What is a Stub stack

A stub stack is a shortened section of soil and waste pipework that serves sanitary appliances such as toilets, basins, baths or showers but does not continue vertically through the property to provide full ventilation. Unlike a full soil and vent pipe (SVP), which extends above the roof to vent foul gases, a stub stack terminates internally, usually just above the highest connected appliance, and is fitted with an air admittance valve (AAV) to maintain proper system function.

Stub stacks are commonly used in modern low-rise buildings, particularly where internal layouts make full vertical stacks impractical or unnecessary. They allow for efficient removal of wastewater while maintaining adequate ventilation through localised solutions, avoiding the need for roof penetrations.

Purpose and Function of Stub Stacks

Stub stacks serve two key functions in a drainage system:

  • Collection of waste water: They provide a local vertical pipe to receive discharges from sanitary fittings on a single floor, such as in ground-floor bathrooms or utility rooms.

  • Ventilation and pressure equalisation: By incorporating an air admittance valve or being positioned near a vented stack, they allow air to enter the pipework to prevent siphonage of water seals in traps.

Stub stacks are especially useful in installations where only one floor of sanitary appliances is present and where full venting to the atmosphere is already provided elsewhere in the system. They contribute to a more flexible, space-efficient drainage layout while maintaining compliance with plumbing and building regulations.

Components and Configuration

A typical stub stack installation includes:

  • Short vertical pipework: Usually 100 mm in diameter, rising just above the highest waste connection

  • Connection to appliances: Branch pipes from toilets, washbasins, baths or showers join the stub stack via junctions or boss fittings

  • Access point for rodding: An access cap or rodding eye is often included for maintenance

  • Air admittance valve (AAV): Fitted at the top to admit air when negative pressure occurs

Stub stacks must be properly sized and positioned to ensure efficient drainage and prevent loss of trap seals. The AAV must be certified, installed above the flood level of connected appliances and accessible for inspection.

Difference Between Stub Stacks and Soil Vent Pipes

Although stub stacks and full soil vent pipes both serve the drainage system, they are not interchangeable and serve different purposes:

  • Soil vent pipes (SVPs): Extend vertically through the building and terminate above roof level to allow foul gases to escape and air to enter the system freely.

  • Stub stacks: Terminate within the building and rely on air admittance valves or connection to vented sections for pressure regulation. They do not allow foul gases to vent to the atmosphere.

Stub stacks are suitable only where Building Regulations allow their use, typically in dwellings with no more than one storey served by the stack and where proper venting is provided elsewhere in the drainage system.

Advantages of Stub Stack Systems

Stub stacks offer several practical and economic benefits:

  • Simplified installation: They require less pipework and fewer roof penetrations

  • Reduced material and labour costs: No need to run a full-height vent stack

  • Greater design flexibility: Useful for layouts where full stacks are not feasible

  • Minimised external alterations: Particularly valuable in refurbishments or extensions

  • Aesthetics and space-saving: Eliminate the need for visible external pipework in some designs

Stub stacks help modern plumbing systems meet both technical and architectural demands efficiently.

Regulatory Requirements in the UK

In the United Kingdom, the use of stub stacks is governed by Approved Document H of the Building Regulations and associated British Standards. Key requirements include:

  • Connection rules: Stub stacks may serve WCs and other appliances on the same floor

  • Ventilation proximity: At least one full vented stack must be present in the system

  • Air admittance valves (if used): Must be tested and compliant with BS EN 12380

  • Installation height: The AAV must be fitted at least 200 mm above the flood level of connected appliances

  • Access: Stub stacks must include an access point for maintenance, especially if they serve a WC

Compliance with these regulations is essential for ensuring the safe and reliable operation of the drainage system.

Typical Use Cases

Stub stacks are commonly found in:

  • Bungalows and single-storey dwellings: Where one vented stack elsewhere in the building is sufficient

  • Ground-floor WC and utility installations: Where a dedicated full stack would be excessive

  • Extensions and renovations: Retrofitting a new bathroom or WC without disturbing the roof

  • Flats and apartments: Where individual units may share a common vented stack but use stub stacks internally

In each case, stub stacks provide a localised drainage solution that connects seamlessly to the broader system.

Limitations and Considerations

While stub stacks offer convenience and flexibility, they have several limitations:

  • Restricted to low-rise applications: Generally not permitted for multi-storey vertical drainage

  • Dependence on air admittance valves: If the AAV fails or becomes blocked, it may lead to odour or siphonage issues

  • No venting of foul gases: They cannot be used where full venting is required to discharge gases

  • Maintenance accessibility: AAVs must be installed in accessible locations, which can be a challenge in some designs

For these reasons, stub stacks must be specified and installed carefully, with consideration of the entire drainage layout and building use.

Air Admittance Valves and Stub Stacks

Most stub stacks are fitted with an air admittance valve (AAV) at the top. This valve opens under negative pressure (e.g. when a toilet is flushed) to admit air and prevent the siphoning of water traps. It closes immediately afterward to stop odours escaping.

Features of a compliant AAV include:

  • Silent operation

  • Durable, corrosion-resistant construction

  • Protection against vermin and insects

  • Resistance to air pressure fluctuations

  • Conformance to EN 12380

Some AAVs are suitable for internal use only, while others may be approved for external or cold loft environments. Product selection must align with installation conditions.

Maintenance and Inspection

Stub stacks require minimal but important maintenance to ensure continued function:

  • Visual inspection of joints and fittings

  • Checking air admittance valves for dust or blockage

  • Ensuring access points are not obstructed

  • Listening for signs of siphonage (e.g. gurgling sounds in traps)

Any failure in the stub stack system may lead to odour complaints, water seal loss or slow drainage.

Integration with Modern Building Systems

In modern construction, stub stacks are frequently integrated into prefabricated bathroom pods, modular builds or compact en-suite facilities. Their use supports offsite manufacturing and faster on-site installation.

With the rising emphasis on airtight building envelopes and minimised roof penetrations, stub stacks provide a neat solution that supports both building performance and plumbing efficiency.

Conclusion

Stub stacks are a practical, space-saving component of above-ground drainage systems. By providing a short vertical connection for sanitary appliances without extending through the roof, they offer installation flexibility while maintaining effective waste removal and system ventilation when used correctly.

They are especially valuable in single-storey or ground-floor applications where a full vented stack is unnecessary or undesirable. When combined with a high-quality air admittance valve and installed in accordance with building regulations, stub stacks provide a reliable and compliant solution in both new builds and retrofit projects.