On 27 January 2023, the night Auckland Airport recorded more than 240 mm of rain in a day, a villa on Valley Road in Mount Eden put a stain the size of a dinner plate on a hallway ceiling. It had never leaked before. It stayed dry for six weeks of ordinary rain, then did it again in a 40-minute burst in March. I was asked to price a new roof. The roof did not need replacing. It needed a capacity problem found, and those are a different kind of leak.
A hole leaks every time. A capacity leak does not.
If water gets in only when rain falls hard, the roof is not holed. Something on it can carry a normal flow and cannot carry a peak flow. In Auckland that peak is real: short bursts of 30 to 50 mm in an hour arrive several times a summer, and the 2023 event ran at over 60 mm an hour for stretches. Every part of a roof has a rate it can handle, and on a hundred-year-old villa most of those parts were sized by eye.
The usual suspects, in the order I check them:
- Spouting that overflows at the back and runs down the fascia into the soffit, because the downpipe outlet is too small or the fall is wrong.
- Sheet laps too short for the pitch. Long-run under 8 degrees needs a 250 mm end lap; many old roofs have 150 mm, which fine rain clears and heavy rain backs up over.
- Apron and side flashings with upstands under 75 mm. Water sheeting down a wall in a downpour builds a head of a few centimetres and goes over the top.
- Valleys that are blocked, too narrow, or buried under too much sheet at the end.
- Wind-driven rain pushed under ridge caps and into unsealed penetrations that never see water in a vertical fall.
The hose test, done properly
Waiting for the next storm is not a diagnostic method. A garden hose is. The sequence matters: one person on the roof, one inside with a torch under the stain, and the roof is wetted in zones from the bottom up. Eaves and spouting first, run for 15 minutes. Then the sheets in the first metre above the eaves. Then flashings, one at a time. Then the ridge. Each zone gets its full 15 minutes before moving up, because water appearing while you wet the ridge may have come from the flashing two zones ago and only now worked through the ceiling.
To reproduce a capacity failure you sometimes overload a component deliberately: a bucket tipped into a valley, or a hose at full flow held to the top of a flashing, shows in thirty seconds whether the detail can cope.
What the Mount Eden roof turned out to be
It was two things, which is common. The back verandah spouting had a 65 mm downpipe serving 28 square metres of roof, fine at 25 mm an hour and hopeless at 60. And the side flashing where the verandah roof met the wall had a 50 mm upstand, so once the spouting backed up the water rose above it and went inside. A new 80 mm downpipe and a re-formed flashing with a 100 mm upstand fixed it for NZ$1,150. The new roof I had been asked to price was NZ$34,000.
What diagnosis and small repairs cost
A proper leak investigation with a hose test takes two people half a day and prices at NZ$350 to NZ$650 in Auckland. Re-forming a flashing runs NZ$400 to NZ$1,200 depending on access and whether cladding has to come off. Sealing and re-fixing short laps on a section of roof is NZ$300 to NZ$900. Replacing a valley is NZ$800 to NZ$2,000. Upsizing a downpipe outlet is NZ$250 to NZ$600. None of these is a roof replacement, and a quote that jumps to one without a hose test has skipped the step that matters.
Who to get on the roof
A downpour-only leak wants someone who will spend the half day with a hose rather than ten minutes with a quote pad. Whether you bring in an Auckland roof repair contractor or a general builder, ask how they intend to find it before they tell you what it costs to fix.
Frequently asked questions
Why does my roof only leak in heavy rain? Because a component is at capacity, not because there is a hole. Overflowing spouting, short sheet laps, low flashing upstands and blocked valleys all pass light rain and fail once the flow exceeds what they can carry.
How is a roof leak that only appears in storms found? With a hose test: wetting the roof zone by zone from the eaves up, 15 minutes per zone, with a second person watching inside. Components suspected of capacity failure are deliberately overloaded to reproduce the leak.
What does a roof leak investigation cost in Auckland? A half-day hose test with two people prices at NZ$350 to NZ$650. Typical fixes that follow, such as re-forming a flashing or upsizing a downpipe outlet, run NZ$250 to NZ$1,200.