The call in June 2024 was about paint. A 1970s brick house in Chartwell, Hamilton, had its fascia and the bottom metre of rendered wall peeling a year after a repaint, and the painter wanted to know if my product had failed. It had not. I found the answer at the bottom of the downpipe on the south-east corner: a concrete splash block angled back toward the house, a liquidambar filling the spouting outlet with leaves, and every winter shower pouring down the wall instead of away from it. The roof was doing exactly what it should. The problem was the last half metre of the system.
How much water comes off a roof
People underestimate this by a factor of ten. A modest 160 square metre roof in a 25 mm per hour downpour, which Hamilton sees several times a year, collects 4,000 litres in that hour. Across a year with 1,100 to 1,200 mm of rain, the same roof sheds 175,000 to 190,000 litres, every one arriving at a downpipe outlet about 100 mm across.
An 80 mm round downpipe handles roughly 100 square metres of roof, a 65 mm one about 60 square metres. Many 1970s Hamilton houses were built with two 65 mm downpipes on a 180 square metre roof, and each extension since has made it worse. If the spouting overflows at the outlet in a heavy shower, that is usually the reason, not a blockage.
What the runoff tells you about the roof
Milky white or grey water in the first minutes of a shower is chalk washing off a degraded roof coating, the earliest reliable sign a painted steel or tile roof needs attention. Grit at the outlet is sand from concrete tiles or granules from a textured coating. Red-brown streaks mean rust from a cut edge or fixing upstream. On the Chartwell house the splash block had a white film, and the roof was chalking badly, about two years from a repaint at NZ$6,000 to NZ$8,500 for its size.
Leaves, outlets and the Hamilton winter
Hamilton winters are wet, foggy and still. Spouting does not dry out between showers, and liquidambar leaves, which are large and slow to rot, mat into a plug that sits over the outlet for weeks. The outlet, or dropper, is the narrowest point on the roof drainage path, and it is where the whole system chokes. A leaf guard on each outlet costs NZ$25 to NZ$60 fitted and is the most cost-effective thing I know on a roof under trees. Full-length spouting mesh works too, at NZ$25 to NZ$45 per metre, but it needs clearing itself or it becomes a shelf for leaf litter.
Clearing the spouting twice a year, once after the autumn drop in May and once before the winter rain properly starts, runs NZ$150 to NZ$350 for a single-storey house if someone is hired to do it. On a two-storey house the price roughly doubles and it stops being a job to do from a stepladder.
Where my part of the story ends
Above the ground, roof drainage is roofing and painting work: outlets, downpipe size and count, the brackets that hold it all on. Adding a downpipe to an overloaded run costs NZ$350 to NZ$700 installed. Once the water leaves the bottom of the downpipe, it is a different trade. In Hamilton that matters more than in most cities, because much of the city sits on flat ground over ash and clay that drains slowly, and a lot of older sections still discharge downpipes onto splash blocks or into soakage pits that were dug in 1975 and have been silting up ever since. The pit under the Chartwell downpipe had simply stopped accepting water, which is why it was running back toward the house.
Whether a downpipe should go to a council stormwater connection, a new soakage pit or a rain garden is a question of soil, section fall and the rules for the site, and I do not pretend to answer it. It is the kind of question a Hamilton drainage contractor deals with weekly, and having one look at the bottom of the downpipe is cheaper than repainting the wall a second time. The Chartwell house had the outlet guarded, a third downpipe added on the long run, and the discharge sorted at ground level. The wall was repainted once more, and this time it stayed put.
Frequently asked questions
How much water comes off a house roof in heavy rain? About 25 litres per square metre of roof for every 25 mm of rain, so a 160 square metre roof sheds around 4,000 litres in a 25 mm per hour downpour and 175,000 to 190,000 litres over a typical Hamilton year.
Why does my spouting overflow at the downpipe in heavy rain? Usually because the downpipe is too small or there are too few of them, not because of a blockage. An 80 mm downpipe handles roughly 100 square metres of roof; a 65 mm one about 60 square metres. Many 1970s houses have half the capacity they need.
What does white or milky water from a downpipe mean? The roof coating is chalking. Pigment and binder are washing off a degraded painted steel or coated tile roof, and it is typically one to three years from needing a repaint. Grit means tile sand or coating granules; red streaks mean rust upstream.