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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.


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 v24.06:Useful-Information; Sheet-Metal-Work-Roofing-Contractors

18.8 Sheet Metal Work For Roofing Contractors. 

When forming various flashings in sheet-metal the Roofing Contractor is required to know how to cut the material in order to obtain the desired shape.

Basic knowledge of geometrical drawing and mensuration is required and this section explains the methods which are employed to ensure accurate results.


A straight line. A straight line is a line drawn in the shortest manner between two given points, so any other line between these points is a curved line.



18.8A Parallel Lines

Parallel lines are lines which, when extended, do not touch. Given a line CD, to draw a parallel line set a compass to the required distance apart and with C and D as centres, describe two arcs. A line drawn as a tangent to both arcs will be a parallel line to CD.


18.8B Perpendicular Line


To draw a perpendicular line. Given a straight line EF, set a compass to any distance more than half the distance EF and, with E and F as centres, describe arcs of radius EG and FH.

A line drawn through the points of intersection of these arcs is perpendicular to EF and bisects the distance EF.


18.8C Dividing a line into equal parts


To divide a line into any number of equal parts. Given a straight line J K, draw another line J L at any suitable angle and no particular length. Set off on JL, at any reasonable distance apart, a number of equal spaces similar in number to the parts into which JK is to be divided.

Connect L and K with a line, and parallel to this draw other lines through points on JL. These divide JK into the required number of equal parts.



18.8D Bisecting and Arc

Given an arc AB, set a compass to a distance a little more than half that between the ends and with A and B as centres, describe arcs of equal radii.

A line drawn through the points of intersection will bisect AB. This method can be employed to divide the arc into any number of even parts by repetition. Further, the method may be used to find the centre of any given arc by further bisecting AC and CB.

Lines taken through the intersecting points of these latter arcs, when produced, will intersect at the centre of the arc AB.


18.8E Bisecting an Angle

Given an angle ABC, set off equal distances BD and BE and with D and E as centres and a compass set at any reasonable radius, describe arcs to intersect in F. A line drawn through B and F bisects the angle.



18.8F Dividing a Circle into Six Equal Parts

 Set a compass to the radius of the circle and step this distance off along the circumference. Further division into 12 parts may be done by bisecting one part, and again stepping off with the radius of the circle


18.8G Developing the Frustrum of a True Cone

Draw the elevation X with base diameter AB, the vertical height CD to the desired cone angle and add the section line EF to the elevation. With centre D and radius DA describe a semicircle Y on the base, and divide the circumference of this into six equal parts.

To draw the development Z: With centre C and radius CB, describe an arc AA¹ whose length equals the circumference of cone base.

This may be obtained by marking off along the arc from A spaces equal to parts in the semicircle Y but double in number.(12)

With C as centre and radius CE, draw the arc EE1 and add the line CA¹.

The figure AA1E1E is the development Z of the frustrum.


To do this, drop a perpendicular from F to F¹ and extend the base line AB.

An offset diagram is now made by measuring distances B¹F¹, B¹G, and B¹H, setting these off from F¹ on base line AB and drawing lines to F.

The lengths FF¹, FG, and FH, etc., are now true lengths.

To draw the development Z:

Draw a centre line C¹O. At right angles to C¹ draw A²B² equal to AB.

From C1, set off distance C1F1, equal to FB.

Join A² and B² to F¹. With centre A² and radius F¹G, draw a short arc to be cut by an arc of F¹G radius struck from F1 to obtain point G1.

Similarly, with A² as centre and radius FH, draw an arc, to be cut by an arc of GH radius struck from point G¹, thus obtaining point H¹.

Draw a line through, A² and H¹ and produce same to intersect the centre line C¹O at O.

Repeat the process with. B² as centre for long radii, thus completing one quarter of the whole development.

To complete the pattern, draw a curve through points H¹G¹F¹ and repeat in the other sections of the development.


18.8H Developing a Square Base to a Circular Top

Draw the elevation X, making the base AB, vertical height CD, and diameter of top EF.

Draw a half plan Y on the base, drawing the semicircle E1F1 and dividing one half of this into a number of equal parts, F¹G, GH, HJ, and JK.

Through points F¹, G, H, J, and K, draw lines to B¹.

Before proceeding to the development it is necessary to find the true lengths of B¹K, B¹J, B¹H, B¹G, and B¹F¹.