Can I Dig a Soak Hole for a Downpipe Myself Without Machinery?

Yes, in many situations a homeowner can hand-dig a simple soak hole (also called a soakpit or soakage pit) for a residential downpipe without machinery — but whether it's legal, effective, and safe depends on your soil type, site conditions, local council rules, and the NZ Building Code. Read on before you reach for the spade.

What Is a Soak Hole and What Does It Do?

A soak hole is an excavated pit, usually filled with coarse gravel or rubble, that receives stormwater from a downpipe and allows it to slowly disperse into the surrounding soil. They are a common and cost-effective solution on properties where connecting to a stormwater drain isn't practical or available.

Under the NZ Building Code, stormwater disposal is covered primarily by Clause E1 (Surface Water). Any system you install must manage stormwater in a way that doesn't cause damage to your property, neighbouring properties, or public infrastructure.

Do You Need a Building Consent?

This is the first question to answer. Many simple soak holes fall under Schedule 1 exemptions of the Building Act 2004, meaning they don't require a building consent — but this is not guaranteed. Key factors include:

• The size and depth of the pit
• Whether it affects foundations or neighbouring land
• Your local council's specific rules and district plan

Always check with your local council before you start digging. Some councils have specific requirements around setbacks from boundaries, buildings, and septic systems. Getting this wrong can be costly to fix.

Checking Your Soil First — The Percolation Test

Before digging anything, you need to know whether your soil will actually absorb water. A percolation test (or perc test) is a simple DIY check:

1. Dig a small test hole about 300 mm deep and 300 mm wide.
2. Fill it with water and let it drain completely (this saturates the soil).
3. Refill it and time how long it takes to drop 25 mm.

If the water drains away in a reasonable time (generally within an hour or two for stormwater soakage), your soil is likely suitable. Clay-heavy soils common in parts of Auckland, Waikato, and Manawatū drain very poorly and may make a soak hole ineffective or even cause waterlogging on your or your neighbour's property. Sandy or loamy soils tend to work well.

BRANZ has useful guidance on stormwater and drainage best practice for NZ conditions.

How to Hand-Dig a Basic Soak Hole

If your soil passes the perc test and your council is happy, here's the general process for a single downpipe soak hole:

Tools you'll need

• Long-handled spade and/or post-hole digger
• Mattock (useful in harder ground)
• Wheelbarrow for spoil
• Tape measure
• Level

Typical dimensions

A standard residential soak hole for a single downpipe is often 600–900 mm in diameter and 1–1.5 m deep, though your council or a drainage professional may specify otherwise. Larger roof catchment areas need bigger pits.

Steps

1. Mark out the pit location — keep it at least 1.5–3 m from foundations, boundaries, and any septic tank or effluent field (confirm exact setbacks with your council).
2. Dig the pit to the required depth. In firm ground, a post-hole digger speeds this up considerably.
3. Line the sides with geotextile fabric to prevent soil migration while still allowing water to pass through.
4. Fill with coarse aggregate — 40–65 mm clean crushed rock or river gravel works well. Avoid fine material that will compact and block drainage.
5. Connect the downpipe using a suitable elbow and pipe to direct water into the top of the pit.
6. Cap the top with geotextile and a layer of topsoil or a grate to keep debris out.

You can find geotextile fabric, gravel, and drainage pipe at Bunnings, Mitre 10, or PlaceMakers.

Safety Considerations

• Call Before You Dig: Before any excavation, contact Dial Before You Dig (0800 248 344) or check with your council to identify underground services — water mains, gas lines, and electrical cables can all be lurking beneath your lawn.
• Trench safety: Holes deeper than 1.5 m can be dangerous and may require shoring — check WorkSafe NZ guidance for excavation safety requirements. For most soak holes this depth isn't needed, but be aware of the risk.
• Spoil placement: Keep excavated soil well away from the pit edge to prevent collapse.

Limitations of Hand Digging

Hand digging is perfectly fine for a single, shallow soak hole in soft to medium ground. However, it becomes impractical when:

• You need multiple large pits for a big roof area
• The ground is very hard, rocky, or full of tree roots
• The required depth exceeds 1.2–1.5 m
• You're working in a confined or awkward space

In these cases, hiring a mini-excavator (often available from hire companies) or engaging a professional earthmover will save you a lot of effort and ensure the job is done correctly.

When to Call a Professional

Consider engaging a qualified earthmover or drainage contractor if:

• Your soil fails the percolation test and you need an alternative drainage solution
• The project involves connecting to existing stormwater infrastructure (this may be restricted work requiring a licensed drainlayer under the Plumbers, Gasfitters and Drainlayers Board)
• You're on a sloping site with potential for erosion or slip
• Your council requires engineered drainage design
• The scale of work is beyond comfortable hand digging

Connecting to a public stormwater drain is restricted drainlaying work and must be carried out by a licensed drainlayer — this is not a DIY task.

You can find experienced local earthmovers and drainage contractors on tradedirectory.co.nz to get quotes and advice specific to your site.

Sources & Further Reading

• MBIE Building Performance — Building Code Clause E1 Surface Water
• BRANZ — Stormwater and Drainage Guidance
• WorkSafe NZ — Excavation Safety
• Plumbers, Gasfitters and Drainlayers Board — Licensed Drainlayers
• Bunnings — Drainage & Landscaping Supplies
• PlaceMakers — Drainage Products