The Ultimate Guide to Soil and Waste Pipes

By / January 11th, 2023 / There are currently 0 comments

If you are looking to repair or replace your waste plumbing but are confused about what types of pipes you need, you’re in the right place. Repairing and replacing waste pipes can sometimes be technical and messy, so if you are unsure, we always advise that you consult a professional before undertaking any DIY plumbing.

This guide will explain the difference between soil and waste pipes, how to plumb them correctly and how to maintain them.

Table of contents

What is a waste pipe vs a soil pipe?

A waste pipe and a soil pipe are different and provide different functions. Waste pipes carry wastewater or greywater from your bathroom sink, shower, and bath and from your kitchen sink, washing machine, and dishwasher to the sewer, or to your soil pipe which then goes to the sewer. A soil pipe takes soiled waste mainly from your toilet carrying both water and solids to the sewer. A soil pipe also often picks up waste pipes in the stack when they are not directly terminated into the sewer often from the first floor of a property up.

Waste pipes were traditionally either copper, iron or lead. These can still be found in older properties however plastic pipe is mostly used now as it is less expensive, easy to install and resistant to corrosion and impact damage, they are also generally less noisy. Waste pipes come in both a solvent form where they require solvent cement to create a sealed connection and a push-fit that requires no tools and is pushed together with a ring seal creating a secure joint. Solvent waste and push-fit are made from different materials and are not interchangeable. They also have slightly different outside diameters.

Traditionally, soil pipes were cast iron. However, like waste pipes, soil pipes are now predominantly plastic because they are cheaper, more lightweight, flexible, tough, and resistant to household cleaning chemicals. Like waste pipes, they also come in a solvent and push-fit form. Unlike waste pipes, solvent soil and push-fit soil are interchangeable.

Other types of soil and waste are also available for more specific installations. Cast iron soil is also still widely available whether looking to connect to existing systems, new installations for specific styling, or period properties which have a specified requirement.

Does size matter?

Another difference between soil pipes and waste pipes is their size.

What sizes do waste pipes come in?

Waste pipes have smaller diameters because they only carry wastewater with no solids. The standard UK sizes are 32mm often referred to as 1.1/4”, suitable for low-volume outlets, like bathroom sinks/basins; 40mm (1.1/2”), which works for most kitchen sinks, showers and baths and 50mm (2”), which is suited to commercial appliances or connecting multiple waste pipes into a single outlet when increased capacity is required.

What is a typical soil pipe size?

Soil pipes need to be bigger than waste pipes to accommodate the solid material that needs to flow smoothly through them. They come in a 110mm (4”) diameter, usually used above ground in domestic households, or 160mm (6”) diameter which is most commonly used in larger buildings or commercially. Soil pipe is also used sometimes as a downpipe on large properties for commercial rainwater systems. 

Vents and valves

A crucial difference between soil pipe and waste pipe systems is the requirement to vent soil pipes. Human waste builds up dangerous and unpleasant gases that need a place to escape. That’s why if you have an internal or external soil stack pipe, you will see a soil vent pipe above the roof line for gases to exit. You will also need a vent terminal so large debris and animals can’t get in the pipe.

However, soil and waste pipes also need a place to allow air to flow into the system to prevent the syphoning effect. For waste pipes to flow, they need to be full of water. However, when you flush or run a tap, you create negative pressure in the pipes. As a result, air draws from the nearest place possible, like a toilet, sink or bath, to equalise the system. Unfortunately, this can cause the drainage to fail and cause unpleasant smells from your drains. It also creates that gurgling sound from your sink and bath drains. 

So, to prevent this, you need an air vent or valve to allow air in to equalise the pressure. If you have a combined soil and waste system, the vent at the top of the soil stack fixes this problem. However, if you have a separate water waste and soil system, you will need to attach an air admittance valve to your wastewater system, which allows air in when the pressure needs equalising.

Some modern buildings use air admittance valves instead of a soil pipe vent to avoid an unsightly pipe sticking out at the top of your house. However, if you install one of these, you must ensure the methane gasses have an alternative place to escape elsewhere.

How to connect waste and soil pipe systems

How to connect waste and soil pipe systems

Depending on the age of your property and your DIY skills, you might be able to replace or repair the waste and soil pipe systems.

External plastic soil stacks are generally easy to access for refurbishment or repair works or new installations. However, if it is an old cast iron soil stack these are much heavier in weight and you might need a professional to inspect it, as it could have asbestos you need to dispose of correctly.

Internal stacks more commonly found on newer properties are generally in stud walls and under floors meaning these will need accessing for any works.

How do you connect two waste pipes?

You will inevitably need to connect wastewater pipes together if you are plumbing a waste pipe system. Couplings connect two pipes together to make a long straight pipe. If you want to change direction use a bend or if you need to join a waste pipe in the middle of another waste pipe, use a single tee, or for three waste pipes cross tees can be used.

You can use push-fit or solvent-weld waste pipes, depending on your preference. However, you will need solvent cement if you use solvent weld pipes.

Remember push-fit and solvent-weld waste pipes are not interchangeable.
If you have traditional waste pipes and need to replace or repair them, you can use a compression fitting or flexible rubber coupling to join two different types of waste pipe or where there is a slight variation in size.

How to connect a waste pipe to a soil pipe

If you have a combined waste system, your waste pipes will terminate into your soil stack. This connection is known as a boss connection and comes in varying forms. You can use a boss pipe with a boss connector/adaptor to take the correct size waste pipe. Alternatively, you can get strap-on/clip bosses and mechanical boss connectors which are especially useful when retrofitting into an existing stack.

How to maintain your pipes

Maintaining your waste pipes will help avoid blockages and clogs and ensure that your pipes will last longer. Here are our top tips on how to keep your soil and waste pipes running smoothly.

  • Don’t use excessive toilet paper, only use what you need. Also, avoid using thick, quilted toilet paper. 
  • Do not flush anything other than toilet paper and bodily waste. That means no plastic, sanitary products, nappies and wet wipes, even if they say “flushable”. These items can get stuck around the bends and cause blockages.
  • Waste water pipes should only channel water, so avoid sending food down the plug hole. Even sending grease and fat down there can cause a blockage.
  • Similarly, avoid clogging your bath and shower by picking up any hair that falls in the drain hole.

Conclusion

Now you know the difference between soil and waste pipes, you can successfully fit or repair your waste plumbing pipes. However, if you’re worried about plumbing the pipes correctly, we recommend you contact a professional for help or guidance. 

Here at Build & Plumb, we want to supply you with the knowledge and tools to help you complete your plumbing jobs to a high standard, so why not browse our selection of high-quality piping and plumbing tools to get you on your way?

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