110mm Soil Pipe: The Pipe That Carries Everything You'd Rather Not Think About

Your extension needs a toilet. The toilet needs a 110mm soil pipe connecting it to the underground drain. Get that pipe wrong, get the gradient wrong, forget the fire collar at the floor penetration, or skip the air test before your plumber boards everything in, and you're looking at ripped-out plasterboard, a failed building control inspection, and a bill that dwarfs the cost of the pipe itself. Soil pipe is cheap. Fixing soil pipe mistakes is not.

What it is and what it's for

A 110mm soil pipe is the large-diameter plastic pipe that carries waste from toilets (and sometimes other appliances) down to the underground drainage system. The "110mm" refers to the outside diameter. You'll hear it called a soil stack, a soil vent pipe (SVP), or just "the stack." It's the backbone of your extension's foul drainage.

The pipe does two jobs. First, it carries waste downward by gravity to the drain. Second, the top section acts as a vent, allowing air into the system so waste flows freely without creating a vacuum that would suck water out of your traps. Without that ventilation, every time someone flushes, you'd hear gurgling from every sink and basin in the house. Worse, the lost trap seals would let sewer gas into your rooms.

In the UK, 110mm is the standard diameter for any system that includes a WC (toilet) connection. Smaller waste pipes (32mm for basins, 40mm for sinks and showers) connect into the soil stack via boss adaptors or branch fittings. The system is governed by Approved Document H of the Building Regulations in England and Wales, and building control will inspect the drainage layout before it's covered up.

Types and variants

Nearly every soil pipe sold at UK retailers is uPVC (unplasticised polyvinyl chloride). Cast iron still exists, mainly for heritage or conservation area work. The practical choice for any extension is uPVC push-fit.

The colour coding reality

Grey for internal, black for external is a convention, not a regulation. Both are PVC-U with UV stabilisers and they're functionally identical above ground. Use whichever your plumber prefers or whichever is in stock.

Below ground is different. You must use brown or terracotta pipe. It's not just a colour change; below-ground pipe has different chemical stabilisers designed for long-term soil contact. Using grey or black pipe underground will eventually degrade.

Push-fit vs solvent weld

For 110mm soil pipe, push-fit ring-seal is the standard. Unlike smaller waste pipes (where push-fit and solvent-weld diameters differ), 110mm push-fit and solvent-weld pipes are the same outside diameter and can be mixed using adaptor fittings.

Push-fit is faster, more forgiving, and allows thermal movement. The rubber ring seal inside each fitting creates a watertight joint when the pipe is pushed home. After pushing in fully, pull the pipe back 10mm from the stop. That 10mm gap is the thermal expansion allowance. Skip it on a long run and the pipe will buckle in summer.

Solvent weld creates a permanent chemical bond. It's neater and slightly more rigid, which some plumbers prefer for visible runs. But a solvent-weld joint that's wrong is wrong forever. You cut it out and start again.

How to work with it

Cutting

Cut 110mm soil pipe with a hacksaw or a pipe cutter designed for large-bore plastic. A fine-tooth hacksaw blade (32 teeth per inch) gives the cleanest cut. The critical thing: cut dead square. An angled cut won't seat properly in the push-fit fitting, and you'll get a leak under test. After cutting, chamfer the outside edge with a file or sandpaper (about 15 degrees) so the pipe slides past the rubber seal without displacing it.

Joining push-fit

Clean the pipe end. Apply silicone lubricant to the spigot end (most manufacturers include a sachet, or use washing-up liquid in a pinch). Push the pipe firmly into the fitting until it hits the internal stop. Then pull back 10mm. Mark the pipe with a pencil at the fitting face before pushing in, so you can see the 10mm gap afterward.

On longer horizontal runs, add an expansion coupling every 4 metres. Push-fit joints handle some movement, but 4m of PVC-U expands roughly 2.5mm per 10 degrees Celsius of temperature change. Over a full-length 3m pipe, that's enough to stress joints if there's no relief point.

Support and fixing

Vertical runs: pipe clips every 2 metres maximum (BS EN 12056). Horizontal runs: clips every 1 metre maximum. Use proper 110mm pipe clips, not improvised brackets. The pipe is heavy when full of water, and sagging between clips creates low spots where solids accumulate.

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Gradient

Horizontal soil pipe runs need a fall of between 18mm and 90mm per metre (roughly 1:55 to 1:11). Too shallow and solids don't move. Too steep and water outruns the solids, leaving them stranded. The practical sweet spot is around 1:40 (25mm per metre).

There's a persistent myth about a maximum gradient in the regulations. Approved Document H specifies a minimum fall but no maximum. A vertical stack works perfectly well, after all. But avoid gradients steeper than about 1:40 on horizontal runs connecting a WC, because the physics of water separating from solids is real. Your plumber will know this instinctively.

Connecting waste pipes to the stack

Smaller waste pipes (basins, sinks, showers) connect to the 110mm soil stack through boss fittings. Three methods:

Strap-on boss is the most common for retrofit. You drill a 54mm hole in the stack using a hole saw, clean the edges, and solvent-weld or push-fit the boss adaptor over the hole. The boss accepts 32mm, 40mm, or 50mm waste pipe via reducing adaptors. Cost: around £12.

Boss pipe is a short section of 110mm pipe with factory-moulded boss connections already built in. Cleaner than a strap-on but requires replacing a section of stack.

Branch fitting (a 92.5 degree Y-junction) is the strongest connection and is required for WC branches. WCs always connect via a proper branch, never a boss.

Position boss connections at least 200mm below a WC branch centreline. This prevents the WC flush from backflowing into the waste pipe.

Building regulations and inspection

Soil pipe installation falls under Approved Document H (drainage and waste disposal) in England and Wales. This is not optional. Building control must inspect the drainage layout before it's hidden behind plasterboard or buried underground.

Key regulatory requirements

The SVP must extend to open air. The traditional approach is to run the pipe through the roof and terminate at least 900mm above any opening (window, door, air vent) within 3 metres. In practice, most extension soil stacks now use an air admittance valve instead (see below).

All horizontal branches connecting to the stack must use 45-degree sweep bends, not sharp 90-degree junctions. A 90-degree bend at the base of the stack where it turns to run underground is acceptable only if it's a long-radius (or two 45-degree) bend.

Rodding access points must be installed at changes of direction and wherever the system can't be cleared from the ends. Without them, a blockage means cutting the pipe.

Fire stopping

Your builder or plumber must install fire collars before the floor screed is poured over the penetration. If the collar isn't in place before the concrete goes down, retrofitting it is a serious problem.

The air test

Before plasterboard goes up, the entire soil and waste system must pass an air test. The test is simple: seal all open ends with inflatable plugs, pressurise the system to 38mm water gauge using a hand pump, and hold for 3 minutes. The pressure must not drop below 25mm water gauge.

A test kit costs £40 – £60 from Screwfix or Toolstation. Your plumber should own one. If they suggest skipping the test or doing it "later," that's a red flag. Once plasterboard is up and painted, finding and fixing a leak means tearing it all down again.

Acoustic treatment

Internal soil stacks are noisy. Every flush sends water cascading down the pipe, and the sound transmits through the plasterboard boxing. NHBC Standards (Section 8.1.10) require all internal soil and vent pipe sections to be wrapped with minimum 25mm unfaced mineral wool at 10kg/m3 density, carried through the full thickness of any sound-insulating floor.

Box the wrapped pipe in with two layers of acoustic plasterboard on an independent timber frame (not fixed directly to the pipe). This detail is cheap to do during first fix and expensive to retrofit. If your builder skips it, you'll hear every flush from the bathroom above for the life of the house.

SVP through the roof vs air admittance valve

This is the decision most homeowners don't know they have. The traditional approach runs the soil stack up through the roof, with a weathering slate and a cage terminal on top. It works, but it means roof penetrations, lead flashing, and potential leak points.

An air admittance valve (AAV) sits on top of the stack inside the building (in the loft space, a cupboard, or boxed in above the highest connection). It opens to let air in when water flows down the stack, then closes to prevent sewer gases escaping. No roof penetration needed.

For a single-property extension, an AAV is accepted by most building control officers. The BPF Pipes Group guidance (the plastics industry body) confirms AAVs are suitable for primary and secondary ventilation in domestic properties. The AAV must comply with BS EN 12380:2002, be installed vertically, and sit at least 200mm above the highest water entry point in the system.

The exception: if your property is on a shared drainage run with neighbours, at least one stack in the group must be an open vent to atmosphere. This is the "rule of five" - one open vent per five houses sharing a drain. For most single-house extensions, it doesn't apply. Check with your building control officer.

An AAV costs around £20 – £30. Compare that to the roofing work for a traditional SVP penetration.

How much do you need

A typical single-storey rear extension with one WC and one basin connecting to a new or existing soil stack needs surprisingly little pipe. The stack itself is one vertical run from the underground drain to above the highest connection.

Worked example for a 3m stack height (ground floor WC connecting to underground drain, stack vented with AAV in loft space):

Item	Quantity	Approx unit cost	Subtotal
110mm soil pipe, 3m length	1-2	£20 – £23	£20 – £46
92.5° single branch (WC connection)	1	£17 – £18	£17 – £18
92.5° bend (base of stack)	1-2	£11 – £14	£11 – £28
Strap-on boss adaptor (basin waste)	1	£12	£12
Double socket coupler	1-2	£9 – £10	£9 – £20
Pipe clips (pack of 5)	1-2	£10 – £11	£10 – £22
Air admittance valve	1	£20 – £30	£20 – £30
Intumescent fire collar	1-2	£15 – £25	£15 – £50
Total materials			£114 – £226

Add 10% for offcuts and the odd fitting you didn't anticipate. Soil pipe fittings are cheap individually but the list adds up. Buy everything in one trip to avoid multiple delivery charges.

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Cost and where to buy

A 3m length of 110mm uPVC soil pipe costs £20 – £23 at national retailers including VAT. Screwfix and Toolstation are both at £20.74 for a FloPlast 3m length (grey or black). Wickes charges £23 for the same pipe. Shorter 1m lengths cost £12 – £13 each, useful for stub runs or where you're cutting to size anyway.

FloPlast is the dominant brand at all major UK retailers. Polypipe and Brett Martin are alternatives stocked at builders' merchants. For a domestic extension, they're all interchangeable as long as everything is PVC-U and to BS EN 1329.

Cast iron is a different price bracket entirely. At £80 – £200 per length, it's only justified where planning conditions demand a traditional appearance (conservation areas, listed buildings) or where noise is the overriding concern in a multi-storey property. Cast iron is quieter than plastic because the mass of the pipe absorbs sound. But proper acoustic treatment on a plastic stack achieves good results at a fraction of the cost.

Alternatives

Cast iron is the only real alternative material for above-ground soil drainage. It's heavier, harder to cut, and far more expensive. Unless you're in a conservation area, stick with uPVC.

For the ventilation question, an air admittance valve replaces a full SVP roof penetration in most domestic situations. It's simpler, cheaper, and avoids weatherproofing complications. But it must be accessible for maintenance (AAVs can stick closed over time) and it's not appropriate for every drainage layout.

If your extension is close to the existing house's soil stack, connecting into that existing stack (via a slip collar or Fernco flexible coupling) is often simpler than running an entirely new stack. Your plumber will assess whether the existing stack has capacity. A single WC and basin addition rarely exceeds the flow rate of a standard 110mm stack.

Where you'll need this

110mm soil pipe appears wherever a WC or major appliance connects to the drainage system:

• Drainage - main drainage runs from inspection chambers to the sewer connection; underground sections use terracotta/brown pipe
• First fix plumbing - installing the soil stack, WC branches, boss connections for waste pipes, fire collars, and acoustic wrapping before plasterboard
• Garden and external works - external drainage connections, inspection chamber installations, and any above-ground external stack runs

Soil pipe work spans groundwork and first-fix phases of any project that adds or moves a toilet. The material cost is modest. The labour and compliance costs (air testing, fire stopping, building control sign-off) are where the real budget sits.

Common mistakes

Forgetting the air test before boarding. This is the single most common soil pipe mistake on forum threads, and it's the most expensive to fix. Once plasterboard is up and painted, finding a leaking joint means tearing the boxing out. Test before your plumber leaves site. A cheap test kit is good insurance.

Missing intumescent fire collars. Every floor or wall penetration through a fire compartment needs a collar. Your plumber may not think of it (it's a building regs requirement, not a plumbing one). Your builder may assume the plumber handles it. Somebody needs to own this. Check before the screed goes down.

No expansion gap in push-fit joints. The 10mm pull-back after pushing a pipe into a fitting is there for thermal expansion. Skip it and the pipe has nowhere to go when it warms up. On a long horizontal run, the stress can push a joint apart. Ten seconds of attention during assembly prevents it.

Wrong gradient on horizontal runs. Too shallow (less than 18mm per metre) and waste sits in the pipe. Too steep (more than about 1:40 on a WC connection) and water outruns solids. Either way, you get blockages. Check gradient with a spirit level and a measured shim.

Using above-ground pipe below ground. Grey and black pipe are not rated for soil contact. Below the DPC and underground, you must use brown/terracotta-coloured pipe with the correct chemical stabilisers. Getting this wrong doesn't cause immediate failure, but the pipe degrades over years.

Skipping the acoustic wrap on internal stacks. Every flush is audible through a single layer of plasterboard. Wrapping the stack with 25mm mineral wool and boxing it in properly takes an hour during first fix. Doing it after the house is finished requires stripping and re-plastering.