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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 v23.12:Structure; Profiles

3.4 Profiles 

Profile familes can broadly be segregated as pierce fastened (corrugate, trapezoidal), and secret fix (tray and trough). Within these broad categories there are subsets.

 

Symmetrical corrugate or trapezoidal profiles have the advantage of being more easily curved around a radius. Because the ribs are necessarily close together, they have the disadvantage of roof traffic loads having to be spread over two crests, and they have a lower run-off capacity.

 

3.4.1 Corrugate Profile 

The corrugated profile has been used in New Zealand for over 150 years and there has been only one significant change during that period.

In the 1960s the steel grade used for roof and wall cladding changed from low-strength steel (250MPa or G250)  to high-strength steel (550 MPa or G550). The number of corrugations also changed from 8 to 10.5, which enabled the sheets to be laid either side up, as opposed to over-and-under.

The standard corrugation profile has 17.5 mm deep crests at 76 mm spacings, but smaller and deeper alternatives are also available.

Corrugate cladding is formed with a slightly asymmetrical overlap profile to a capillary barrier.

3.4.2 Trapezoidal Profile 

The trapezoidal shape provides greater water carrying capacity and provides greater spanning capabilities than corrugate profile.

The angle and height of the trapezoid rib determines the profiles performance under compression. A deeper rib generally gives improved performance, but lowers the effective coverage of the sheet.

Trapezoidal profiles are available with different ribs heights (see 3.4A Pierce Fastened Profiles).

 

3.4.3 Miniature Profiles 

Various miniature cladding profiles are manufactured in New Zealand, the most common being known variously under the names of mini-corrugate, sparrow iron, baby iron and mini-iron.

Mini-corrugate is sometimes used for small roof areas, such as spires and awnings. It is most commonly used for wall cladding, parapets and internal linings where studs are normally spaced at 600 mm centres or less.

Some miniature trapezoidal profiles are also manufactured specifically for wall cladding.

3.4.4 Secret-fixed Profiles 

Secret fixed profiles are attached to the structure by clips rather than by pierce fasteners. This allows for freer thermal movement and minimises the number of penetrations in the cladding. The steep sided ribs and wide pans typical of secret fixed profiles gives greater water carrying capacity, and facilitates watertight flashing design.

Traditionally they were used for pitches as low as 1° but due to durability issues caused by deflection and resultant ponding the recommended minimum pitch is now 3°.

Trough sections generally have 2 to 3 pans or trays 180 – 250 mm in width. Tray roofing has a single pan of up to 550 mm in width.

Standing Seam is a subset of tray roofing. Standing seam roofs are based on traditional manufacturing methods (using folding and hand tool rather than roll forming), but now they are available roll-formed in most itirations. (See 15.4 Tray Roofing)

3.4.5 Material Grade 

To test for tensile strength the material is subjected to a longitudinal (stretching) load, and values are taken for yield strength (when it permanently deforms) and tensile strength (when it breaks). Elongation is also measured during this test.

The minimum tensile strength defines the grade of steel, eg, G550 for high-strength light gauge steel, but to comply with this grade the yield strength and elongation must also fall within defined parameters.

Yield strength is an important determinant of the strength of a profile, along with profile shape and material thickness. High tensile material will have more resistance to failure such as buckling around the fastener under wind uplift, pull-through of the fastener head, or buckling under foot traffic load. However, tensile strength has a negligible effect on deflection under load.

Where 0.55 material is specified for straight corrugate or trapezoidal roofing, it is unacceptable to substitute G300 for G550 grade material as the resultant profile will have little strength advantage over 0.40 mm G550.

Aluminium is defined by a hardness grade ranging from H32 to H38.  Typically, H34 is used for flashings, severe profiles such as trough sections, and profiles that are to be curved. Most corrugated and Trapezoidal profiles are manufactured using H36.

It must be remembered that the alloy also affects strength. H36 aluminium in 5005 or 5025 alloys, which are typically used in New Zealand, will have considerably greater tensile strength than the same grade in a 3000-series alloy.

3.4.5A End Use for Typical Alloys

MaterialGradeTypical End Use
SteelG300Flashings, ridging, spouting, curving, some trough sections.
G550Corrugated and trapezoidal profiles, some trough sections.
Aluminium 5505/5025H32Lock seaming
H34Flashings, curved roofing, trough sections, and tray roofing.
H 36Flashings and profiled roofing, trapezoidal sections, and corrugate.
 

 

 

 

 

3.4.6 Material Thickness 

Material thickness has a great bearing on load capacity and buckling resistence. For residential buildings, 0.40 mm thickness material is most commonly used for corrugated and trapezoidal profiles, and this will normally be sufficient to withstand the statutory wind loads at typical spans and fastener spacings in up to High Wind Zones. In higher winds speeds, it is often more effective to use 0.55 mm material, rather than reducing purlin and fastener spacings.

Material with a 0.40 mm thickness is very vulnerable to foot traffic damage in most profile configurations and requires careful and accurate foot placement to avoid buckling. In residential buildings with high foot traffic expectancy or highly visible roofs, eg, multi-level mono-pitch roofs, roofs with UV collectors, flues, aircon devices, chimneys that need servicing, or prestige housing 0.55 mm material should be selected.

For commercial and industrial applications, 0.55 mm is almost universally used on the roof, and 0.40 mm is most common on walls.

0.40 mm and 0.55 mm are not the only thicknesses available; 0.48 mm G550 is often used for high tensile trough sections, where it will often compare in strength to similar profiles manufactured from 0.55 mm G 300 material. 0.75 mm is often specified for heavy duty industrial roofs, and 0.63 mm is often manufactured for the Pacific Islands and other hurricane-prone regions. Other thicknesses are also available subject to minimum order quantities