Internal gutters: the leaky-home risk you should design out

Concealed internal gutters are the top cause of leaky-home claims in New Zealand — avoid them where you can, and if you must use one, build it strictly to E2/AS1 §8.1.6.

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An internal (concealed) gutter runs inside the building envelope rather than hanging off the eaves, and it’s the single biggest reason leaky-home claims get filed. When one fails — and they eventually do — water rots the frame and skirting right at the floor line, where it’s hardest to find and dearest to fix. This page sums up the plain-English rules from E2/AS1 §8.1.6 so you know what you’re signing up for.

Design them out first

The best internal gutter is the one that isn’t there. They fail over time through debris, ice and expansion, and a concealed failure means rotting frame and skirting at the floor line. If it’s at all possible, design internal gutters out of the building. Only use one where it’s genuinely unavoidable — and then follow §8.1.6 to the letter.

Sizing and slope

If you must use an internal gutter, size it for the extra load it has to carry and give it enough fall to keep water moving:

Capacity: design for 2× the rainfall intensity of an external gutter.
Minimum size: 300mm wide × 70mm deep, plus 20mm freeboard.
Minimum cross-section: 21,000 mm².
Slope: 1:300 absolute minimum, with 1:100 recommended to allow for sag over time.

Overflows and outlets

Overflows are your early-warning system — put them where a blockage shows itself before it soaks the framing:

Membrane internal gutters need dedicated overflow outlets, sized at least as large as the downpipes.
Position overflows somewhere obvious — over a doorway, for example — so a problem is spotted early.

Materials and valley discharge

Material choice depends on how flat the gutter runs, per E2/AS1 §8.1.6.1:

At a slope under 3°, only listed materials are acceptable, and AZ150/Z275 is allowed only with a factory finish.
Where a valley discharges into eaves spouting that catches more than 50m², install a downpipe within 2m of the valley.

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

Why are internal gutters such a leaky-home risk?

Concealed internal gutters are the top reason for leaky-home claims. If one ever fails, water rots the frame and skirting at the floor line. They will fail eventually through debris, ice and expansion, which is why the advice is to design them out of the building wherever possible.

What size does an internal gutter need to be?

Under E2/AS1 §8.1.6 the minimum is 300mm wide by 70mm deep, plus 20mm freeboard, with a minimum cross-section of 21,000 mm². It must also be sized for 2× the rainfall intensity of an external gutter.

What slope should an internal gutter have?

The absolute minimum slope is 1:300, but 1:100 is recommended because it allows for sag in the gutter over time.

Where should internal gutter overflows go?

Membrane internal gutters need dedicated overflow outlets sized at least as large as the downpipes, positioned somewhere obvious — such as over a doorway — so a blockage gives early warning before it damages the framing.

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