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Disclaimer

Although the information contained in this Code has been obtained from sources believed to be reliable, New Zealand Metal Roofing Manufacturers Inc. makes no warranties or representations of any kind (express or implied) regarding the accuracy, adequacy, currency or completeness of the information, or that it is suitable for the intended use.

Compliance with this Code does not guarantee immunity from breach of any statutory requirements, the New Zealand Building Code or relevant Standards. The final responsibility for the correct design and specification rests with the designer and for its satisfactory execution with the contractor.

While most data have been compiled from case histories, trade experience and testing, small changes in the environment can produce marked differences in performance. The decision to use a particular material, and in what manner, is made at your own risk. The use of a particular material and method may, therefore, need to be modified to its intended end use and environment.

New Zealand Metal Roofing Manufacturers Inc., its directors, officers or employees shall not be responsible for any direct, indirect or special loss or damage arising from, as a consequence of, use of or reliance upon any information contained in this Code.

New Zealand Metal Roofing Manufacturers Inc. expressly disclaims any liability which is based on or arises out of the information or any errors, omissions or misstatements.

If reprinted, reproduced or used in any form, the New Zealand Metal Roofing Manufacturers Inc. (NZMRM) should be acknowledged as the source of information.

You should always refer to the current online Code of Practicefor the most recent updates on information contained in this Code.

Scope

This Code of Practice provides requirements, information and guidelines, to the Building Consent Authorities, the Building Certifier, Specifier, Designer, Licensed Building Practitioner, Trade Trainee, Installer and the end user on the design, installation, performance, and transportation of all metal roof and wall cladding used in New Zealand.

The calculations and the details contained in this Code of Practice provide a means of complying with the performance provisions of the NZBC and the requirements of the Health and Safety at Work Act 2015.

The scope of this document includes all buildings covered by NZS 3604, AS/NZS 1170 and those designed and built under specific engineering design.

It has been written and compiled from proven performance and cites a standard of acceptable practice agreed between manufacturers and roofing contractors.

The drawings and requirements contained in this Code illustrate acceptable trade practice, but recommended or better trade practice is also quoted as being a preferred alternative.

Because the environment and wind categories vary throughout New Zealand, acceptable trade practice must be altered accordingly; in severe environments and high wind design load categories, the requirements of the NZBC will only be met by using specific detailing as described in this Code.

The purpose of this Code of Practice is to present both Acceptable Trade Practice and Recommended Trade Practice, in a user-friendly format to ensure that the roof and wall cladding, flashings, drainage accessories, and fastenings will:

  • comply with the requirements of B1, B2, E1 E2 and E3 of the NZBC;
  • comply with the design loading requirements of AS/NZS 1170 and NZS 3604 and with AS/NZS 1562;
  • have and optimised lifespan; and
  • be weathertight.

COP v24.03:Roof-Drainage; Outlets-And-Downpipes

5.7 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.
Downpipe retaining brackets, otherwise known as stand-off clips as astragals, are used to align the downpipe between the dropper and and discharge sump. Normally they are situated near both ends of the downpipe, with intermediate brackets placed in high traffic areas or where span is greater than 3 m.

 

 

 

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.

5.7.1 Placement of Downpipes 

Placement of downpipes significantly affects gutter and downpipe calculations.

 

 

5.7.2 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 5.4.7 Gutter Capacity Calculator.

5.7.2A 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

 

 

5.7.3 Downpipe Capacity Calculator 

The values for  5.7.2 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 5.3 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 5.3.2 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 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 and is not to scale.
 
HeadFreeboardDepth
Describe the product: this does not control the calculation which relies on you entering accurate data
 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
 
Describe the product: this does not control the calculation which relies on you entering accurate data
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