Yes, digging a rubble drain (also called a French drain or soakage drain) is a job many NZ homeowners can tackle themselves — no special licence is required for basic garden drainage work. That said, there are important rules around where the water discharges, potential building consent requirements, and safety considerations you need to understand before you pick up a spade.
What Is a Rubble Drain?
A rubble drain is a trench filled with coarse aggregate (crushed rock or gravel) that collects and redirects excess groundwater away from a waterlogged area. Water percolates through the rubble and either soaks into the surrounding soil or flows to a discharge point. They are one of the most cost-effective and DIY-friendly drainage solutions for NZ gardens.
Planning Before You Dig
Good planning saves a lot of rework:
- Identify the source of the problem. Is water pooling because of a high water table, compacted soil, a blocked existing drain, or runoff from a neighbour's property? A rubble drain works best for groundwater and surface runoff — it won't fix a broken stormwater pipe.
- Work out where the water will go. This is the most critical step. Water must discharge to an approved outlet — typically your property's stormwater system, a legal soakage area, or a watercourse. You generally cannot direct water onto a neighbour's property or into the sanitary sewer.
- Check with your council. Stormwater discharge rules vary by region. Some councils require a resource consent or building consent for new drainage connections. Always confirm with your local council before connecting to any public system. The MBIE Building Performance website has useful guidance on what work may be exempt under Schedule 1 of the Building Act 2004.
- Call Before You Dig. Before any excavation, contact Dial Before You Dig (0800 248 344) or check with your council to locate underground services — gas, water, electrical cables, and existing drains can all be lurking beneath your lawn.
- Check your property boundaries. Don't dig close to boundaries, foundations, or retaining walls without understanding the implications for your structure and your neighbour's land.
Tools and Materials You'll Need
- Spade, mattock, or hire a small trenching machine for longer runs
- String line and spirit level (to set your fall)
- Geotextile filter fabric (drainage cloth) — this stops silt clogging the rubble
- Coarse aggregate — 20–40 mm crushed rock or drainage chip (available from landscape suppliers or Bunnings and Mitre 10)
- Optional: slotted agricultural pipe (ag pipe) laid in the base of the trench for higher-flow situations
- Topsoil and turf or seed to reinstate the surface
Step-by-Step: Digging Your Rubble Drain
1. Mark and Excavate the Trench
Mark your drain line with string. A typical rubble drain is 300–500 mm wide and 500–900 mm deep, depending on how deep the waterlogged zone is. Maintain a minimum fall of 1:100 (1 cm drop per metre of run) toward your discharge point — more fall is better.
2. Line with Geotextile Fabric
Lay geotextile filter fabric into the trench, leaving enough overlap to fold back over the top. This is a crucial step — without it, fine soil particles will migrate into the rubble and block the drain within a few years.
3. Add Aggregate (and Ag Pipe if Needed)
For a simple rubble drain, fill the trench with coarse aggregate to within about 150 mm of the surface. If you're dealing with significant water volumes, lay a slotted ag pipe in the base of the trench first, with the slots facing down (or as recommended by the pipe manufacturer — some installers prefer slots facing up; check with your supplier), then surround it with aggregate.
4. Fold Over the Fabric and Reinstate
Fold the geotextile fabric over the top of the aggregate, then backfill with topsoil and re-grass or replant.
5. Check the Outlet
Make sure your discharge point is clear and functioning. If connecting to an existing stormwater pit or pipe, ensure the connection is watertight and compliant with your council's requirements.
Key NZ Considerations
| Factor | What to Check |
|---|---|
| Building consent | Check with your council — minor soakage drains are often Schedule 1 exempt, but connections to public stormwater are not |
| Healthy Homes (rental properties) | Drainage is relevant to the Healthy Homes Standards for rental properties |
| Soil type | Clay soils drain slowly — a soakage-only drain may not work; you may need a piped outlet |
| Sloping sites | Steep sites can have erosion and stability implications; seek advice |
| BRANZ guidance | BRANZ publishes practical guidance on site drainage for NZ conditions |
When to Call a Professional
While basic rubble drain digging is DIY-friendly, call a qualified excavator or drainage contractor if:
- The waterlogging is severe, widespread, or affecting your home's foundations
- You need to connect to the public stormwater network (this may require a licensed drainlayer under the Plumbers, Gasfitters and Drainlayers Board rules)
- Your site is steep, unstable, or has complex underground services
- You're unsure of the cause of the waterlogging
- Excavation is needed near existing structures, retaining walls, or boundaries
Connecting to a public drain or council stormwater system is restricted work that must be carried out by a licensed drainlayer. Don't attempt this yourself.
You can find qualified local excavators and drainage contractors on tradedirectory.co.nz to get the job done safely and to code.
Safety Note
Trenches of significant depth require shoring or battered sides under WorkSafe NZ excavation safety rules — check current WorkSafe guidance for the applicable thresholds. Even shallower trenches can collapse — never enter an unsupported trench, and keep children and pets well clear of open excavations.
Licensed Building Practitioner (LBP)
