Stormwater drainage: getting rain off the roof and away

Stormwater is the rain off your roof and hard surfaces — a separate system to foul drainage, governed by NZBC clause E1 and your consent.

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Stormwater is rain off the roof and hard surfaces — a completely separate system from foul drainage. It has one job: carry surface water away so it doesn’t pond against the building, flood the neighbours, erode the site or undermine foundations. Because council rules vary a lot, the consent is what governs your discharge point and any detention.

What E1 asks of you

NZBC clause E1 Surface Water requires stormwater to be carried away from the building so it can’t cause harm. In practice that means keeping water clear of:

Ponding against the building.
Flooding onto neighbouring sites.
Erosion of the site.
Undermining of the foundations.

Sizing it: catchment → intensity → size

The size of pipe or downpipe you need comes from the catchment area multiplied by the local rainfall intensity. A steep coastal Bay of Plenty site sheds far more water than a flat Canterbury one for the same roof, which is why councils publish a design rainfall figure for their area.

Work out the catchment area draining to that pipe.
Apply the council’s local rainfall intensity for the site.
Size the pipe or downpipe from the result.

Typical downpipes are 80 mm or 100 mm round, or 100×75 rectangular. A common rule of thumb is roughly one downpipe per 80 m² of roof. Surface water off the site is collected via sumps and catchpits, while subsoil and ag drains handle groundwater.

Where it discharges

The consent approves where your stormwater goes. The usual options are:

Council stormwater main — the usual urban path, with the connection approved in the consent.
Soakage / soak pit — where ground permeability allows; common on rural or free-draining ground, including parts of Auckland.
Detention / attenuation tank — holds a storm and releases it slowly to limit peak flow; mandatory in many Auckland catchments to cut downstream flooding.
Rainwater tank — can supply toilets and laundry, trimming council water charges and storm peaks.

Design the secondary (overflow) path

Systems are sized for a normal storm — so you also have to show where the water goes in a bigger one. This is the overland flow path: it needs to run safely away from the house rather than into it. Councils increasingly require it on the plans, and it’s a common consent condition.

Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.

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Common questions

Is stormwater the same system as foul drainage?

No. Stormwater is rain off the roof and hard surfaces, and it’s a completely separate system from foul drainage. NZBC clause E1 Surface Water requires it to be carried away so it doesn’t pond against the building, flood neighbours, erode the site or undermine foundations.

How do I size a stormwater pipe or downpipe?

Required pipe or downpipe size comes from the catchment area multiplied by the local rainfall intensity. Because a steep coastal site sheds far more than a flat one for the same roof, councils publish a design rainfall figure. Typical downpipes are 80 mm or 100 mm round, or 100×75 rectangular, with a rule of thumb of roughly one downpipe per 80 m² of roof.

Where can stormwater discharge to?

The consent governs the discharge point. Common options are the council stormwater main (the usual urban path), a soakage or soak pit where ground permeability allows, a detention or attenuation tank that releases a storm slowly, or a rainwater tank that can supply toilets and laundry.

What is the secondary or overland flow path?

Systems are sized for a normal storm, so you must also show where water goes in a bigger one. That overland flow path needs to run safely away from the house rather than into it. Councils increasingly require it on the plans, and it’s a common consent condition.

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