New spouting that overflows or ponds is usually working to an old stormwater layout. Fall, outlet spacing, internal gutters, and what to ask before your reroof starts.
Short answer: new spouting is usually fitted to the existing stormwater layout — the same outlets, in the same places. If that layout was never capable of draining the roof properly, new spouting won't fix it. It just makes the limitation visible.
New spouting goes on. It looks straight, clean and properly finished. Then the first serious Auckland downpour arrives and water sheets over the front edge, or sits in a puddle at one end long after the rain has stopped.
The natural assumption is that it was installed badly. Sometimes that's true. More often it was fitted competently to a system that was already at its limit.
All gutters should have a minimum fall of 1:500 towards the outlet — that's 2mm of drop for every metre of run.
A fall of 1:200, or 5mm per metre, moves water faster and self-cleans better. It's the better performing option, and it has a visual cost: on a long run of external spouting a steeper fall becomes visibly out of line with the fascia, and the ends of the roof sheets become more exposed. On a 12-metre run, 1:200 drops 60mm from one end to the other. People notice.
So there's a genuine trade-off between drainage performance and how it looks, and it's worth deciding deliberately rather than discovering it afterwards.
Brackets hold that gradient. The Code of Practice sets bracket spacing at a maximum of 750mm for gutters under 180mm wide, and 600mm for gutters 180 to 300mm wide, with brackets close to all stop-ends and at both ends of sumps and rainwater heads.
This is the part most people never think about, and it matters.
Spouting should be installed with the back lower than the fascia board or cladding, so that when the gutter is overwhelmed the overflow drains through the gap between the back of the gutter and the fascia — away from the building rather than into it.
The Code of Practice notes that residential spouting installed 5mm below the top of the fascia at the high point, with a 3mm gap between fascia and spouting, may be considered to have sufficient overflow. Larger spouting needs bigger gaps. And that gap only works if the spouting is maintained and clear of debris — a designed outlet is better.
Less than most people assume from the Building Code alone.
Acceptable Solution B2/AS1 requires spouting to have a durability of just 5 years. The Code of Practice is candid that this is rarely commercially acceptable, and notes that with sound design and reasonable maintenance a spouting life of 10 years or more is usually achieved when using the same material as the roof.
Spouting that's difficult to access for replacement should be specified in more durable, compatible materials.
Because when an internal gutter overflows, the water goes into your building rather than over an edge. The requirements tighten accordingly.
Materials. Where internal gutters are difficult to replace and their failure could cause major disruption to the building below, the Code of Practice states they must be made from materials that will last 50 years to comply with the Building Code — and that metallic coated steel is not recommended for internal gutters that are difficult to replace. Common materials are butyl or other membranes, fibreglass, or non-ferrous metal: 0.9mm aluminium, 0.6mm stainless steel, or 0.6mm copper. Contact between coated metal products and copper or stainless steel has to be avoided, because it causes early corrosion — and so does runoff or splashback from copper onto coated metal.
Shape. Internal box gutters must have a minimum depth of 50mm at their lowest point including freeboard. A width to height ratio of 2:1 plus freeboard gives maximum flow, because it minimises the wetted surface area for a given cross-sectional area. A deep narrow gutter and a shallow wide one of the same capacity do not perform the same.
Fall. Here the two main reference documents diverge. Acceptable Solution E2/AS1 requires internal gutters to have a minimum fall of 1:100 — 10mm per metre. MRM's technical guidance acknowledges that 1:100 is often unachievable in practice, and the Code of Practice works from a 1:500 minimum with 1:200 recommended, letting designers calculate the extra capacity a steeper fall provides. So if you see 1:100 quoted, that's the Acceptable Solution figure rather than someone being awkward. BRANZ has written on internal gutters and flow capacity if you want a third view.
Detailing. Upstands must be hooked or returned. Gutters that return under the eaves aren't recommended, because it makes replacement harder later. The sole needs support — plywood lining, or close-ribbed roof cladding separated by underlay — strong enough to hold the weight of water at capacity, and if the gutter is over 300mm wide, strong enough to take foot traffic. There should be an expansion joint at the stop-end.
Direction changes. A sharp change in flow direction affects discharge capacity. Where two buildings meet at an angle, each gutter must be drained separately, or a specific discharge capacity calculation applied.
Yes, and it's often the thing nobody looks at.
A gutter's discharge capacity increases with the depth of water over the outlet, which is why discharging an open gutter end into a rainwater head or sump performs better than a simple dropper. Every bend reduces capacity — the Code of Practice accounts for each 90-degree bend as a specific loss in its calculations.
Where an internal gutter discharges sideways through a wall, that's a scupper. If a scupper's opening is smaller than the gutter, it's the opening — not the gutter — that determines the effective capacity. Scupper apertures block easily, so they should be fitted with an overflow that alerts people inside to a problem.
Gutter length is limited by thermal expansion. Through-fastened gutters should be restricted to 12 metres, while gutters with an expansion allowance can run to 25 metres in steel, or 12 metres in copper or aluminium, with an expansion joint that can double as a sump or rainwater head. Through-fastened gutters aren't recommended, because they're difficult to replace.
The Code of Practice publishes an online capacity calculator covering all of this — including rainfall intensity for your specific location, drawn from NIWA's HIRDS tool, with allowances for climate change. It isn't guesswork, and a competent roofer can show you the working.
Because unless your quote says otherwise, the new spouting goes back to the existing stormwater layout. Same outlet positions, same number of downpipes. Nobody adds new underground stormwater on a reroof unless you've asked for it and paid for it.
On older houses that layout often predates current guidance. There may be too few outlets, or outlets positioned where the required fall can't be achieved. The same problem shows up where a fascia has sagged, where the building has moved over decades, or where the roof height has changed — which can happen when a ventilation system goes in.
Work to a layout like that and the finished spouting may pond, may not fall evenly, or may overshoot in heavy rain. That's a limitation of the original design rather than a fault in the new work.
Everything above describes what good design looks like. Existing buildings are a different problem.
On a building that's already there — particularly a commercial one, where the roof form, the parapets and the stormwater layout were all set decades ago — altering the underlying design often isn't feasible or affordable. Sometimes the outlet simply can't go where the calculation says it should without work that costs more than the entire roof.
That doesn't make the standard irrelevant. It means the conversation has to be an honest one, and it's worth having before the work starts rather than after the first storm. What does the standard ask for? What can this building practically achieve? And what's the gap between those two in heavy rain?
Often a modest change gets most of the way there for a fraction of the cost — one more outlet, a rainwater head instead of a dropper, a deeper gutter. Sometimes the limitation stays, and the sensible thing is to know it exists rather than assume it's been fixed.
Either way, it's a discussion to have with whoever is quoting the work. A roofer who raises it unprompted is telling you something useful about how the rest of the job will go.
Yes, and more than people assume.
Water sitting in a gutter collects dirt. Dirt holds moisture against the metal. That's how premature corrosion starts, and it's why ponding affects warranty cover.
That last one is the whole article in a single question. For what else a good quote should cover, see what should a roofing quote include?
What is the minimum fall for spouting in New Zealand?
The Code of Practice sets a minimum of 1:500, or 2mm per metre, with 1:200 recommended for better flow and self-cleaning.
Why does my new spouting overflow?
Most often because it's been fitted to an existing stormwater layout with too few outlets, or outlets positioned where the required fall can't be achieved.
What material should an internal gutter be?
Where it's difficult to replace, it must last 50 years to comply with the Building Code. Metallic coated steel isn't recommended — membranes, fibreglass, aluminium, stainless steel or copper are the usual choices.
Is water sitting in my gutter a problem?
Yes. Ponding collects dirt, dirt holds moisture against the metal, and that causes premature corrosion.
Does new spouting come with new downpipes?
Usually it goes back to the existing layout — same positions, same number. Adding outlets is separate work and should be quoted as such.
