My son’s first house is a 1990s two-storey plaster-and-tile place in Henderson, off Lincoln Road, with the internal spouting fashionable then: the gutter hidden behind the fascia so the roofline looks clean. In winter 2023 a brown stain appeared on the upstairs bedroom ceiling. He painted over it, and in March it came back bigger. By the third time, after a night when Auckland got 60 millimetres in a few hours, water was running down the inside of the wall. Three people had looked at the roof and found nothing wrong with it. They were right. The roof was fine. The gutter behind it was drowning.
What an internal gutter is and why it fails differently
An ordinary external spouting hangs off the fascia; when it blocks, water spills over the front and everybody notices. An internal gutter sits inside the roofline, a trough of folded Colorsteel or membrane-lined ply behind a parapet or fascia, with the roof sheets discharging into it. When it blocks, the water has two ways out: over the front lip, or over the back edge inside the building. It rises past the turn-up, into the soffit and down the wall framing. From below it looks exactly like a roof leak, which is why three people found nothing.
The gutter on my son’s house was 150 millimetres wide, 90 millimetres deep and 11 metres long, with one 80 millimetre downpipe at the far end and no fall. It served about 70 square metres of roof, roughly a third of what E2/AS1 would size for that catchment, and it had no overflow provision at all.
Sizing, fall and the overflow nobody fitted
West Auckland rain comes hard. The design intensity used for gutter sizing around Auckland is around 100 to 130 millimetres an hour for a ten-minute burst, and the Waitākere foothills get more. A 70 square metre catchment at 120 millimetres an hour delivers about 2.3 litres a second to the gutter. One 80 millimetre downpipe on a flat gutter will barely pass that when it is spotless, and it is never spotless under a canopy of liquidambar and pōhutukawa.
Three things an internal gutter must have, and this one had none:
- Fall. A minimum of 1:500 towards the outlet, and 1:200 is kinder. A flat gutter holds a permanent puddle that grows moss and rots the lining.
- An overflow. A slot or spitter through the fascia set below the level of the back edge, so that when the downpipe blocks the water goes outside where you can see it. E2/AS1 requires this for internal gutters, and it is the single most skipped item in the whole trade.
- Enough outlet. Two downpipes at 100 millimetres, or a rain-head and a larger drop, rather than one 80 millimetre pipe at the end of an 11 metre run.
Finding the leak on a dry day
A hose is the diagnostic tool. On a dry Saturday we blocked the downpipe outlet with a rag, ran the garden hose into the gutter at the far end and waited. In four minutes the water reached the back turn-up, and in six it was dripping through the bedroom ceiling. Ten minutes of hose reproduced what a dozen storms had done, and it settled the argument about whether the roof or the gutter was at fault.
While we were there we cleaned the gutter properly. It held about a bucket and a half of compost and a healthy fern. The downpipe was clear below the outlet, which is the point where my interest in the water ends; where it goes after the pipe is a different subject.
What the fix costs and who does it
Cleaning an internal gutter properly, from a ladder with the outlet cleared, is NZ$150 to NZ$350 and should happen twice a year under trees. Cutting an overflow through the fascia and fitting a spitter is NZ$300 to NZ$700. Adding a second downpipe with a new outlet and rain-head runs NZ$600 to NZ$1,400. Relining a membrane gutter that has cracked is NZ$1,500 to NZ$4,000 for a run like this. Replacing the whole trough in new folded Colorsteel with correct fall, two outlets and an overflow, which is what my son eventually did, was NZ$4,800 including the scaffold and the repair to the soffit.
In Auckland, spouting and downpipes are plumbing work. The work on my son’s house was done by a West Auckland plumber who put a level on the gutter before quoting, which is how you know they have met an internal gutter before. The ceiling was replastered in August and has been dry through two winters since.
Frequently asked questions
Why does my ceiling only leak in heavy rain? Because the fault is capacity, not a hole. An internal gutter or spouting that is undersized, flat or partly blocked copes with light rain and overflows backwards into the ceiling when intensity passes what the downpipe can carry, typically above 60 to 80 millimetres an hour.
What is an overflow on an internal gutter? A slot or pipe through the fascia, set lower than the gutter’s back edge, so that when the downpipe blocks the water spills outside instead of into the building. E2/AS1 requires one on every internal gutter, and fitting one costs NZ$300 to NZ$700.
How often should internal spouting be cleaned? Twice a year at minimum, and three times under liquidambar, pōhutukawa or pine. A clean costs NZ$150 to NZ$350 and a blocked internal gutter can put water through a ceiling in a single storm.