Gutter & Downpipe Size Calculator NZ
Find the minimum gutter size and how many downpipes your roof needs, using the method behind the NZ Building Code's E1/AS1. No sign-up.
House footprint including the eaves overhang
Most Auckland homes sit between 15° and 30°; this guide covers up to 55°
Only for a two-level house with a wall above a lower roof
Where is the downpipe in each gutter run?
Design rainfall (10-minute storm)
Peak run-off
284 L/min
4.7 L/s off 170 m² of roof
Minimum gutter size
5,217 mm²
Cross-section per run, each draining 43 m²
Downpipes for your roof
| Size | Roof per pipe | You need |
|---|---|---|
| 63 mm round | 60 m² | 3 · your count is enough |
| 74 mm round | 85 m² | 3 · your count is enough |
| 100 mm round | 155 m² | 2 · your count is enough |
| 65 × 50 mm rectangular | 60 m² | 3 · your count is enough |
| 100 × 50 mm rectangular | 100 m² | 2 · your count is enough |
| 75 × 75 mm rectangular | 110 m² | 2 · your count is enough |
Keep downpipes no more than 12 m apart along a gutter, even when the numbers above allow fewer.
10,000 mm² is about the inside of a gutter 125 mm wide and 80 mm deep. Compare your result with the cross-section on your spouting's spec sheet, or let us check it on site.
How big should your gutters be?
Every gutter run in New Zealand needs at least 4,000 mm² of cross-section, and more as the roof area behind it grows. A typical 160 m² Auckland home with four downpipes needs about 5,200 mm² per run and three 74 mm downpipes at the standard design rainfall of 100 mm/h.
Enter your own roof to check it. If your gutters overflow in heavy rain even when they are clean, they are usually too small, too flat or short of downpipes, and the numbers here show which.
What you need
- Roof plan area: length × width of the house plus the eaves
- Roof pitch: on your plans, or ask us when we visit
- Downpipe count: walk round the house and count them
How the calculator works
The calculator follows BRANZ Bulletin 509, Sizing gutters and downpipes, which is the E1/AS1 method (NZ Building Code clause E1 Surface Water) with one change: it uses the sloping area of the roof rather than its plan area. A steeper roof catches more driving rain, so this is the more conservative of the two.
- Roof area. Your plan area is divided by the cosine of the pitch to give the sloping area. Half of any wall that drains onto the roof is added.
- Run-off. Rainfall in mm/h divided by 60 gives litres per minute for every square metre of roof: 1.67 L/min per m² at 100 mm/h.
- Gutter size. The flow reaching each downpipe gives the minimum cross-section, A = (flow ÷ 0.0016)0.8 mm², never less than 4,000 mm². A downpipe in the middle of a run splits the flow, so each side carries half.
- Downpipes. The area one downpipe can serve comes from the bulletin's downpipe table (E1/AS1 Table 5 on sloping area) for your pitch, scaled down when you design for heavier rain.
We check our results against the bulletin's own worked example: 172.5 m² of roof at 100 mm/h gives 290 L/min and a minimum gutter of 16,092 mm².
Worked example: a 160 m² Auckland home
A single-storey house with 160 m² of roof in plan, a 20° pitch and four downpipes, each at the end of a gutter run:
- Sloping roof area: about 170 m²
- Peak run-off at 100 mm/h: about 284 L/min (4.7 L/s), or 43 m² of roof per downpipe
- Minimum gutter per run: about 5,200 mm²
- Downpipes needed: three 63 mm or 74 mm round, or two 100 mm round, so the four on the house are enough
Designed for 150 mm/h instead, the same roof needs about 7,200 mm² per run, and 63 mm downpipes would need five rather than four. That is why we ask where you live and how exposed the roof is before we quote new spouting.
When the numbers say your gutters are too small
Water pouring over the front of a clean gutter, a waterfall at one corner, or a downpipe that backs up in every downpour are the usual signs. Often the gutter itself is fine and the fix is an extra downpipe or a corrected fall, which costs far less than new spouting. Where the run really is undersized, the answer is a larger profile.
We measure, check the fall and quote the cheapest fix that solves it. See gutter installation, downpipe repair and replacement and our gutter cost calculator for prices, or read choosing the right gutter size for an Auckland home.
Frequently Asked Questions
What size gutter do I need for my house in NZ?
It depends on the roof area draining to each gutter run, the roof pitch and the design rainfall. E1/AS1 sets a minimum gutter cross-section of 4,000 mm². A typical 160 m² single-storey Auckland home at 20° pitch with four downpipes at the ends of each run needs about 5,200 mm² per run at the standard 100 mm/h, and about 7,200 mm² if you design for 150 mm/h.
How many downpipes does a house need?
Divide the roof area by what one downpipe can serve. At the E1/AS1 base rainfall of 100 mm/h and a pitch under 25°, a 63 mm round downpipe serves about 60 m² of roof, a 74 mm round about 85 m² and a 100 mm round about 155 m². Keep downpipes no more than 12 m apart along a gutter even when the area allows fewer.
What rainfall intensity should I use for gutters in Auckland?
External gutters in New Zealand are sized for a 10-minute storm with a 10% chance of happening in any year. E1/AS1 allows a minimum of 100 mm/h, but some locations need more, so check your address on NIWA's HIRDS tool or E1/AS1 Appendix A. BRANZ recommends at least 200 mm/h for internal and box gutters, where an overflow can reach the inside of the house.
How much fall should a gutter have?
E1/AS1 itself gives no minimum fall for external gutters. The NZ Metal Roof and Wall Cladding Code of Practice sets a minimum of 1:500 (2 mm per metre) and recommends 1:200 (5 mm per metre) for better drainage and self-cleaning.
Is this calculator a substitute for a building design?
No. It is a homeowner guide for external gutters on a house, based on BRANZ Bulletin 509, which uses the sloping roof area and is more conservative than E1/AS1. Internal gutters, box gutters, commercial roofs and anything needing building consent should be designed specifically.
Sources: BRANZ Bulletin 509 Sizing gutters and downpipes (2009); MBIE Acceptable Solution E1/AS1; BRANZ, “Designing gutters to cope with high rainfall”; NZ Metal Roof and Wall Cladding Code of Practice 5.4.2.1.
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