What is a Rest bend

A rest bend is a specially shaped pipe fitting used to connect a vertical downpipe — typically from a rainwater or soil system — to the horizontal section of underground drainage. It features a 90-degree change in direction but unlike a sharp right-angle bend, it has a long, sweeping curve designed to ease the transition of flow and minimise the impact on the drainage system.

The name “rest bend” comes from the inclusion of a small integral footing or base, which allows the fitting to rest directly on the bedding material within the trench. This base helps to absorb the hydraulic force of falling water and supports the pipework structurally.

Rest bends are an essential component in above-to-below-ground drainage connections, ensuring waste or rainwater is conveyed safely and efficiently from the building into the underground system without causing damage or flow disruption.

Function and Purpose

The rest bend is installed at the bottom of a vertical stack or downpipe where the pipe transitions from vertical to horizontal. Its primary function is to reduce the energy and turbulence caused by fast-moving water, particularly in gravity-fed systems.

Without a properly designed rest bend, the water or waste flowing down the pipe could strike the base of a conventional right-angle bend with force, potentially causing:

  • Hydraulic shock

  • Back-splash or blow-back up the vertical pipe

  • Premature wear or failure of the pipe joint

  • Noise and vibration in the drainage system

The sweeping profile of the rest bend slows and redirects the flow smoothly, allowing waste to continue through the underground drainage system with minimal turbulence. The inclusion of a stabilising foot at the base further prevents movement or disconnection under pressure.

Where Rest Bends Are Used

Rest bends are typically used in domestic and commercial drainage applications where vertical soil stacks or rainwater downpipes discharge directly into the underground system without passing through an intermediate gully. Common situations include:

  • Rainwater downpipes from roofs discharging into soakaways or combined drains

  • Soil stacks carrying waste from WCs, sinks or appliances

  • Discharge points from vented stacks in multi-storey buildings

  • Connection points between external rainwater systems and below-ground drainage

Their use is particularly important where high-volume or high-velocity flow is expected, such as from toilets, large roof areas or pressure-flushed appliances.

Characteristics of a Rest Bend

Several specific features distinguish rest bends from standard pipe fittings:

  • Swept 90-degree curve: Unlike tight bends, a rest bend has an extended arc which reduces flow resistance and impact.

  • Integral base or support foot: A flattened section or leg at the bottom of the bend allows the fitting to rest on compacted bedding without shifting.

  • Socketed connections: Most rest bends are supplied with push-fit or solvent-weld sockets at each end for secure jointing to vertical and horizontal pipes.

  • Material compatibility: Rest bends are available in uPVC, HDPE, and clay, depending on the system being used.

Rest bends typically come in standard diameters — such as 110 mm for domestic systems — but are also available in larger sizes for commercial or industrial drainage.

Why a Swept Bend Is Important

The importance of the sweeping angle in a rest bend lies in its ability to control the momentum of water descending under gravity. In a vertical stack, liquid waste can reach significant velocities, especially in taller buildings. When this high-speed flow hits a sharp bend, several problems can occur:

  • Hydraulic noise: The force of impact can generate banging, gurgling or vibration sounds, which are transferred through the building structure.

  • Air pressure fluctuations: Rapid flow changes can create negative or positive pressure zones that disrupt water seals in traps, leading to odours or siphoning.

  • Joint stress: Repeated impact at the base of the stack can strain or misalign pipe joints.

  • Turbulent flow: Sharp changes in direction cause eddies and turbulence, which may lead to blockages or reduced flow capacity.

A rest bend mitigates these issues by gradually redirecting the flow, preserving the laminar nature of movement and reducing pressure fluctuations.

Installation Guidance

Correct installation of a rest bend is essential to its function. Standard good practice includes:

  1. Excavation and bedding: The trench must be excavated to a suitable depth with a compacted granular bed (such as pea gravel or 10 mm shingle) to support the pipe.

  2. Level positioning: The rest bend should be positioned so the vertical pipe drops directly into the socket, and the base of the bend rests flat on the bedding.

  3. Secure jointing: Use push-fit or solvent-weld techniques according to the manufacturer’s instructions. Ensure joints are clean and properly aligned.

  4. Backfilling: Once installed and checked, the trench should be backfilled carefully, avoiding sharp objects or large stones that could damage the fitting.

  5. Access and inspection: If the bend is installed near a property boundary or inspection chamber, adequate access for future inspection or rodding should be ensured.

Rest bends must not be installed in a way that places weight or stress on the socket joints, especially where external pipes are not fully supported.

Regulations and Best Practices

In the UK, the use of rest bends is covered under Building Regulations Part H (Drainage and Waste Disposal). Key considerations include:

  • Access for rodding or jetting: Where rest bends are used at the base of soil stacks, provision must be made for maintenance access, either via rodding eyes or inspection chambers.

  • Gradient of outfall pipes: The horizontal pipe connected to the rest bend should maintain a minimum gradient (typically 1 in 40 for 110 mm pipe) to ensure self-cleansing flow.

  • Separation from foundations: Rest bends and connected pipework should be located at least 150 mm from building foundations unless properly sleeved or bridged.

  • Materials and compatibility: Rest bends must match the pipework system being used, with uPVC fittings only used in systems of the same material.

Following industry-approved installation standards — such as those from the Chartered Institute of Plumbing and Heating Engineering (CIPHE) — ensures both compliance and performance.

Alternatives and Misapplications

While rest bends are specifically designed for high-velocity vertical discharge, some installers mistakenly use standard 90-degree bends or multiple 45-degree fittings in place of a rest bend. These alternatives may be acceptable in low-flow or low-rise scenarios but carry risks in taller buildings or where waste volume is significant.

In general:

  • Do not use tight 90-degree bends at the base of soil stacks.

  • Avoid unsupported changes in direction near pipe entries to underground systems.

  • Use access fittings where the rest bend is not easily reachable.

For vertical-to-horizontal transitions where a gully or hopper is used, a rest bend may not be necessary, as the water flow is broken at the surface before entering the underground system.

Benefits of Using Rest Bends

Using rest bends in drainage systems offers several advantages:

  • Smoother flow: Reduces turbulence, blockages and air pressure fluctuations.

  • Less noise: Quieter operation due to the gradual transition of flow direction.

  • Structural support: The base of the fitting provides added stability at a critical junction point.

  • Reduced maintenance: Lower risk of failure or leakage at the bend over time.

  • Compliance: Meets current regulatory requirements and best practice for soil stack discharge.

Their design has remained largely unchanged because they reliably address the specific hydraulic and structural needs at a key point in the drainage system.

Conclusion

The rest bend is a specialised but essential fitting in building drainage systems, facilitating the safe and efficient transition from vertical downpipes to horizontal underground pipes. Its sweeping shape, stabilising foot, and durable construction ensure it can manage high-velocity waste flow without introducing shock, noise or damage. Installed correctly, rest bends provide long-term reliability, support regulatory compliance, and enhance the overall performance of domestic and commercial drainage installations.