110mm Soil Pipe: A Trade Guide to Push-Fit and Solvent Weld Above-Ground Drainage

110mm Soil Pipe: A Trade Guide to Push-Fit and Solvent Weld Above-Ground Drainage

Soil pipe connects WCs, baths, showers, and washing machines to the underground drainage system. Get the stack configuration right and you have a silent, odour-free drainage system that will last decades. Get it wrong and you are back on site dealing with slow drainage, gurgling traps, or smells coming back through the building. This guide covers 110mm PVC soil pipe systems — push-fit vs solvent weld, key fittings, stack venting, and the Building Regulations requirements that shape every installation.

Grey vs Black: Which to Use

Both grey and black soil pipe are 110mm OD and use the same fitting range — they are interchangeable. The choice comes down to application:

• Grey — traditionally used for concealed pipework in ducts, voids, and within buildings. Looks cleaner in airing cupboards and service areas.
• Black — preferred for external above-ground runs where the pipe is visible on the building exterior. Black has superior UV resistance compared to grey, which can become brittle and discolour with prolonged sun exposure. Always specify black for any externally exposed soil stack.

Never mix push-fit and solvent weld fittings in the same run without a compatible adaptor — push-fit ring seal fittings and solvent weld fittings have different socket depths and sealing methods.

Push-Fit Ring Seal vs Solvent Weld

Push-fit (ring seal) is the standard for most domestic above-ground soil work. A rubber ring seal inside the socket grips the pipe spigot and creates a watertight seal. The joint allows limited angular deflection (useful for minor alignment corrections) and thermal expansion movement — important for long vertical stacks where temperature differences between summer and winter can cause significant pipe movement.

Solvent weld creates a rigid, permanent joint. The solvent dissolves the pipe and fitting surfaces and they fuse together. Used where a fully rigid joint is required (commercial applications, where vibration is a concern, or where the fitting geometry does not allow a ring seal joint). Not suitable for long exposed runs where thermal expansion must be accommodated.

For most domestic installations, push-fit ring seal throughout is correct practice. Use solvent weld only where specifically required.

Key Fittings and Their Uses

92.5° swept bend — the most important fitting in a soil system. Used at the base of the soil stack where it turns from vertical to horizontal to connect to the underground drain. The swept (long-radius) version is essential here — a tight 90° knuckle bend at the base of a stack creates turbulence and increases the risk of trap seal siphonage. Always use a swept 92.5° double socket bend at the stack base, not a tight 90°. The Aquaflow 110mm 92.5° Double Socket Swept Bend is the correct fitting for this application.

90° knuckle bend — used for right-angle changes of direction elsewhere in the system (not at the stack base). Branch connections joining at 90° to the stack. Available in single socket and double socket versions. The Aquaflow 110mm 90° Knuckle Bend Double Socket suits tight spaces where a swept bend cannot be accommodated.

45° branch (swept tee) — used for connecting WC branches and other lateral connections to the main stack. The 45° swept entry reduces turbulence compared to a perpendicular connection. On a two-storey stack, offset branches so that they do not oppose each other — connections from both sides of the stack at the same height cause cross-flow and can siphon traps. The Aquaflow 110mm 45° Double Socket Branch covers this.

Adjustable bend (0–45° and 0–90°) — invaluable for awkward stack positions where the branch connection angle to a WC pan outlet or wall does not align with standard fitting angles. The Aquaflow 110mm Adjustable Bend 0–45° removes the need to cut and re-route pipework to hit standard angles.

Access pipe coupling — provides a rodding access point in the stack for clearing blockages. Required by Building Regulations (Approved Document H) at every change of direction in the drainage stack and at the base. The Aquaflow 110mm Access Pipe Coupling has a screw-cap access cover that can be removed for rodding.

Mushroom vent cowl — fitted at the top of an open vent stack above the roof. Prevents birds and debris entering the open pipe end while allowing air flow for stack ventilation. The Aquaflow 110mm Mushroom Vent Cowl clips or pushes onto the pipe end above the roof line.

Stack Venting: Open Vent vs Air Admittance Valve

Every soil stack must be vented to atmosphere to prevent trap seal siphonage. Without venting, flushing a WC creates a pressure wave that can suck the water seal from adjacent basin and bath traps, allowing sewer gas into the building.

Two options:

• Open vent stack — the stack pipe extends above the roof line and terminates open to atmosphere (with a cowl to prevent ingress). Required by Building Regulations H1 for the primary stack on most buildings. The open top allows air in and out freely during drain flow.
• Air admittance valve (AAV) — a one-way valve that opens under negative pressure (inflow of air when water drains) and closes under positive pressure (preventing sewer gas escaping). Permitted by Building Regulations on secondary stacks and branch connections, but NOT on the primary stack in most cases, and not where the stack connects to a shared drain or where positive pressure could occur. The 110mm Air Admittance Valve is suitable for secondary stacks and long branch runs where a full open vent to the roof would be impractical.

Always check Approved Document H requirements before specifying an AAV as the sole ventilation on a primary stack. In most domestic cases the primary stack must have an open vent terminating at least 900mm above any opening window within 3 metres.

Pipe Support and Fixing

110mm soil pipe requires pipe clips at maximum 1.8m centres on vertical runs and 1.2m on horizontal. Use dedicated 110mm soil pipe clips — standard conduit clips are not rated for the pipe weight and thermal movement. Leave a minimum 10mm gap between the pipe and the wall at each clip to allow thermal expansion movement. Do not over-clamp — the pipe must be able to slide longitudinally through the clip.

In Stock — Same-Day Collection Acton

We stock the Aquaflow 110mm push-fit soil pipe fitting range in black at 24 Western Avenue, Acton W3 7TZ. Call 020 8702 8080 for availability on less common fittings.

Browse: Soil Pipes & Fittings at APM | Plumbing Fittings