Waste Pipe Guide: Choosing the Right Pipes for Your Project

When it comes to home improvement and plumbing upgrades, understanding the different types and sizes of waste pipes is essential. These unsung heroes of your plumbing system are responsible for carrying greywater away from your sinks, showers, baths, and appliances. Choosing the correct type of waste pipe—whether it’s push-fit, solvent weld, or soil pipes—can prevent leaks, clogs, and costly repairs down the line.

In this guide, we’ll walk you through waste pipe sizes, connection types, and how they differ from soil pipes, helping you choose the right building materials for your next plumbing project.

Waste Pipe Sizes and Their Uses

Waste pipes come in several standard sizes, each designed for specific fixtures based on how much water they typically drain. Picking the right size pipe is critical to ensuring your plumbing system functions efficiently.

32 mm Waste Pipes

Best for:

• Bathroom basins

32 mm pipes are typically used in areas with low water volume. They’re perfect for bathroom hand basins where the flow of water is minimal.

40 mm Waste Pipes

Best for:

• Baths

• Showers

• Kitchen sinks

The most commonly used waste pipe in residential settings, 40 mm pipes handle larger volumes of water. You’ll find them under your kitchen sink, bath, and shower.

21.5 mm Waste Pipes

Best for:

• Toilet overflow systems

• Condensate pipes from boilers

These small-diameter pipes are used for specific low-flow applications. You’ll usually see 21.5 mm pipes used to route excess water from toilet cisterns or to drain condensate from modern combi boilers.

50 mm Waste Pipes

Best for:

• Commercial plumbing applications

Larger waste pipes like 50 mm are typically reserved for commercial environments or large residential setups with multiple fixtures draining into a single pipe.

Push-Fit Waste Pipes vs Solvent Weld Waste Pipes

Once you’ve selected the right size, the next step is choosing a connection method. There are two main types of connections used in waste pipe systems: push-fit waste pipes and solvent weld waste pipes.

Push-Fit Waste Pipes

Push-fit waste pipes are ideal for beginner DIYers or quick installations. These systems work by simply pushing the pipe into a fitting to create a water-tight seal. No adhesives or special tools are required.

Advantages:

• Quick and easy to install

• No chemical bonding required

• Easily disassembled for corrections or modifications

Best for:
DIY projects, small plumbing upgrades, or where future disassembly may be necessary.

Solvent Weld Waste Pipes

Solvent weld waste pipes use a special solvent cement that chemically fuses the pipe and fitting together. This creates a solid, permanent bond that’s highly resistant to movement and leaks.

Advantages:

• Strong, durable connections

• Ideal for fixed installations

• Better suited for long-term reliability

Drawbacks:

• Requires more precision

• Difficult to undo once bonded

Best for:
Professional installations, high-traffic areas, or where pipe movement is a concern.

Can You Mix Push-Fit and Solvent Weld Pipes?

Since push-fit and solvent weld waste pipes have slightly different external diameters, they’re not directly compatible. However, if you're updating your plumbing system or switching connection types, you can bridge the gap with a universal compression fitting or conversion coupler. These adapters allow for a smooth transition between systems without compromising the seal.

Waste Pipes vs Soil Pipes: What’s the Difference?

A common source of confusion in plumbing is understanding the difference between waste pipes and soil pipes. Though they look similar, they serve very different functions.

Soil Pipes

Soil pipes carry blackwater—the waste from toilets—safely away from your home and into the sewer or septic system. They’re typically larger in diameter and must comply with strict regulations to prevent contamination and odour leakage.

Waste Pipes

Waste pipes, on the other hand, carry greywater from showers, sinks, washing machines, and dishwashers. They’re generally smaller than soil pipes and do not carry human waste.

Tip: Always check local building regulations when working with soil pipes, as incorrect installation can pose serious health risks and legal consequences.

Choosing the Right Building Material for Home Plumbing

The materials you use for your waste system matter just as much as the size and type. Waste pipes are typically made from plastic (like PVC or ABS), making them lightweight, durable, and resistant to corrosion—ideal building materials for both interior and exterior use.

When choosing a system for your home improvement plumbing project, consider:

• The ease of installation (push-fit vs solvent weld)

• The risk of future pipe movement

• Access to fittings and adapters

• Whether the installation is temporary or permanent

Final Thoughts

Upgrading or installing a waste system is a key component of any plumbing-related home improvement. Whether you’re fixing a kitchen sink, adding a new bathroom, or upgrading an old boiler line, understanding the differences in waste pipe sizes, connection types like push-fit waste pipes and solvent weld waste pipes, and the role of soil pipes will help you make informed decisions.

By choosing the right pipe size, connection method, and building materials, you’ll ensure your plumbing is efficient, compliant, and built to last.