The bedding material you use when laying pipes in a trench is one of the most important factors in how long your pipework lasts. Using the wrong material — or skimping on depth — can lead to pipe cracking, settlement, and costly repairs down the track. As a general rule, clean, well-graded granular material (such as pea gravel or crusher dust) is the go-to choice for most pipe types in New Zealand.
Why Pipe Bedding Matters
Pipes are not designed to sit directly on hard, uneven ground. Bedding material does three key jobs:
- Distributes the load from the soil and surface traffic evenly around the pipe
- Prevents point loading — sharp rocks or hard lumps pressing into the pipe wall
- Provides drainage around the pipe to reduce hydrostatic pressure
Getting this right is especially important in New Zealand's variable soils, from Auckland's heavy clays to Canterbury's stony river gravels, where ground movement and moisture levels can vary significantly.
Common Bedding Materials
Pea Gravel (Pea Metal)
Pea gravel — rounded, washed aggregate typically 6–10 mm in diameter — is the most widely used bedding material in NZ. It compacts well around the pipe, is free-draining, and is easy to work with. It's suitable for stormwater, sewer, and drainage pipes.
Crusher Dust (Screenings)
Fine crusher dust (also called chip dust or screenings) is sometimes used, particularly under rigid pipes. It compacts firmly and provides good support. However, it can retain moisture and is less free-draining than pea gravel, so it's not always ideal in wet ground.
Concrete Haunching
For pressure pipes or pipes in unstable ground, concrete haunching (placing concrete around the lower portion of the pipe) may be specified by an engineer. This is a more permanent solution and is sometimes required under roads or driveways.
Sand
Clean, coarse sand can be used as bedding in some situations, but it must be free of clay, silt, and organic material. Fine sand can wash away or migrate in wet conditions, so it's generally less preferred than pea gravel for NZ conditions.
What NOT to Use
Avoid using:
- Topsoil or clay — these compress unevenly and retain water
- Subsoil with large rocks or clods — these create point loads on the pipe
- Recycled fill of unknown composition — can contain contaminants or sharp debris
Bedding Depth and Zones
A standard pipe bedding setup has three zones:
| Zone | Description | Typical Depth |
|---|---|---|
| Foundation | Below the pipe, levelled and compacted | 100 mm minimum |
| Bedding | Surrounds the lower half of the pipe | To pipe centreline |
| Haunch/Surround | Above pipe to 150–300 mm over the crown | Granular fill |
| Backfill | Above the surround to surface | Compacted native or imported fill |
As a guide, aim for at least 100 mm of granular bedding below the pipe barrel, with the material brought up to at least the pipe's centreline (and ideally to 150 mm above the crown) before switching to general backfill.
NZ Building Code and Standards
Pipe bedding requirements in New Zealand are governed by several standards and codes depending on the pipe type:
- NZS 3725 covers the design and installation of buried flexible pipes
- NZS 4404 relates to land development and subdivision infrastructure
- NZ Building Code clauses E1 (surface water), G12 (water supplies), and G13 (foul water) all have relevance depending on what you're installing
For building work, always check whether a building consent is required. Some drainage work is exempt under Schedule 1 of the Building Act, but this depends on the scope and your local council's interpretation — confirm with your council before starting. See MBIE Building Performance for guidance on consents and the Building Code.
BRANZ also publishes technical guidance on drainage and earthworks that is well worth reviewing.
Pipe Type Matters
Different pipe materials have different bedding requirements:
- PVC and polyethylene pipes (flexible) rely heavily on the surrounding soil and bedding for structural support — good granular surround is essential
- Concrete and vitrified clay pipes (rigid) are stronger but still need even bedding to prevent cracking at joints
- Copper and HDPE pressure pipes need protection from sharp objects and corrosive soils — a sand or pea gravel surround is standard
Always check the pipe manufacturer's installation instructions, as they may specify minimum bedding requirements for their product.
Compaction and Backfilling
Once the pipe is laid and the surround material is in place, backfill should be compacted in 150–200 mm layers using a plate compactor or hand tamper. Avoid heavy compaction directly over the pipe until you have at least 300 mm of cover — this can deform flexible pipes. In areas under driveways or roads, engineered compaction may be required.
When to Call a Professional
Some pipe-laying work in New Zealand is restricted work that must be carried out by a licensed tradesperson:
- Plumbing and drainlaying (including sewer and water supply connections) must be done by a licensed drainlayer or plumber registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Homeowners can do their own plumbing on their own home in some circumstances, but drainage connections to the public network must be done by a licensed professional.
- Excavation near services (power, gas, water, telecoms) requires care — always call Before U Dig (0800 248 344) to locate underground services before digging.
- If your project involves significant earthworks, retaining walls, or work near boundaries, consult a Licensed Building Practitioner (LBP) — find one at LBP.
For larger excavation and pipe-laying projects, it's well worth engaging a qualified excavator operator or drainlayer. You can find experienced local tradespeople on tradedirectory.co.nz.
Safety note: Trenches of sufficient depth may require shoring or battered sides to prevent collapse — check WorkSafe NZ guidance for the specific requirements that apply to your situation. Refer to WorkSafe NZ for excavation safety requirements before you dig.
Licensed Building Practitioner (LBP)
