Question 1

What trap size do I need?

Accepted Answer

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 110 mm for WCs. Using an undersized trap will restrict flow and cause slow drainage. The trap connects to waste pipe of the same diameter.

Question 2

What is depth of seal and why does it matter?

Accepted Answer

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.

Question 3

When should I use a bottle trap instead of a P-trap?

Accepted Answer

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).

Question 4

What is a running trap?

Accepted Answer

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.

Type	Shape	Outlet Direction	Best For
P-trap	P-shaped	Horizontal (to wall)	Most common — waste exits through wall
S-trap	S-shaped	Vertical (downward)	Waste exits through floor
Bottle trap	Cylindrical	Horizontal or swivel	Exposed pipework, compact spaces
Bath/shower trap	Shallow profile	Horizontal	Low clearance under baths/shower trays
Running (inline) trap	Inline U-bend	Horizontal	Ground floor runs, condensate

Overall Depth (mm)	Seal Depth (mm)	Application
~85	50	Standard shower tray, raised bath
~60	38	Low-profile shower tray (check if compliant)
~50	30	Ultra-low trays (may need HepvO valve)

UK Trap Sizes — Waste Pipe Traps

Trap Sizes by Appliance

Trap Types

Shallow Shower/Bath Traps

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Appliance	Trap Size (mm)	Min. Depth of Seal	Waste Pipe Size
Wash basin	32	75 mm	32 mm
Bidet	32	75 mm	32 mm
Bath	40	75 mm (50 mm if ventilated)	40 mm
Shower	40	75 mm (50 mm if ventilated)	40 mm
Kitchen sink	40	75 mm	40 mm
Washing machine / dishwasher	40	75 mm	40 mm
WC	110 (integral)	50 mm (built into pan)	110 mm soil pipe