Why Auckland gutters overflow
Leaves are one cause. So are a small outlet, a downpipe that cannot keep up, a gutter with no fall, or an internal gutter with nowhere safe for surplus water to go.
What overflow means
Water coming over the front or end of a gutter means it could not get out fast enough. From the ground it all looks the same. On the roof, the cause can be different.
This is not a sizing calculator, and it is not a reason to climb on the roof. If the overflow is a fault we can fix, that is a repair.
What decides whether the guttering can cope
- roof catchment — area, pitch, how many roof planes dump into one gutter
- Auckland rainfall — storms are short and hard
- gutter profile and how much water that shape can carry
- outlets — number, size, and whether they are clear
- fall — water sitting in the gutter is often this
- downpipe size and layout
- overflow provision, especially on concealed and internal gutters
- how the roof and guttering are put together
The parts, in plain language
Catchment
A bigger or more complicated roof does not automatically need a bigger gutter. What matters is how fast water arrives and how it leaves.
Gutter and spouting
In New Zealand, gutter is the umbrella word. Spouting is the everyday external eaves gutter. Size, shape and fall all matter.
Outlets, downpipes and rainwater heads
A lot of overflow starts here. Internal and concealed gutters rely on these even more than a simple eaves gutter.
Overflows
On concealed and internal systems there needs to be a controlled overflow. Without that, water can end up inside the house.
The official documents
Rainfall intensity and gutter/downpipe sizing for New Zealand work sit in the documents below. Publishing guessed millimetres here would be unsafe DIY advice.
- NZ Metal Roofing Manufacturers Code of Practice — gutters and roof-drainage terminology
- New Zealand Building Code clause E1 Surface Water
- NIWA HIRDS — high-intensity rainfall data used by designers
If someone quotes a downpipe size without looking at the roof, that is a guess. Internal gutters need the same thinking, and more care around overflows.