What is a Inline trap

An inline trap is a section of pipework, typically incorporating a U-bend or similar water-seal design, that is installed directly into the main run of a drainage or sewer pipe. Its primary purpose is to prevent foul odours, sewer gases, and vermin from travelling back from the communal sewer system into properties or public spaces. Unlike conventional traps beneath sinks or appliances, inline traps are embedded within the underground drainage system, often serving as a final barrier before waste discharges into the public sewer.

Although inline traps offer effective odour control, they are often considered problematic in modern drainage design due to maintenance challenges and their tendency to block over time.

Function and Purpose

The core function of an inline trap is to create a water seal that prevents gases or pests from migrating up through the drainage system. This is achieved through a U-shaped bend or similar configuration that retains a standing volume of water at all times. As wastewater flows through the trap, the water seal remains intact, acting as a physical barrier between the user environment and the sewer.

In many legacy drainage systems — particularly in older UK properties — inline traps were commonly used as the final component before a drain connected to the main sewer line. Their purpose was especially important before the widespread use of ventilated stacks and interceptors, when the risk of odours entering buildings was more prevalent due to limited system ventilation.

Design Characteristics

Inline traps are usually located beneath ground level, between the last inspection chamber on private property and the public sewer connection. They often include:

  • A U-bend, P-trap, or interceptor-style trap to hold a permanent water seal
  • Installation inline with the main pipe run, meaning that flow must pass through the trap
  • No direct access point, making it difficult to inspect or clear the trap if a blockage forms
  • No rodding eye or maintenance hatch, particularly in older installations

This design creates a significant constraint for modern maintenance, as any blockage occurring within or near the trap can be very difficult to resolve without excavation.

Common Problems and Limitations

While inline traps serve a useful sealing function, they are not favoured in modern drainage design due to their tendency to cause operational issues. The main problems associated with inline traps include:

  • Frequent blockages: Solids, grease, and debris tend to accumulate in the low point of the trap, especially in under-used or poorly flushed systems.
  • Lack of rodding access: Because inline traps are often inaccessible from nearby manholes or inspection chambers, they cannot be cleared using traditional drain rods or jetting equipment.
  • Standing water issues: In cold weather or long dry periods, the water seal can evaporate or freeze, eliminating its effectiveness and potentially causing gas ingress.
  • Excavation requirements: Blockages often require digging up the trap to access and remove obstructions, leading to disruption and high costs.
  • Obsolescence in new systems: Modern sewer systems rely on ventilation and interceptors rather than inline traps, making them largely redundant and discouraged in current design standards.

Due to these limitations, many property owners or water companies opt to remove inline traps during sewer upgrades or property renovations.

Alternatives and Modern Practices

Contemporary drainage systems are designed to prevent odour and pest ingress through stack ventilation, backflow valves, and manhole trap connections, rather than through inaccessible inline traps. Key alternatives include:

  • Intercepting traps: Installed in accessible manholes and designed with cleaning access
  • Air admittance valves (AAVs): Used to balance air pressure in the system and reduce siphoning
  • One-way valves or non-return valves: Installed to prevent backflow of water and gases
  • Ventilated stacks and discharge pipes: Providing air pathways to regulate pressure and allow gas dispersion safely above roof level

Where odour control is still needed near the connection to the public sewer, it is preferable to use accessible components that can be maintained easily without excavation.

Regulatory Context and Best Practice

In the UK, Building Regulations and water authority guidance generally advise against the use of inaccessible inline traps in modern foul drainage design. Key documents include:

  • Building Regulations Part H1 – Sets out drainage requirements and discourages designs that are difficult to maintain or prone to blockage.
  • Sewerage Sector Guidance (SSG) – Used by water companies to assess adoptable drainage, favouring accessible and maintainable systems.
  • BS EN 752 – Emphasises the need for proper inspection and maintenance access in below-ground drainage systems.

Where inline traps exist in legacy systems, property owners should be aware of their location and potential for maintenance issues, especially when planning renovation or repair works.

Conclusion

An inline trap is a type of underground drainage component that provides a water seal to prevent sewer gases, smells, and vermin from entering buildings or public areas. While historically useful, these traps are known for causing blockages and maintenance problems due to their inaccessibility and design limitations. As such, they are generally avoided in modern drainage systems, with current best practice favouring maintainable, accessible alternatives. For property owners and drainage engineers, awareness of the presence and condition of inline traps is crucial for effective system performance and long-term maintenance planning.