COP v3.0:roof-drainage; roof-drainage-design

5.6 Roof Drainage Design 

The objective of roof drainage systems is to maintain a weatherproof building, to minimise the risk of injury or inconvenience due to flooding, and to avoid potential monetary loss and property damage — including to the contents of buildings.

Roof drainage design requires consideration of:

  • Type of gutter (external, internal, valley, or roof gutter),,
  • rainfall intensity,
  • catchment area,
  • gutter fall,
  • gutter-cross-sectional area and wetted surface area, and 
  • outlet and downpipe capacity.

This section details specific requirements for the sizing of all drainage components.

5.6.1 Catchment Area 

The effective catchment area for a gutter is determined not only by the plane area of the roof itself but also by the walls adjacent to the roof. When a wall is discharging on to a roof, half the surface area of that wall (up to a maximum height of 10 m), must be added to the catchment calculation. Roof Pitch 

The COP calculations are based on the plane area of the roof (which is the sloping surface area of the roof), not the plan area (which is the area covered by the roof).



Wind action can influence effective catchment area, and the COP assumes the worst case scenario, i.e., rain striking the roof at an angle perpendicular to the roof plane.


5.6.2 Gutter Fall 

All gutters must have a minimum fall of 1:500 (2 mm in 1 m), the COP recommends 1:200 (5 mm in 1 m), as it will improve drainage and self-cleaning. 


5.6.3 Rainfall Intensity 

Rainfall intensity can be taken off the maps for 50-year average return intervals (ARI). When the co-ordinates of a site are known, site-specific values can be obtained using NIWA’s HIRDS tool at

As NZBC E1 requires that rainwater from events having 2% likelihood of occurring annually shall not enter buildings, the COP uses figures for 50-year Average Return Interval, rather than the 10% probability figures published in E1/AS1. Duration 

Rainfall intensity figures quoted on the NIWA site are for maximum intensity over a ten-minute duration. Intensity may vary within this period, and roof drains can overflow quickly when demand exceeds capacity. A 1-minute rainfall intensity can be as much as 4.2 times higher than the 10-minute intensity.

To account for short-term rainfall intensity, various factors should be applied to internal and external gutters, and to drains depending on their location and consequence of overflow. See Short-Term Intensity Multiplication Factors. Allowance for Short-Term Intensities 

The COP drainage calculator multiplies the ten-minute maximum intensity by a factor to allow for short-term fluctuations. This minimum factor varies by gutter location as follows. Short-Term Intensity Multiplication Factors

ApplicationGutter MultiplierDownpipe Multiplier
With OverflowNo Overflow
Internal Gutters Residential3.11.63.1
Internal Gutters Commercial2.21.12.5
External Gutters — no Overflow2.51.32.5
External Gutters — with Overflow111 Short-term Intensity Factor Explanation

These are minimum factors; higher factors may be applied at the designer’s discretion.

  • Valleys, Penetrations, and Internal Gutters Residential have a minimum factor of 3.1 because failure of these gutters is likely to cause damage to internal elements. Where a 2% probability of flooding is unacceptable, a higher figure should be used.
  • Internal Gutters Commercial have a minimum factor of 2.2 as failure of these gutters is less likely to cause severe damage and water run time may be longer. Short runs and steep pitches will reduce run time. (At 250 mm/hr intensity and 3 degrees pitch, rain will take 2 minutes to travel 15 metres). For short runs, steeper pitches and where the probability of flooding of 2% is unacceptable, a higher figure should be used.
  • External gutters no overflow have a minimum factor of 2.5, providing the building has a soffit. Otherwise, they should be treated as an internal gutter.
  • External Gutters with overflow have a minimum factor of 1.5 as occasional overflow is not likely to cause damage. To qualify as drained, the back of the gutter must be below the fascia height and it must have a gap of at least 3 mm between the gutter and the fascia or cladding.

For convenience, ARI maps are included in the calculation section which includes tables for gutter and valley capacity for different rainfall intensities.


5.6.4 Minimum Freeboard Values 

In gutters where overflow can enter the structure, it is necessary to have freeboard to allow for wave action, obstructions, and other unforeseen circumstances. 5.7 Capacity Calculationsallow the thse minimum freeboard values.

5.6.4 Minimum Freeboard Values

5.6.4A Minimum Freeboard Values

Gutter TypeFreeboard
Internal gutters20 mm
Secret gutters 
Valleys with a pitch less than 8° 
External gutters with no overflow15 mm
Valleys with a pitch of 8° or more 
External Gutters with OverflowNo freeboard required