COP v3.0:roof-drainage;

Roof Drainage  

This section considers the design of water drainage from the time it hits the roof cladding to the time it enters the downpipe. As the design considerations are similar, this also includes the discharge from gutters and troughs within the roof plane, valleys, internal gutters, and external spouting.

The Roof Drainage section gives guidance for compliance with NZBC Clause E1 Surface Water. It describes how to drain rainwater effectively from roofs and gutters.

 

The Code of Practice provides several interactive calculators:

5.1 NZBC: Clause E1 — Surface Water 

Objective

Safeguard people from injury or illness, and property from damage, caused by surface water

Functional Requirement

Buildings and sitework shall be constructed in a way that protects people and other property from the adverse effect of Surface Water.

Performance

Surface water resulting from an event with a 2% probability of occurring annually shall not enter buildings.

5.2 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.

5.2.1 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.

5.2.1.1 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.

 

5.2.2 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 https://hirds.niwa.co.nz/

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.

 

5.2.2.1 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 5.2.2.2A Short-Term Intensity Multiplication Factors.

 

 

5.2.2.2 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.

 

5.2.2.2A Short-Term Intensity Multiplication Factors

ApplicationGutter MultiplierDownpipe Multiplier
With OverflowNo Overflow
Valleys3.1n/an/a
Penetrations3.1n/an/a
Internal Gutters Residential3.12.13.1
Internal Gutters Commercial2.21.52.5
External Gutters — no Overflow2.51.72.5
External Gutters — with Overflow111

 

5.2.2.2B 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, provided the building has a soffit, 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. This gap must be maintained in all areas, including internal angles. External gutters to buildings without soffit must be provided with a 10 mm drainage gap or be designed as an internal gutter.

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

 

5.2.3 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. 5.3.7 Gutter Capacity Calculator allow the these minimum freeboard values.

5.2.3A Minimum Freeboard Values

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

5.3 Gutters 

The term “gutters” can be applied to all roof drains, but “spouting” refers specifically to external gutters.

Types of gutter:

  • External gutters – positioned outside the building envelope.
  • Concealed Fascia-Gutter Systems – gutters installed directly behind a fascia.
  • Internal Gutters – formed inside a parapet wall or where two connected gables meet at an internal draining point.
  • Valleys – where two roof planes meet at an angle of less than 180°.
  • Roof Gutters – where a penetration obstructs and concentrates the flow of water, often into a single pan.
  • Secret Gutters – where a roof discharges into a raked barge.

The definition of gutters in the COP includes the troughs of a profile adjacent to an obstruction (such as a penetration) or where a secret gutter is required, i.e., at the barge line of a swiss gable roof.

5.3.1 External Gutters (Spouting) 

NZBC clause B2/AS1 requires spouting to have a durability of 5 years. In practice, this is rarely commercially acceptable. However, with sound design and reasonable maintenance, a spouting life of 10 years or more is usually achieved when using the same material as the profiled metal roof.

Spouting that is difficult to access for replacement should be specified in more durable, compatible materials.

5.3.2 Gutter Capacity Design 

Spouting should be installed with the back lower than the fascia board or cladding to allow for draining of overflow water through the gap between the gutter back and the fascia.

A 2 mm gap between the back of the gutter and the fascia will give a discharge area equal to the diameter of a 75 mm downpipe for every 2.2 m of gutter run.

This gap is only totally effective if the spouting is correctly maintained and the gap is free of debris.  A designed outlet is preferred, either a gutter bracket creating a minimum 6 mm space stop end, a weir, a raised outlet above the spouting sole, a slotted front, or a low fronted gutter.

A weir stop-end, or an outlet with a top edge above the sole of the gutter, can be used to increase outlet capacity.

5.3.2.1 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. 

 

5.3.2.2 Maximum Gutter Length 

All gutters are subject to expansion. Maximum gutter-length is determined by the type of metal and its colour. Where gutters have an allowance for expansion (such as an external gutter on a typical gutter bracket or an internal gutter with sliding clips), lengths should be restricted to 25 m in steel and 12 m for copper or aluminium.

An expansion joint can be either a sump, rainwater head or a saddle flashing. Gutters that are directly through-fastened to the fascia or eaves purlin will not be free to move and should be restricted to a maximum of 12 m. Through-fastened gutters are not recommended as they are difficult to replace.

5.3.2.3 Gutter Support Systems 

The spouting bracket system must withstand the potential weight of a gutter full of water. In snow load areas, spouting may be fitted with snow straps and brackets at a maximum of 600 mm centres to withstand the additional potential weight of any snow build-up.

Brackets should be made using compatible material or non-ferrous metal. Brackets for pre-painted external gutters should be painted or powder coated before installation.

Brackets for external gutters should be located close to all stop-ends, at both ends of sumps and rain-heads at a maximum of 750 mm spacing for gutters less than 180 mm wide, and at 600 mm for gutters 180 – 300 mm wide. Brackets must be installed to provide a 1:500 (2 mm per metre) minimum gutter gradient towards the outlets.

5.3.3 Internal Corners 

When the back of a gutter is cut down to allow the valley to discharge into it, the gutter capacity is affected. In these cases, gutter calculations should allow for 20 mm less water height, and a min 3 mm spacer should be attached to the back of the gutter (or fascia) at the internal corner to maintain the clearance between the gutter and the fascia.

 

5.3.4 Concealed Fascia Gutters 

Concealed gutter systems are bespoke or proprietary systems that run inside the fascia.

The concealed gutter design must ensure that water cannot enter the soffit or overflow into the building if the gutter system outlet becomes blocked.

Overflows must be provided for concealed gutter systems within 1 m on either side of the downpipe to discharge through the soffit, immediately behind the fascia, and be capable of discharging the total catchment area served by the downpipe.

See 5.5.3 Overflows.

5.3.5 Internal Gutters 

When internal gutters are difficult to replace and their failure could cause major disruption to the building below, they must be made from materials that will last 50 years to comply with the NZBC; metallic coated steel is not recommended for internal gutters that are difficult to replace.

Common internal gutter materials are butyl or other membranes, fibreglass, or non-ferrous metal. Where butyl gutters are used, the metal and flashings should be separated from wet contact with the butyl rubber.

 

Suitable non-ferrous metals include 0.9 mm aluminium, 0.6 mm stainless steel, and 0.6 mm copper. Contact between coated metal products and copper or stainless steel must be avoided because it will lead to early corrosion. Splashback or runoff from copper onto coated metal can have the same effect.

5.3.5.1 Internal Gutter Design Features 

All internal gutters must have upstands that are hooked or returned. Gutters that return under the eaves are not recommended as this design makes removal for replacement more difficult.

To prevent permanent deflection of the gutter, support for the sole of an internal gutter should be provided by either a plywood lining or by close ribbed sheets of roof cladding, separated by a layer of roofing underlay. Internal gutter support must be strong enough to support the weight of water when at capacity, and if over 300 mm wide, be able to support foot traffic.

Internal box gutters must have a minimum depth of 50 mm at their lowest point, including freeboard. A width to height ratio of 2:1 plus freeboard gives maximum flow as it minimises wet surface area for a given cross-sectional area.

A sharp direction change in flow of an internal gutter will affect discharge capacity. Where two buildings meet at an angle, each gutter must be drained separately, or a specific discharge capacity calculation must be applied.

Internal gutters should have an expansion joint at the stop-end.

Outflows from internal gutters may be scuppers or weirs.

Scupper outlets should be avoided where possible. They are difficult to weatherproof, and they can inhibit expansion. At the outlet end, a weir overflow should discharge into a sump or rainwater head.

5.3.6 Secret Gutters 

A secret gutter is used where the roof edge runs at an angle of less than 90° to a wall, barge, or parapet.

 

 

 

 

Secret gutters should be wide enough to allow for cleaning and must be designed in accordance with 5.3.5.1 Internal Gutter Design Features

5.3.7 Gutter Capacity Calculator 

A responsive online tool for calculating gutter capacity is available at  www.metalroofing.org.nz/cop/capacity-calculations.

Before using this calculator, please read 5.2 Roof Drainage Design.

To calculate gutter capacity, select the type of building, type of gutter, and overflow (yes or no). Complete the rest of the data by changing the val in the designated fields.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1: =  mm per metre
  rads
 bends
 
 m
 
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only and is not to scale.
 
WFBthingD
Illustration is for explanatory purposes only and is not to shape or scale.
 
Cross-Sectional AreaNett of FreeboardWetted Perimeter
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile

5.4 Valleys 

A valley is a gutter at the internal intersection of two sloping panes of roof cladding.

5.4.1 Valley Fixing 

Valleys should not be positively fixed, except at the head, because that would inhibit expansion and can produce noise.

Alternative means of securing the valley gutter to the substrate include:

  • A clip system allows for thermal movement and security.
  • A compatible washered nail or screw or a galvanised nail, provided they do not penetrate the sole of the gutter.

5.4.2 Valley Design 

Valley gutters must discharge into a rainwater head, sump, or an eaves gutter. The discharge point must be within 2 m of a downpipe if the catchment area exceeds 50 m².

When the roof pitch is less than 12°, the valley should be made in one piece or the joints must be sealed. To ensure snug fitting, the valley angle should be matched to the pitch of the valley support. Having the valley too open will result in a diminished capacity, and too sharp an angle will make installation difficult.

5.4.2B Internal Valley Angle

Roof PitchInternal Angle
176°
173°
10°166°
15°159°
20°152°
25°145°
30°139°
35°132°
40°126°
45°120°
50°114°
60°104°

5.4.2C Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h

Roof Pitch10°12.5°1520°25°30°
A 3-fold  1218294170106146
B standard  2534476399140184
C Deep6086152180215251321389452
D Tile    1722334557
Free Board: 15 mm for pitches 8° and above
                      20 mm for pitches below 8°

For other pitches, rainfall intensity, and valley shapes refer to the 5.4.7 Valley Capacity Calculator tool.

For information about internal corners, refer to 5.3.3 Internal Corners.

5.4.3 Bifurcated Valleys 

The maximum recommended catchment area for a bifurcated valley is 10 m². 

 

5.4.5 Changing Angles in Valleys 

A change of roof pitch in a valley run will usually result in the change of angle in plan view. The change is acceptable, but the freeboard of the lower valley must be at least 20 mm to allow for turbulence.

5.4.6 Asymmetrical Valleys 

Where opposing roofs of different pitches discharge into a valley an asymmetrical valley is required. They may be designed so the side under the flatter roof is at the same height as the steeper side and 20 mm freeboard is required. When this is impractical, the valley must be sized as an internal gutter in accordance with E1/AS1 Surface Water.

5.4.7 Valley Capacity Calculator 

The values for  5.4.2C Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h can be found in this PDF document. A responsive online tool for calculating valley capacity is available at 5.4.7 Valley Capacity Calculator.

Before using this calculator, please read 5.2 Roof Drainage Design.

To calculate valley capacity, insert the required values in the designated fields. All valleys require freeboard.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1: =  mm per metre
  rads
 bends
 
 m
 
Minimum 1°, Maximum 60°
 
 °
  rads
Secondary pitch only needs to be entered manually if it is different to the main Roof Pitch
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only.
 
B=159°D=20Width=130Freeboard=20
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 °
  rads
= max ( RS , RS2 )
 °
  rads
= min ( RS , RS2 )
Using Martindales Formula:
 °
  rads
= atan ( tan ( A1 ) / tan ( A2 ) )
 °
  rads
= asin ( cos ( A1 ) * cos ( A2 ) ) + pi()/2
 
= cos ( A2 ) * cos ( A1 )
 °
  rads
= asin ( sC7 )
 
= tan ( A2 ) * sin ( aD )
 °
  rads
= atan ( tR1 )
 
= tan ( aD ) * csc ( R1 )
 °
  rads
= atan ( tC6 )
 
= tan ( pi()/2 - aD ) * csc ( R1 )
 °
  rads
= atan ( tC6' )
 °
  rads
= pi()/2 - C6'
 °
  rads
= pi() - C6 - C6' - C5'
 °
  rads
= C6 + C6'
Using WSP Sketch:
 
 
=W * sin ( C5' )
 
=D * cos ( C5' ) - FB
 
=IF ( ( h1max + h3 ) < h1max , h1max + h3, h1max )
 
=W * sin ( C5' )
 
=IF ( ( h1max + h3 ) < h2c,h1max + h3,h2max )
 
=IF ( ( h1max + h3 ) < h2max,0,h1max + h3 - h2max )
 
=0.5 * h1 * tan ( PI()/2 - C5 ) * h1
 
=0.5 * h2 * tan ( Beta - PI()/2 + C5; ) * h2
 
=IF ( ( h3 > 0) , ( W * cos ( C5; ) - 0.5 * h3 * tan ( C5; ) ) * h3 , 0 )
 
=( W * cos ( C5' ) - 0.5 * h4 * tan ( C5' ) ) * h4
 
=A1 + A2 + A3 + A4
 
=h1 / sin ( C5 )
 
=h2 / sin ( C5' )
 
=IF ( ( h3 > 0 ) , h3 / cos ( C5 ) , 0 )
 
=h4 / cos ( C5' )
 
=WP1 + WP2 + WP3 + WP4
 
=h2 * tan ( PI()/2 - C5 ) - IF ( ( h3 > 0 ), h3 * tan ( C5 ) , 0 )
 
=h2 * tan ( Beta - PI()/2 + C5 ) - h4 * tan ( C5')
 
=FWSW13 + FWSW24
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile

5.5 Rainwater Heads, Sumps, and Overflows 

Rainwater heads are situated outside the building envelope and sumps are internally located.

They both serve to increase the head of water entering a downpipe, and to provide an overflow capacity to safely discharge water when downpipe capacity is compromised or exceeded. The overflow should be obvious so discharging water warns the occupant that downpipe capacity has been exceeded or the primary downpipe is blocked.

 

 

 

5.5.1 Rainwater Heads 

Rainwater heads must be at least as wide as the gutter and have an overflow (normally a weir type). The cross-sectional area of the overflow must be at least equal to that of the required downpipe size for the catchment being served. The lower edge of the overflow must be at least 25 mm below the sole of the gutter, and the upper edge must be at least 25 mm below the upper edge of the gutter.

5.5.2 Sumps 

 

Sumps must be at least the same width as the gutter and have an outlet positioned below the sole of the gutter to increase the head of water at the outlet.

Internal sumps must have overflows. These are often a secondary pipe overflow with the outlet height positioned above the level of the primary outlet.

An internal sump should have a guard that prevents debris from blocking the outlet. A removable aluminium expanded-metal box can be fitted at a minimum of 40 mm below the sole of the gutter. Because the top is flat, it is unlikely that the entire surface area of the outlet can become blocked, so it is preferable to balloon-type guards. A leaf guard should have a horizontal surface area of at least four times the size of the downpipe outlet area and should be installed at roughly mid-height of the sump depth. Gratings can cause sump blockage, and this can reduce the outlet capacity.

Gratings or guards should be designed so that any debris will float, and hail, or obstructions, such as a tennis ball, will not wedge and block the guard. Gratings or guards should be cleared of accumulated debris regularly as part of normal maintenance.

5.5.3 Overflows 

Overflows must discharge clear of the building to clearly show that downpipe capacity has been exceeded; it should be an obvious indication that the gutters need maintenance.

The overflow opening of a rainwater head from an external gutter must have a cross-sectional area equal to that of the downpipe. The bottom of the overflow must be no higher than 25 mm below the bottom of the spouting.

Where the position of an outlet of a parapet wall gutter is on an outside wall, any scupper outflow should discharge into a rainwater head.

5.6 Outlets and Downpipes 

A gutter’s discharge capacity increases with the depth of water over the outlet. The best way to increase the head is to discharge the open end of the gutter into a rainwater head or sump. Swirl at the outlet reduces its performance, so positioning of the outlet is important.

Outlets must be placed at a distance less than or equal to the outlet diameter from the nearest vertical side of the sump.
Where they are connected directly to the drain, all internal downpipes must be sealed to internal sumps by a compression ring, or similar fitting, and must have access for cleaning at the base. All sump downpipes must be able to withstand a water pressure test with an applied head of 1.5 m of water without leakage.

To avoid any water back-up if the drain capacity is overloaded or obstructed, an air-break should be provided for all downpipes to ensure that water does not back up the downpipe.

All exterior downpipes must discharge freely over a grated gully trap or into an oversize pipe which must be a minimum of 50 mm above the adjacent ground level.

 

 

 

Downpipes fixed at an included angle of less than 105° must have a cross-sectional area equal to that of the gutter or be sized by calculation.
Downpipes must be compatible with the roof and gutter material and must comply with the 15-year durability requirement of the NZBC.

Discharging water off an inert surface onto unpainted galvanised rainwater goods can cause corrosion. See 4.11B Normal Catchment.

Horizontally run PVC downpipes and gutters require a greater provision for expansion than metal, particularly if they are painted a dark colour. Horizontally run PVC downpipes and gutters should have a maximum length of 9 m.

When rainwater is collected into a water tank, there is often not enough distance to obtain adequate fall for one downpipe outlet. In such cases, or whenever the roof design pre-empts a continuous spouting to the tank, it is possible to have several sealed downpipes (some of which can run underground) to discharge into the tank. The outlet discharging into such pipes should be a rainwater head to avoid flooding.

5.6.1 Placement of Downpipes 

Placement of downpipes significantly affects gutter and downpipe calculations.

 

 

5.6.2 Capacity Table for Common Size Downpipes 

Use this table to select the correct internal dimensions of common downpipe sizes for use in the online calculator at 5.3.7 Gutter Capacity Calculator.

5.6.2A Downpipe Capacity Table

MaterialSizeNominal
Diameter
(mm)
Internal
Dimension
(mm)
x-Section
Area
(mm²)
PVC65 x 50 65 x 523380
100 x 50 102 x 515171
 65633138
 80764537
 110987626
 16014316157
 20017825157
 25022439840
 28025350823
 31527459610
Steel 75754466
 1001007940
 90x50 90 x 504400

 

 

5.6.3 Downpipe Capacity Calculator 

The values for  5.6.2 Capacity Table for Common Size Downpipes can be found in this PDF document. A responsive online tool for calculating downpipe capacity available at https://www.metalroofing.org.nz/downpipe-capacity-calculator.

Before using this calculator, please read 5.2 Roof Drainage Design.

To calculate downpipe capacity, select the type of building, type of gutter and overflow (yes or no). Complete the rest of the data by changing the values in the designated fields.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1: =  mm per metre
  rads
 bends
 
 m
 
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only and is not to scale.
 
HeadFreeboardDepth
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile

5.7 Downpipe Spreaders 

All downpipes that discharge onto a lower roof must have a spreader to dissipate energy and ensure wide distribution of the water. A spreader should be used over multiple troughs.

For corrugate, a spreader should not discharge into a lapped trough. When using the COP calculator, discharge may be into a lapped trough of a trapezoidal or a trough profile.

The area of discharge holes from a spreader should equal the cross-sectional area of the downpipe.

The 5.7.1 Maximum Area Above Spreader Calculator enables users to determine the maximum upper roof area that a lower roof can discharge for a given combination of rainfall intensity, roofing profile, and lower roof pitch.

 

5.7.1 Maximum Area Above Spreader Calculator 

Before using this calculator, please read 5.2 Roof Drainage Design.

Calculate the maximum area above a spreader by entering the values in the designated fields.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

A responsive online tool for calculating Maximum Area Above Spreaders is available at https://www.metalroofing.org.nz/maximum-area-above-spreader-calculator.
Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1: =  mm per metre
  rads
 bends
 
 m
 
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only and is not to shape or scale.
 
Wetted Perimeter=49Pitch=76Cross Section Area=272Free Surface Width=42Depth=17Wetted Perimeter=85Pitch=76Cross Section Area=585Free Surface Width=76Depth=17
Illustration is for explanatory purposes only and is not to shape or scale.
 
DepthPitchCrestCapillary DepthPanDepthPitchCrestPan
Illustration is for explanatory purposes only and is not to shape or scale.
 
Wetted PerimeterPitchCross Section AreaFree Surface WidthWetted PerimeterPitchCross Section AreaFree Surface Width
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile