Connecting soil and waste pipes to below ground drainage

min read time
2023-02-01 12:05:38

Sewage systems play a fundamental role in every domestic, civic and commercial building. A robust, reliable and sustainable plumbing system is needed to successfully and safely discharge wastewater.

When properly designed and installed, drainage and waste systems prevent foul air from entering a building, minimise the risk of blockages or leaking, allow appropriate ventilation and reduce the likelihood of flooding.

In this blog, we’ll explore the principles of soil and waste pipes and will explain how to connect both to a below ground drainage system.

What is the main difference between a soil pipe and waste pipe?

Although soil pipes and waste pipes both transport wastewater out of a property and into the sewer, there are a few key differences between the two. The first is the type of waste they carry and the second is the size of the pipe.

Soil pipe overview
soil and waste system

As its name suggests a soil pipe transports soiled water (blackwater) and solid waste from the toilet to the sewer.

Unpleasant and other dangerous gasses, like methane, are produced from bodily waste so soil systems are designed to release these gasses from the system.

With the type of waste soil pipes transports, the size of the pipe is another important factor, and in the UK 110mm is the most commonly used soil pipe size.


Waste pipe overview

Waste pipes carry wastewater (greywater) from showers, sinks, baths, washing machines and dishwashers. The most common sizes of waste pipes in the UK are 32mm, 40mm and 50mm.

Typically, 32mm waste pipes are used in bathroom hand basins, 40mm is used for kitchen sinks, showers and bathrooms whereas the largest, at 50mm, is for commercial applications or systems which connect multiple waste pipes into one single flow.

The anatomy of branch pipework and soil stack

Understanding the basic anatomy of branch pipework and the soil stack is important before taking a closer look at connecting soil and waste pipes to below ground drainage.

The stack is the vertical pipework which is normally 110mm in diameter and occasionally 160mm in larger buildings.

Branch discharge pipework is the horizontal pipework connecting baths, showers, hand basins, urinals and kitchen sinks to the stack. The branch should always discharge either into another branch pipe, the main discharge stack or a gully.

Soil stacks are designed with either an external balloon grating to allow pressure fluctuations in and out or an air admittance valve to allow air in to balance air pressure. Both methods will prevent siphonage in water traps.

How to connect a soil pipe and waste pipe to below ground drainage

When it comes to connecting soil and waste pipes below ground, the Wavin Osma systems make it much simpler to enable direct connection of Osma Above and Below Ground with the use of couplers, connectors and adaptors. Incredibly versatile, these fixtures also work well with other systems made of PVC-U, cast iron and clay.

Connecting to PVC-U

110mm Osma Soil Pipe connections to PVC-U drainage pipes are incredibly straightforward with the ring seal/push-fit Jointing and Osma Below Ground Drainage fittings via the OsmaDrain Bend 4D581 and OsmaDrain Coupler 4D205.

OsmaDrain Bend 4D581

To connect the 110mm PVC-U soil pipe to a drain, simply push the pipe spigot into the socket of an OsmaDrain Bend (4D581).

connecting PVC-U pipe diagram

OsmaDrain Coupler 4D205

Alternatively, fit a coupler to the below soil pipe spigot to guarantee complete joint integrity.

Connecting Osmadrain pipe diagram

Waste to drain connections

Connecting waste pipes to below ground drains couldn’t be easier with Boss Socket Adaptors.

Boss Socket Adaptors are available with either push-fit sockets or solvent weld sockets. The Push-fit enables the connection to a Polypropylene pipe manufactured to BS EN 1451-1, and copper waste pipes manufactured to BS 659 or BS 2871.

Whereas solvent weld allows connection to ABS pipe which is manufactured to BS EN 1455-1 and PVC-C pipe manufactured to BS EN 1566-1.

Typical internal waste pipe connection to drain

To connect a typical internal waste pipe to a drain, fit the waste pipe to either a 32mm, 40mm or 50mm single socket boss adapter and then push it into an OsmaDrain Coupler.

Connecting a bottle gully diagram

Typical waste pipe connection to Bottle Gully

Connect the waste pipe to a 32mm single sock boss adapter, and place this inside a closed OsmaDrain Plain Ended Socket Plug. Then fit this to a drainage double socket short radius bend to successfully connect to the bottle gully.

Connecting soil pipes to cast iron or clay below ground pipes

When replacing an old soil pipe, the chances are that it connects to either a clay or cast-iron pipe and with Osma’s easy-to-use adaptors it’s a very easy task.

If you’re working with a cast-iron pipe, grind through part way to remove the old soil pipe. Then fit the adaptor. With a flexible base, it’s simple to push the adaptor down into the existing pipe.

And that’s it!

It works the same way for a clay pipe. First, you’ll need to fit an adaptor and then you can press in the rest of the new soil pipe.

Part H requirements for connecting soil and waste to below ground drains

There are several requirements that must be followed when connecting soil and waste pipes to below ground drains which are set out in the Approved Documents of the UK building regulations.

Part H relates to the drainage and waste disposal from buildings.

In relation to conveying foul water from an appliance to a discharge stack, Part H specifies that on the ground floor, the branch pipe from the WC should only discharge directly to a drain if the depth from the floor to the invest of the drainpipe is 1.3m or less.

It’s also important to be aware that Part H sets out the minimum distances between the invert of the bend at the foot of the stack, to the lowest branch connection into the stack – depending on the number of storeys in the building, as explained in the table below.

Building Type

Minimum distance (mm) from discharge into stack to invert of bend at foot of stack

Single dwellings up to 3 storeys


Multi-storey building up to 5 storeys

750mm (or ground floor discharge can connect direct to a drain or its own stack)

More than 5 storeys

Ground floor appliances must discharge into gully, drain or own stack

More than 20 storeys

Ground floor and first floor appliances must discharge into gully, drain or own stack

Part H also specifies what needs to be done to reduce the risk of blockages and recommends that the use of bends is minimised as much as possible.

It states:

  • Avoid bends in branch pipes where possible. But if bends are installed, make sure they have a large radius to reduce the risk of blockages.
  • Junctions on branch pipes should have a 25mm swept radius or a 45° junction should be used.
  • Large branch pipe connections to the soil stack (82mm and 110mm) should have a swept radius of 50mm or a 45° junction should be used.
  • If branch pipes discharge to a gully, they should terminate between the gully grating and the water level. Doing this will help to reduce the risk of freezing, and therefore, blockages.

Managing wastewater with Wavin

The Wavin Osma soil range offers a comprehensive choice of pipe and fittings. To connect your soil system, we offer push-fit and solvent weld waste ranges, traps, over-flow and condensate ranges to cover every installation need.

For managing wastewater we have a great range of systems that feature specialist seal technology, high performance pipes made with a minimum of 50% recycled content and quality fittings.

Wavin Osma Soil & Vent

A soil vent pipe (SVP) or soil stack pipe is a vertical pipe that removes sewage from a building.
Wavin Osma Soil and Vent offer you secure and durable connections by push-fit or solvent jointing.

HDPE Soil, Waste and Vent

For larger commercial or high rise projects Wavin HDPE is a tough and durable system of pipes and fittings manufactured from high-density polyethylene (PEHD). It offers incredible chemical resistance and great impact resistance.

Wavin AS+ Acoustic Soil System

Wavin AS+ is a high-spec plastic sound-insulating Soil & Waste system manufactured using high density mineral reinforced polypropylene (PP).

Due to the unique material composition for improved noise performance, premium reduction of unwanted sound emissions by the drainage system inside the building is assured.

To view the full Wavin soil range click here


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