If you're planning a new build, a renovation, or just repairing a broken
water line, you've likely encountered PEX pipe. Its flexibility,
cost-effectiveness, and corrosion resistance have made it a favourite for indoor
plumbing across Australia. But a common question we hear from homeowners and
builders alike is: Can PEX pipe be used underground?
The short answer is yes, but with very important caveats. PEX pipe is
approved for underground use in Australia, but it must be installed correctly
and in compliance with Australian Standards to ensure a long-lasting and
reliable system.
Let's dive into the specifics of how to use PEX pipe underground the right
way.
Understanding PEX Pipe and Its Strengths
PEX (cross-linked polyethylene) is a durable, flexible plastic pipe. Its
key advantages include:
Freeze Resistance: PEX can expand and contract, making it more resistant to
cracking if water inside freezes—a concern in some colder Australian
regions.
Corrosion Proof: Unlike copper, it won't corrode from acidic water or
soil.
Flexibility: It can bend around obstacles, often requiring fewer fittings
and reducing potential leak points.
The Golden Rule: Use a Protective Conduit (Sleeving)
The single most important rule for underground PEX installation in
Australia is that it must be run inside a protective conduit or sleeve. This is
not just a recommendation; it's a requirement under the Australian Plumbing
Standard AS/NZS 3500.1.
The protective sleeve, typically a solid PVC or HDPE pipe of a larger
diameter, serves several critical functions:
Physical Protection: It shields the PEX pipe from sharp rocks, gravel, and
potential damage during backfilling or future digging.
UV Protection: PEX is not rated for continuous UV exposure. The sleeve
protects it from sunlight during storage, installation, and any future
exposure.
Thermal Expansion: It provides a channel for the pipe to expand and
contract freely without the constraint of surrounding soil.
Future Proofing: If the PEX pipe ever needs to be replaced, it can be
easily pulled through the conduit without excavating the entire trench.
Australian Standards and Compliance
All plumbing materials used in Australia must comply with the WaterMark
Certification Scheme. This ensures the product is fit for purpose and meets
strict Australian standards for health, safety, and performance.
When purchasing PEX pipe for any use, especially underground, always look
for the WaterMark certification logo on the pipe itself and its packaging. Using
non-certified products can void your insurance and is not compliant with
Australian law.
The relevant standard for underground installation is AS/NZS 3500.1:2021
Plumbing and drainage - Water services, which explicitly outlines the
requirement for sleeving.
Best Practices for Installing PEX Pipe Underground
Trench Depth: The trench must be deep enough to protect the pipe from
surface activity and frost. Local council regulations vary, but a common minimum
depth is 450-600mm. Always check with your local authorities.
Bedding: The trench should have a smooth, level bed free of sharp stones.
Sand is often used as a bedding material to create a soft, uniform base for the
conduit.
Running the Pipe: Feed the continuous, un-fixed length of PEX pipe through
the pre-laid conduit. Avoid stretching the pipe. Ensure there are no fittings or
joints inside the conduit itself, as these are potential failure points that
should only be accessible at endpoints.
Backfilling: Carefully backfill the trench, first with soft soil or sand
around the conduit to avoid displacing it, followed by the excavated soil. Be
sure to remove any large rocks.
Direct Burial PEX: Is It an Option?
You may hear about "direct burial" PEX. While some manufacturers produce
tougher grades of PEX (like PEX-b or PEX-c with a thicker wall), the Australian
Standard (AS/NZS 3500.1) still requires sleeving for all plastic pressure pipes
underground. Therefore, direct burial of PEX is not compliant in Australia,
regardless of the pipe's specific rating.
Conclusion
So, can PEX pipe be used underground? Absolutely. It is a excellent, modern
material for underground water services in Australia, provided it is installed
within the guidelines set out by the Australian Plumbing Standards.
The key takeaway is simple: always use a continuous, WaterMark-certified
PEX pipe inside a protective conduit or sleeve. By following this rule, you will
ensure a durable, efficient, and compliant plumbing system that will serve your
home reliably for decades to come.
For any major plumbing work, always engage a licensed plumber who
understands these local standards and can ensure the installation is performed
correctly and certified.
