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July 18, 2026 APM Electricals

110mm Soil Pipe: The Complete Guide to Above-Ground and Underground Drainage

110mm Soil Pipe: The Complete Guide to Above-Ground and Underground Drainage

Soil pipe carries the waste from WCs, urinals, and any appliance with a high risk of sewage contamination directly to the underground drain — without any of the intermediate traps and small-bore runs used for basin and bath waste. Getting the installation right matters: an incorrect branch angle, missing access point, or inadequate ventilation causes blockages, smells, and compliance failures. This guide covers the complete 110mm Aquaflow range in stock at APM — above-ground solvent and push-fit in black and grey, and underground terracotta.

110mm Double Socket Knuckle Bend Grey Soil Pipe

Soil Pipe vs Waste Pipe: What Goes Where

System Pipe size Connects to Needs trap?
Soil pipe 110mm WC, urinal, combi-WC No — WC pan incorporates its own water seal
Waste pipe 32–50mm Basin, bath, shower, sink Yes — trap required at every appliance

The 110mm stack is also used as the primary ventilation pipe for the drainage system. Under Approved Document H, above-ground drainage systems must be ventilated to prevent trap siphonage and allow drainage gases to escape safely. The open vent at the top of the stack — or an air admittance valve (AAV) if the stack cannot be carried above the eaves — provides this ventilation.

Above-Ground Soil Pipe: Solvent Weld vs Push-Fit

  • Solvent weld: Permanently bonded with solvent cement. For concealed runs inside walls and floor voids. Joints are rigid and reliable but permanent. Grey and black available.
  • Push-fit (black only): Ring-seal socket connections. Demountable for future access. Preferred for surface-mounted stacks and any installation where access for blockage clearance may be needed.

Colour: Grey is traditionally used internally (inside the building fabric, in duct shafts); black for external stacks and visible runs. Functionally identical — colour is convention only.

Pipe

  • Aquaflow 110mm × 3m Plain End Soil Pipe Black — browse live plumbing fittings for available alternatives.

Straight Couplings and Slip Couplings

Bends — Solvent Weld

Bends — Push-Fit Black

Branches — Connecting Waste to the Stack

Branches allow 50mm waste pipe to connect into the 110mm stack. All Aquaflow branches are swept — waste enters in the direction of flow to prevent turbulence and backpressure. The branch angle refers to the entry angle of the waste arm: 92° (perpendicular-ish) or 135° (shallow sweep). For single-storey connections, 135° is preferred; 92° where the waste must drop steeply to connect.

Waste-to-stack reducers:

Access Points

Building Regulations require rodding access at the base of every stack and at any change of direction on a branch longer than 2 metres. Access plugs and access couplings fulfil this requirement — the screw cap can be removed to rod or jet the pipe clear.

Strap-On Bosses

Strap-on bosses allow a 50mm waste connection to be added to an existing stack without cutting in a branch fitting. A hole is cored in the stack and the strap-on boss clamps over, providing a 50mm push-fit or solvent socket. Useful on refurb and extension work where the original stack is not being replaced.

110mm Black Solvent Soil Double Socket Pipe Coupling

Air Admittance Valve

Where the soil stack cannot be open-vented through the roof, a BS EN 12380 AAV can be fitted at high level in lieu of the open vent. The valve opens under negative pressure when drainage flows, admits air to prevent trap siphonage, then closes. It must be installed above the highest connection and remain accessible — it must not be boxed in permanently.

Socket Plugs and Pipe Clips

Underground Drainage — 110mm Terracotta

Once the soil stack reaches the underground drain at the foot of the building, the system transitions to underground-grade pipe. These runs are buried below ground from the stack foot to the inspection chamber or sewer connection. Aquaflow underground fittings are terracotta (orange) — the standard colour for underground drainage. Underground joints use ring-seal sockets (push-fit) to accommodate ground movement.

Underground Pipe

Underground Couplings

Underground Bends

Underground Branches

Underground Access and Gullies

Guttering and Downpipe Accessories

Installation Notes

Above-ground: branch angles and WC connections

WC outlets connect directly to the 110mm soil pipe — no trap is required as the WC water seal provides the barrier. Where waste pipes connect at a branch, use swept (not square) entries. All Aquaflow branches have swept entries in the direction of flow.

Ventilation

The stack must be ventilated to prevent trap siphonage on connected waste appliances. Approved Document H requires the vent to terminate at least 900mm above any opening window within 3m. Where an AAV is used instead of an open vent, it must be installed above the highest connection and must remain accessible.

Underground falls and bedding

Approved Document H recommends a minimum 1:40 fall (25mm per metre) for 100mm underground drains. Pipe must be bedded in granular material with 100mm bed depth and 150mm cover above the pipe crown. Access must be provided within 22m of any straight run and at every change of direction.

All Aquaflow 110mm soil pipe, fittings, and underground drainage products are stocked at APM's Acton trade counter — available for same-day collection.

APM Plumbing & Electrical | 24 Western Avenue, Acton, London W3 7TZ | 020 8702 8080 | apmi.uk

Filed in: 110mm, above-ground drainage, Aquaflow, drainage, push-fit soil pipe, soil pipe, soil stack, solvent weld soil pipe, underground drainage, WC drainage
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