What Is a Bentonite Clay Plug and Why Is It Used in Plumbing?

If you’re building a new home or doing major plumbing work, you might come across the term “bentonite clay plug.” While it might sound technical, it plays a very simple and important role: keeping unwanted water and dampness from getting under your home.

Let’s break it down.

So, What Is a Bentonite Clay Plug?

Bentonite is a natural clay that swells up when it gets wet—like a sponge, but even denser. It forms a sticky, waterproof barrier. In plumbing, we use it to block any gaps or pathways that might let water sneak through the ground.

A bentonite clay plug is just a packed section of this clay placed in a trench—usually where underground pipes run under your slab or through the foundations of your house.

this shows how and where the Bentonite Clay Plugs are installed in a trench. Typically the 'biscuit' part is a stiffened cardboard box when assembled, then filled with bentonite clay that typically comes in 25kg bags.

Why Is It Used in Plumbing?

When plumbers dig a trench to lay your sewer or drainage pipes, it creates a loose, open path in the soil—kind of like a tunnel. If left open, water from rain or groundwater can easily travel along that trench and end up under your slab, bringing moisture with it.

Over time, this moisture can lead to:

• Rising damp in your floors or walls

• Mould and mildew issues

• Slab movement or cracking due to shifting soil

That’s where the bentonite plug comes in. It’s packed into the trench (usually just outside the slab edge) and swells up to block the path, stopping water from flowing under the house.

Why Do the Plumbing Codes Require It?

Australian plumbing codes—including the National Construction Code (NCC) and AS/NZS 3500—require a water-tight barrier where pipes pass through or near your house slab. Specifically, they want to make sure water can’t flow back toward your home along the pipe trench.

A bentonite clay plug:

• Meets this code requirement

• Doesn’t rely on plastic or concrete seals that might crack

• Works naturally by swelling and sealing the trench

It’s an approved and reliable solution that plumbers use to protect your home’s foundations from moisture, damp, and long-term damage.

This shows an example illustration of where Bentonite clay plug sits within the ground in relation to the plumbing and the foundation (image courtesy of Plastec)

Simple, Affordable Protection

Bentonite clay is a natural product, easy to install, and highly effective. It’s a small part of your plumbing system, but it plays a big role in:

• Keeping your home dry and structurally sound

• Preventing water damage and slab movement

• Meeting Australian building and plumbing codes

If you have any questions regarding the use of Clay Plugs or need to chat about Articulation Reports, send us an enquiry or give us a call.