If you’re not a qualified plumber or drainage engineer, soil pipes and waste pipes can be a minefield to navigate. After all, there are lots of pipes inside your property and lots of pipes that exit it. And unless you have prior experience, understanding which pipe is which can be a nightmare.

One of the most common problems is confusion between a soil pipe and a waste pipe. Is a soil pipe the same as a waste pipe? And if not, how do they differ?

Here’s the thing:

In the simplest terms, both pipes carry waste from your home to the sewer – but there’s a crucial difference between the two types.

On the one hand, a soil pipe is designed to carry soiled water from the toilet, urinal or bidet (blackwater) to the sewer.

On the other hand, a waste pipe carries water from your sinks, shower, washing machine or bath (greywater).

Let’s take a closer look at the two.

Table of contents:


What is a soil pipe?

As the name suggests, a soil pipe is for soiled water. This type of pipe will carry water and solids into the sewer. While any pipe could physically perform the task, the soil pipe, also known as a soil vent pipe, has certain specific features.

Here’s the deal:

Firstly, a soil pipe is vented high at the top or near the top of a building with the help of a soil pipe stack. This allows gasses produced by waste to vent safely into the atmosphere.

Why is that important?

Such gasses can be harmful to health, so venting them high keeps them out of the way, reduces nasty odours and helps protect the environment. This is a vital feature of soil pipes, and it’s also part of UK building regulations.

Secondly, a soil pipe needs to be of a dimension sufficient to allow solid waste to pass.

The question is:

What size is a soil pipe?

A 110mm soil pipe is most common in the UK. However, other sizes are also available, both larger (160mm in diameter) and smaller (82.4mm in diameter).

What is a waste pipe?

Whether it’s a washing machine waste pipe or a sink waste pipe, this is often a smaller diameter pipe that carries greywater from any appliance you may have that uses water. It can be narrower than a soil pipe, as the waste pipe is only designed to carry water.

Additionally, it doesn’t need the same venting system as a soil pipe. That’s because wastewater doesn’t typically generate harmful gasses, so it doesn’t need to be vented high above ground level.

What is the difference between a soil stack and a waste stack?

Soil stacks are larger than waste stacks because soil pipes need to allow for the passage of solids commonly found in soil water. The most common soil pipe dimensions in the UK are 110mm and 160mm in diameter, although others are also available. Waste pipe sizes also tend to vary, with the 32mm flexible waste pipe option and the 40mm flexible waste option among the most popular choices in the UK.

You might be wondering:

Can you connect waste water to a soil pipe?

Indeed, you can use various waste pipe connectors and waste pipe fittings to connect a waste pipe to your soil pipe. One of the simplest and most popular is a strap on boss.

Here’s how:

How do I connect a plastic waste pipe to a cast iron soil pipe?

In order to connect a plastic waste pipe to a cast iron soil pipe, you should cut off a small section of the cast iron pipe below the joint you want to make. You can then replace the section with a flexible pipe connector that will act as a bridge between the smaller 32mm waste pipe/40mm waste pipe and the larger soil pipe.

Soil pipes and waste pipes in older and new properties

How your property handles waste depends on its age.

For instance, older properties may use separate soil and waste pipes to exit into the sewer. In contrast, newer houses may use a single system.

What’s more:

Older properties tend to use both a waste and a soil pipe that exit separately until they reach the drain. They would then combine and exit your property underground to the sewer. Newer properties, on the other hand, use a simpler single-pipe system that utilises an internal soil stack. This is a thick vertical pipe that reaches above your gutter to provide safe venting.


Both soil and waste pipes will run from your toilet, sinks, shower, washing machine, urinal, bidet and any appliance that voids water to join the soil stack. The soil stack pipe will run directly into your underground drain.

Converted older homes may have a mixture of the two systems. They often use an external soil stack connected to the main drain, with all pipes joining the stack. That’s why using the right pipe is critical, especially for soiled water. If in doubt, you should always contact a professional.

Now that we know the main differences between a soil pipe and a waste pipe, we’re going to address the most common issues related to the two types.

How to connect toilet to a soil pipe

A toilet pan connector is used to connect the toilet to the soil pipe. Make sure you’ve measured the diameter of your soil pipe before buying a new toilet pan connector, so you get the appropriate size.

Before you begin, cover the toilet waste pipe with some cloth or a plastic bag and seal it so that no malodorous gasses come up from the sewer. Then, apply some silicone lubricant along the bottom end of the pan connector and fit it securely into the soil pipe. It’s not absolutely necessary to use lubricant to do this, but it will make things a lot easier if you do.

Once you’ve pushed the toilet connector into place, you can put your toilet back in its location and proceed to screw the toilet pan and cistern down. The process will vary depending on the type of toilet.

How to connect a soil pipe to sewer


The pipe which takes soiled water from your soil pipe to the sewer system is called an underground drain. Connecting to it is a time-consuming project that requires strong DIY and plumbing skills, which is why it’s typically best left to professions.

To give you a general idea:

You’ll have to carefully expose the underground drain with a shovel so as not to damage it. After that, you’ll need to measure and cut the drain, fit a bend, and add a piece of pipe long enough to reach slightly above concrete level. At this point, you should be able to connect your soil pipe.

How to unblock a soil pipe

In order to avoid blocked soiled pipes, you shouldn’t put any household items in your toilet.

Some of the most common culprits are:

  • Cleaning wipes
  • Baby wipes
  • Sanitary items
  • Nappies
  • Cotton buds
  • Syringes
  • Razors

Still, even the most diligent of us find ourselves having to deal with soil pipe blockage every once in a while.

But don’t worry!

Unless your soil pipe is made of cast iron – in which case it’s best to call a professional engineer – you can fix it yourself without too much hassle. All you need is a drain auger, also known as electric eel, snake tool or toilet jack.

Here’s how you go about it:

  1. Find the cleaning eye (an access opening to the interior of the plastic soil pipe).
  2. Unscrew it and insert the drain auger.
  3. Pass the drain auger through the pipe until you locate the blockage.
  4. Crank the handle of the drain auger to release a coiled wire.
  5. Push and pull the drain auger several times until water begins to flow freely.
  6. Disinfect the cleaning eye and screw it back into place.

How to disguise an outside soil pipe

Unlike connecting your soil pipe to an underground drain, disguising it is a much more pleasant DIY project. It’s also one where you can let your imagination run wild.

Here are a few tips on how to hide a soil pipe (or any other type of external pipe, for that matter):

  • Paint the pipes using a colour that matches that of your external walls.
  • Fit pipe covers that complement your landscape design.
  • Use plants, such as evergreen cloud cover for horizontal or low vertical pipes, or ivy for higher vertical pipes.
  • Industrialise your pipes by adding a contemporary, urban finish.

Final thoughts

Pipes can be confusing, and it’s not always immediately clear which pipe does what exactly.

That being said:

It’s important to have a basic understanding of the main differences between soil pipes and waste pipes so you can maintain them properly and avoid blockages and other issues. We hope our guide has shed sufficient light on the matter.

Was this guide useful?

Need more Help & Advice? Speak to our Super team on 01752 692 221

Can’t find the Help & Advice you need? Speak to our Super team today on 01752 692 221