UK Building Regulations for Soil and Waste Pipes

Approved Document H of the UK Building Regulations sets out the requirements for soil pipes and waste pipes systems to ensure effective drainage, prevent foul odours, and maintain hygiene. 

This guide summarises the key takeaways from Section 1: Sanitary Pipework, ensuring full compliance. Let’s get into it.

General requirements for soil and waste pipe systems

Under Requirement H1, all buildings must have an adequate drainage system that:

• Efficiently conveys wastewater to an appropriate outfall, such as a public sewer, private sewer, septic tank, or cesspool.
• Minimises the risk of blockages or leaks.
• Prevents foul air from entering the building.
• Allows air to circulate within the system to prevent pressure imbalances that could cause water seals in traps to be lost.
• Provides access for clearing blockages.

The system should be large enough to handle expected flow rates. Minimum pipe sizes and gradients are defined in the regulations.

Traps and water seals

Every appliance connected to the underground drainage system must have a trap with a water seal to prevent foul air from entering the building. The required minimum trap sizes and seal depths are:

Traps should be removable or include a cleaning eye to allow maintenance. Appliances discharging directly into a gully may have a reduced water seal depth of 38mm.

To prevent trap seals from being broken by pressure fluctuations, branch discharge pipes must be properly designed with adequate ventilation.

Branch discharge pipes

Branch pipes carry waste from appliances to a discharge stack or drain.

• Pipes should discharge into a discharge stack or a gully, not directly into open hoppers.
• Branch pipes from WCs should not discharge directly into a drain unless the depth from the floor to the drain is 1.3 metres or less.
• Pipes must not be positioned to cause crossflow in stacks.
• Bends in branch pipes should be avoided where possible or have a large radius to minimise blockages.

The maximum allowable lengths of unventilated branch pipes are:

If these lengths are exceeded, the branch must be ventilated with an air admittance valve or a connection to a ventilating stack.

Discharge stacks

Discharge stacks are vertical pipes that receive waste from branch discharge pipes.

The minimum stack diameters are:

• Urinal stack: 50mm
• WC outlet less than 80mm: 75mm
• WC outlet greater than 80mm: 100mm

The bend at the foot of the stack should have a radius of at least 200mm to improve flow.

Offsets in discharge stacks should be avoided, but where unavoidable:

• There should be no branch connections within 750mm of the offset in buildings up to three storeys.
• Buildings over three storeys may require additional ventilation above and below the offset.

Ventilation of sanitary pipework

Ventilation ensures proper airflow and prevents pressure imbalances.

• Vent pipes must terminate at least 900mm above any building opening within three metres, such as windows or doors.
• In areas where rodent control is an issue, ventilating pipes should be fitted with metallic, rodent-proof terminals.
• Air admittance valves may be used in place of open vent pipes, provided they comply with BS EN 12380.
• Ventilation stacks should not reduce in size in the direction of flow.

Stub stacks

A stub stack is a short discharge stack that does not extend to roof level. It can be used if:

• It connects to a ventilated discharge stack or a ventilated drain not subject to surcharging.
• It serves a WC with a floor level no more than 1.3 metres above the connection.
• No other branch into the stub stack has a centreline more than two metres above the connection.

Air admittance valves must be used at the top of a stub stack to prevent foul air from escaping.

Rodent prevention

Drainage pipework should be designed to minimise rodent entry and movement.

• Sanitary pipework connected to WCs must not allow light to be visible through the pipe wall, as this may attract rodents.
• Ventilation pipes should be fitted with metal grilles or mesh to prevent rodents from entering the system.
• Gullies should be covered with solid plastic or fixed metal gratings to prevent rodent access.

Rodding access

Rodding points allow access for clearing blockages.

• They should be positioned above the spillover level of appliances.
• All discharge stacks should have an accessible rodding point.
• Rodding eyes should be capped extensions of pipes to allow maintenance access.

Boiler condensate drainage

Boiler condensate pipes may be connected to the sanitary drainage system.

• The connection should be made using a pipe with a minimum diameter of 22mm.
• The connection must be made through a 75mm deep condensate trap.
• All sanitary pipework receiving condensate should be made of materials resistant to pH values of 6.5 or lower.

Materials for soil and waste pipes

Materials must comply with British Standards:

Pipework must be securely supported and properly joined to prevent leaks and movement.

Testing and air tightness

• Pipes, fittings, and joints should withstand an air test of at least 38mm water gauge for three minutes.
• Every trap should maintain a water seal of at least 25mm under working conditions.
• Smoke testing may be used to detect leaks, but is not recommended for PVC-U pipes.

Alternative compliance methods

Instead of following Approved Document H, compliance may be achieved by adhering to:

• BS EN 12056 (gravity drainage systems inside buildings)
• BS 8000-13 (workmanship for above-ground drainage)