External Soil Pipes: An Installation Guide

A soil pipe transports water containing human waste from toilets and bidets and shouldn’t be confused with a waste pipe; if you’re unsure of the difference, read our article that explains this.

Installing a soil pipe isn’t a job for you unless you’ve done it before or have the skills and confidence to have a go. We would strongly recommend using a professional to install and connect a soil pipe to an internal soil stack, but if your soil pipe is going straight through an external wall and dropping to a connection with a sewer – and you think you’re skilled and confident enough – you could give it a go.

Still reading? Okay, well if you really want to try it, here’s how. Remember this is a guide, and don’t tell us you weren’t warned!

What you’ll need to install an external soil pipe

Let’s start with some good news: most of the tools you need are probably already in your toolbox.

• Suitable PPE: gloves, eye protection, hearing protection
• A tape measure
• Something to cut plastic pipe (a hacksaw or fine-toothed hand saw should be okay; a pipe-cutting tool if you have one would be better)
• A power drill (preferably an SDS drill)

One item that may not be in your toolbox is a hole saw to cut through whatever your external wall is made out of.

You’ll also need all the pipe lengths, bends, etc. to assemble the complete pipe, and if you’re using solvent weld soil pipe, some plastic solvent cement (again, if you’re a newbie to this, we would recommend using push-fit rather than solvent weld).

How to install an external soil pipe

The soil pipe should be assembled and installed before it is connected to the sewer and the toilet, urinal or bidet is connected to the soil pipe. If you are using push-fit pipes and connectors, apply silicone lubricant to the sockets and pipe ends to make them fit together more easily.

Step one: Cut a hole through the outside wall

Careful measuring is essential here; the hole must be exactly in the right place for the connector inside the building to precisely align with the sanitaryware. It’s also best to make the hole diameter as close as possible to the external diameter of the pipe, as that will make it easier to seal around the pipe for insulation.

Drill the hole from inside the building, until the smaller guide drill bit has penetrated the outer face of the wall, then finish the drilling from outside the building for a neater result.

Step two: Fit the external soil pipe

The soil pipe will rise from the underground drain to a branch, which will sit below the level of the hole you have just drilled. 

To establish how long the soil pipe needs to be, push a length of pipe into the underground drain connection, applying firm pressure until it rests against the stop inside the connector. Now take another length of pipe and insert it into the branch of the branch connector. Hold this pipe diagonally so that the main section of the branch connector is vertical and aligned with the soil pipe while the other end is aligned with the hole in the wall. This will show you where the soil pipe should be cut (remembering to compensate for the extra pipe that will be inside the branch connector).

Step three: Cut the pipe that will pass through the wall

To calculate how long this length of pipe needs to be, take the distance between the diagonal pipe and the wall, the thickness of the wall – a cavity wall is generally 300mm thick – and another 300mm to allow for the toilet pan connector.

Step four: Finish the connection to the soil pipe

Outside the building again, push the third length of pipe into a bend connector and insert the other end of the pipe through the wall. Rotate the bend connector to align with the diagonal pipe and cut the pipe to length so it will fit neatly into the bend (again allowing for the amount of pipe that will be inside the connector). Push the diagonal pipe into the bend connector.

Step five: Add a vent pipe

Add another section of soil pipe to the top of the branch connector to create a vent pipe. The top of this vent pipe must be at least 900mm higher than any window or opening that is within 3 metres of the vent pipe. Add a cowl to the top of the vent pipe to prevent birds from using it as a convenient nesting place.

Step six: Finish the joints

Now you can double-check that all pipework is correctly aligned and securely connected. 

• If you have used a push-fit soil pipe system, all you need to do is secure the soil pipe to the outside wall of the building with brackets and seal the gap between the pipe and wall with a suitable mastic.
• If you are using a sealant-weld soil pipe system, you must now disassemble all the joints and apply solvent cement to each of them, ensuring a complete seal between the sockets and pipes. The pipework should then be left for at least 24 hours to allow the cement to dry and harden before moving or working on the pipe.

If you aren’t fitting the toilet immediately, temporarily seal the exposed end of the pipe with some old cloths or a plastic bag to prevent unpleasant smells in the house.

Connecting a toilet to a soil pipe

If you temporarily sealed the soil pipe to prevent smells, remove the cloths or plastic bag used to seal it.

The toilet pan connector will feature a rubber gasket that allows for some movement between it and the soil pipe without affecting the seal, and you will find the next stage easier if you apply silicone lubricant to the inside of this gasket.

Attach the toilet pan connector to the toilet then place the toilet in its final position, attaching the back end of the toilet pan connector to the soil pipe and connecting the toilet to the cistern.

Fix the toilet pan in place. Depending on the design of the pan and your personal preference, this can be done with screws, mastic or a combination of the two.

Can a wastewater pipe be connected to a soil pipe?

Yes, there’s no reason why a wastewater pipe from a basin, bath or shower shouldn’t be connected to the soil pipe serving a toilet or bidet, and it makes sense if you are adding another bathroom or toilet.

We hope you have found this information helpful and interesting. As always, we recommend seeking professional advice before embarking on any projects that you are unfamiliar with, and if you have any further questions you are always welcome to call our friendly team of experts on 01420 555600 or email [email protected]. 

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!