Maximum length of a WC branch to the stack

6 m

For a 100 mm branch serving a single WC, with no more than 90 mm/m fall

Long unvented WC branches lose their trap seal, so the run to the stack is limited.

Basis in regulation

Part H
Approved Document H
Read the source

How to check it on site

Measure the actual routed length, not the straight line on the drawing.

Common failure

Branch extended around a stud wall during first fix, taking it past the limit.

Effective from 22 August 2026. 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

Minimum Hot Water Storage Capacity

60 °C

Domestic hot water storage systems must provide a minimum capacity to adequately supply hot water at a target temperature, typically 60°C

Minimum

Electrical accessory spacing

100 mm

Electrical socket outlets and switches should be installed with a minimum spacing of 100mm between their centres to ensure adequate access and compliance with wiring regulations

exact

32mm waste pipe support

32 mm

Independent 32mm polypropylene discharge or waste pipes, conforming to relevant British Standards such as BS 5254 or BS EN 1451, must be adequately supported with pipe clips throughout their run to the soil stack or manifold

Minimum

Clearance from combustible materials

75 mm

Appliances that generate heat, such as solid fuel burners, must maintain a minimum separation distance of 75mm from any combustible materials to prevent fire

Minimum

Headroom over a stair

2000 mm, reducing to 1900 mm at the centre of a loft conversion stair and 1800 mm at the side

Stairs and landings within a dwelling

Maximum

Smoke alarm siting distances

Within 7.5 m of the door to every habitable room, within 3 m of every bedroom door, and at least 300 mm from any wall or light fitting

Ceiling-mounted mains-powered alarms on circulation routes