Getting your trench depth right is one of the most important steps in any drainage project. Dig too shallow and your pipe won't have enough cover or fall; dig too deep and you're wasting time, money, and effort. The correct depth depends on several factors: the required fall (gradient) of the pipe, the minimum cover over the pipe, the pipe diameter, and the starting and finishing invert levels of your drain.
Understand the Key Terms First
Before you grab a shovel or call in an excavator, it helps to know the terminology:
- Invert level – the inside bottom of the pipe at any given point.
- Cover – the depth of soil (or other material) above the top of the pipe.
- Fall – the slope of the pipe, expressed as a ratio (e.g. 1:60 means the pipe drops 1 mm for every 60 mm of horizontal run).
- Soffit – the outside top of the pipe.
Minimum Cover Requirements in New Zealand
Under the NZ Building Code (particularly clauses E1 for surface water and G13 for foul water drainage), pipes must have adequate cover to protect them from damage and to maintain structural integrity. As a general guide:
- Under garden/lawn areas: minimum 300 mm of cover over the top of the pipe is typical.
- Under driveways or areas subject to vehicle loading: minimum 600 mm of cover is commonly required, though this can vary depending on pipe material and loading conditions.
- Under buildings or slabs: specific engineering requirements apply — always check with your local council and a licensed drainlayer.
Always confirm minimum cover requirements with your local council, as territorial authorities can have their own standards on top of the Building Code.
How to Calculate Trench Depth Step by Step
Here's a straightforward method to work out how deep to dig at any point along your trench.
Step 1 – Establish Your Start and End Invert Levels
You need to know where your pipe starts (e.g. at a gully trap or inspection chamber) and where it finishes (e.g. at the council connection point or soakage system). The invert level at the outlet is usually fixed — it's dictated by the council connection or existing infrastructure.
Step 2 – Calculate the Required Fall
For foul water (sewage) drains, standard trade practice typically requires a minimum fall of 1:60 for 100 mm pipes. For stormwater drains, a minimum of 1:100 is often acceptable, but steeper is generally better for self-cleansing velocity.
Fall formula:
Drop (mm) = Length of run (mm) ÷ Fall ratio
For example, a 10-metre run at 1:60 fall:
10,000 mm ÷ 60 = 167 mm of drop over the run
Step 3 – Work Out the Invert Level at the Start
Start invert level = End invert level + Total drop
If your outlet invert is 500 mm below ground level, and you need 167 mm of drop over 10 m, your start invert is 500 + 167 = 667 mm below ground level at the upstream end.
Step 4 – Add Pipe Diameter and Minimum Cover
The trench depth at any point = invert depth + pipe wall thickness (or simply use the outside diameter of the pipe) + minimum cover required.
For a 100 mm PVC pipe with 4 mm wall thickness (outside diameter ~110 mm) and 300 mm minimum cover:
| Point | Invert Depth | Pipe OD | Min Cover | Trench Depth |
|---|---|---|---|---|
| Start (upstream) | 667 mm | 110 mm | 300 mm | ~1,077 mm |
| End (downstream) | 500 mm | 110 mm | 300 mm | ~910 mm |
Always add a bedding layer (typically 100 mm of compacted sand or pea gravel) beneath the pipe, so your actual excavation depth will be trench depth + 100 mm bedding.
Pipe Bedding and Surround
Proper bedding is essential. Most drainage installations in NZ use:
- 100 mm of compacted sand or pea gravel as a bedding layer under the pipe.
- Granular surround to at least 150 mm above the pipe before backfilling with excavated material.
This protects the pipe from point loading and helps maintain alignment and fall. Check the pipe manufacturer's installation guidelines for specific requirements.
Soil Conditions and Trench Safety
In unstable or sandy soils, trench walls can collapse — this is a serious safety hazard. WorkSafe NZ requires that trenches deeper than 1.5 metres must be shored, battered back, or otherwise made safe before anyone enters. Even shallower trenches can be dangerous in loose or wet ground. If you're hiring an excavator operator, discuss trench safety with them before work begins.
Do You Need a Building Consent?
Drainage work often requires a building consent, particularly for new connections to the public sewer or stormwater network. Some minor drainage work may fall under Schedule 1 exemptions of the Building Act, but this is not always straightforward. Always check with your local council before starting work. You can find general guidance at MBIE Building Performance.
Restricted Work — Licensed Drainlayers Only
In New Zealand, drainlaying is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. Any work on sanitary drains (foul water) connecting to the public sewer must be carried out or supervised by a licensed drainlayer. Stormwater drainage rules can vary, but it's always safest to use a licensed professional. Check licensing at the Plumbers Gasfitters & Drainlayers Board.
When to Call a Professional
While understanding the calculations is useful for planning and communicating with tradespeople, most drainage work in NZ should involve a licensed drainlayer and a qualified excavator operator. Call a professional when:
- You're connecting to the public sewer or stormwater network.
- Trenches will exceed 1.5 m in depth.
- You're working near existing services (gas, water, power, telecoms).
- Ground conditions are unstable, waterlogged, or rocky.
- A building consent is required.
You can find qualified local excavators and drainlayers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
