 mm and 50 mm sizes.