Water seal depth at appliance traps

75 mm for baths, basins, sinks and showers on a branch; 50 mm for a WC trap

Appliance trap on a branch discharge pipe

Depth of water seal that must be retained at each appliance trap to keep drain air out of the dwelling.

Basis in regulation

Part H
Approved Document H
Read the source

How to check it on site

Test at second fix by filling and discharging each appliance, then checking the seal has not been drawn out by the run.

Common failure

Shallow-seal traps fitted to gain fall under a bath, so the seal siphons when an upstairs appliance discharges.

Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Hearth Provision and Floor Finishes

225 mm

A hearth may not be necessary if an appliance's flame or incandescent material is positioned at least 225mm above the floor

Minimum

Soundproofing Measures for Soil Pipes

25 mm

Sound insulation for soil pipes involves either encasing them in boxing made from a material with a minimum mass of 15kg/m², or wrapping them with at least 25mm of unfaced mineral wool, typically 10kg/m³ density

Minimum

Electrical Accessory Clearance from Water Diagrams

300 mm

Diagrams illustrating the required 300mm minimum horizontal clearance for electrical accessories from water sources, such as sinks and wash basins, are commonly used in guidance documents

exact

Conditions for Pipe Freezing Calculations

6 °C

Calculations for pipe insulation to prevent freezing are based on specific assumed conditions, typically including an external air temperature of -6°C and an internal water temperature of +7°C

Minimum

Stair Handrail Height and Clearance

600 mm

A handrail is mandatory for flights of stairs where the rise exceeds 600mm

Maximum

Maximum gap in guarding

No opening should allow a 100 mm sphere to pass

Guarding to a stair, landing, balcony or floor edge in a dwelling