COP v3.0:structure;

Structure 

Load Design discusses design and installation elements to ensure roofs are structurally sound and meet the objectives for the NZBC B1 Structure.

Topics include:

  • AZ/NZS 1170:2011
  • NZS 3604
  • Understanding loads
  • Fastener performance
  • Profile shape
  • Wind load span graphs and fixing patterns.

3.1 NZBC Clause B1 (Extract) 

Source: New Zealand Building Code Clause B1 Structure

3.1.1 Objective 

B1.1 The objective of this provision is to:

  • Safeguard people from injury caused by structural failure.
  • Safeguard people from loss of amenity caused by structural behaviour.
  • Protect other property from physical damage caused by structural failure.

3.1.2 Functional Requirement 

B1.2 Building elements shall withstand the combination of loads that they are likely to be subjected to through construction or alteration, and throughout their lives.

3.1.3 Performance 

B1.3.1 Building elements shall have a low probability of rupturing, becoming unstable, losing equilibrium or collapsing during construction or alteration, and throughout their lives.

B1.3.2 Building elements shall have a low probability of causing loss of amenity through undue deformation, vibratory response, degradation, or other physical characteristics throughout their lives.

B1.3.3 Account shall be taken of all physical conditions likely to affect the stability of building elements including:

  1. Self-weight.
  2. Imposed gravity loads arising from use.
  3. Temperature.
  4. Earth pressure.
  5. Water and other liquids.
  6. Snow.
  7. Wind.
  8. Fire impact.
  9. Differential movement.
  10. Influence of equipment services and non-structural elements.
  11. Time dependant effects including creep and shrinkage.

B1.3.4 Due allowance shall be made for

  • the consequences of failure,
  • the intended use of the building,
  • effects of construction activities,
  • variation in the properties of materials, and
  • accuracy limitations inherent in the methods used to predict the stability of building elements

3.2 General 

The information in this section explains the various factors used in calculating a design loading and in resisting that load.

The designer must be familiar with the performance requirements of the New Zealand Building Code (NZBC). Loads can be calculated in accordance with the appropriate standard and maximum spans and fastening patterns specified according to manufacturers’ literature or the generic tables listed in this Code of Practice.

It is the responsibility of the roofing contractor to install roof cladding according to the design and raise any concerns with the designer before commencement.

3.3 AS/AZS 1170.2:2011 

Roof and wall cladding must structurally comply with the requirements of the NZBC Clause B1 Structure. Strength demand may be calculated in accordance with AS/NZS 1170:2 (which is called the “Loadings Code” in this Code of Practice) or NZS 3604.

Designers should know about changes in requirements of the current Loadings Code and amendments to the code. Manufacturers' printed technical literature, using different criteria or test values and a previous Loading Code's design can cause confusion when it is compared to the latest requirements.

The Loadings Code identifies four load categories relevant to metal roof and wall cladding.

  • Wind actions:
    Wind loads are the result of local changes in wind speed as the wind flows over and around the building. High positive forces (pressure) apply where the wind is slowed, high negative pressures (suction) apply where the wind accelerates.  Wind force varies with the shape and position of the building. It also increases with height because the influence of groundsurface drag decreases.
  • Permanent action:
    Dead load is the permanent weight of the roof structure and the permanent part of an imposed load, such as an air conditioning unit.
  • Imposed action:
    Live loads are variable loads imposed on the building by its occupants and contents, such as a person standing on the roof (point load).
  • Induced actions:
    Loads such as wind, snow or ice, and ponding rainwater.

When a structure or part of it, fails to fulfil its expected basic functions, it is said to have reached a limit state. There are two limit states—Serviceability and Ultimate. 

3.3.1 Modes of Failure 

Serviceability limit is a state when a building, or any part of it, becomes unfit for its intended use due to deformation or deflection.

Ultimate limit is a state associated with collapse or failure, or when a building or any part of it becomes unstable or unsafe.

These limit states are not limited to the metal roof and wall cladding, but are intended to be applied to the entire building structure.

Because the prime function of metal roof and wall cladding is to exclude water from the structure, irreversible failure at the serviceability limit state, for example permanent distortion around the fastener head, is generally the governing limit state for pierce fastened roof and wall cladding. This Code of Practice treats serviceability as the criterion of failure for pierce fastened roofs, as these failure levels are far lower than those at which ultimate limit state failure is experienced.

3.4 NZS 3604:2011 

NZS 3604 Timber Framed Buildings  is an acceptable solution to comply with the NZBC for light timber frame buildings not requiring specific design.

It contains prescriptive dimensions for purlin spacing and fasteners, based on maximum design wind speeds of Low (32 m/s), Medium (37 m/s), High (44 m/s), Very High (50 m/s), or Extra High (55 m/s). The load calculations for NZS3604 were based on a simplified interpretation of AS/NZS 1170.  These values can be used for calculation of loads on the cladding of structures designed using NZS 3604.

Some of the limitations in the scope of NZ S3604 are:

  • Timber frame construction.
  • Height from lowest ground to the highest point on the roof may not exceed 10 m.
  • A snow load may not exceed 1.0 kPa, although Section 15 of NZS 3604 does provide additional criteria for 1.5 kPa and 2.0 kPa snow loads.

NZS 3604 includes:

  • private dwellings, hostels, hotels and nurse's homes;
  • factories with restricted floor loadings; and
  • institutional and educational buildings with restricted floor loadings.

NZS 3604 excludes:

  • buildings dedicated to the preservation of human life;
  • buildings which may host crowds;
  • publicly owned buildings containing high value contents; and
  • curved roof construction.

Classification of Wind Zones in NZS 3604 are specific to the site. Because the buildings covered by this standard are limited in size, design tables (but not design wind speed) include a local pressure factor of 1.5 kPa over the entire structure, rather than varying factors according to the position on the roof as required by AS/NZS 1170. 

3.5 Understanding Loads 

The performance of profiled metal cladding under wind, snow and point loads depends on its ability to resist the tension (pulling), compression (squashing), and shear (sliding) forces that it is likely to be subjected to during the lifetime of the building.

 

 

3.5A Profiled Steel — Flanges and Webs

A structural steel member typically comprises of sections named webs and flanges. In a roofing profile the sides of the rib act as a web, and the pan and top of the rib acts as a flange

 

Profiled metal cladding acts as a beam, which derives its strength from the ability of its flanges (pan and the crest), separated by the web to resist tensile and compressive forces. This strength can only be maintained while those parts in compression are restrained from buckling.

When a beam is subjected to bending, one flange will be under compression and the other under tension.

3.5.1 Point Load 

 

 

 

When a roof sheet deflects downwards under point load, the pan of the profile is placed under tension and the crest under compression, but in the adjacent continuous span, this condition reverses.
Metal is relatively strong under tension and weak under compression.  Because of this, excessive point load will cause failure by compression buckling of the rib.  Wherever possible, temporary point load such as roof traffic should be placed with the weight in the pan of the profile, so that the load is shared by the two adjacent ribs, across two ribs, or at the purlin line. Permanent point load should be placed across multiple ribs.

 

 

 

3.5.2 Wind Load 

Under wind uplift loading, when all spans of the roof sheet are under upward loads, the crest of the profile is placed under tension and the pan under compression. The deflection and stress patterns are the reverse of those for point load.

 

Failure under wind load for a clip fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.

Initial failure under wind uplift for pierce-fastened cladding is usually local buckling of the rib crest adjacent to the fastener. While the cladding can still resist a load, this permanent deformation is liable to cause leakage at that point; therefore, it is a serviceability failure.

 

 

The design load relates to a specific building and is calculated by the engineer. The failure load relates to a specific product or system and is supplied by the manufacturer. Engineers calculate both the Serviceability load and the Ultimate Load. They compare these values with the maximum failure loads of the products and systems they are considering.

  • In design, the serviceability load is no more than 0.72 of the ultimate load.
  • In testing, the serviceability load is about 0.5 of the ultimate load for a pierce fastened standard corrugate or low rib trapezoidal products.

Therefore, if a standard corrugate or low rid trapezoidal product passes for serviceability it will comfortably exceed ultimate design load requirements. In the above example, the product has failed against serviceability but still exceeds ultimate load requirements.

For medium and high ribbed trapezoidal, both serviceability and strength loads should be checked. Clip fastened products are typically restricted by strength load only.

Refer to manufacturer's load/span tables for all other profiles, which should give the maximum recommended load for end and continuous spans when tested as described in 17.7 Wind and Point Load Testing.

To determine the performance of corrugated and low rib trapezoidal profiles, see 3.17 Steel Cladding Wind Load Span Graphs.

3.5.3 Deflection 

Deflection is a measured deformation of roof or wall cladding under a load, but there is difference between temporary deflection and permanent deformation.

Temporary deflection of cladding under load is within the elastic limit of the steel; the cladding will regain its original shape and strength properties when the load is removed. Permanent deformation that affects the shape, strength or performance of the cladding is serviceability failure.

With high strength claddings deflection failure is often quite sudden and severe once the point of elasticity is reached, but progressive deflection under repeated loads within the material’s elastic limit is minimal.

Roof design should consider the effect of repeated loads, when expected, because low-strength steel or non-ferrous metals can progressively yield under wind loads or repeated constant foot traffic. Machine roll-curved, crimp curved cladding, and metal roof tiles are usually made from low strength steels.

 

 

 

 

 

 

 

3.5.4 Load Distribution 

The distribution of the load greatly affects the deflection. A load distributed equally along the length of a beam (line load) will cause less deflection than the same load being applied to the centre of the beam.

 

 

 

 

 

 

 

The depth of the profile of metal cladding is another important criterion in the design and use of metal roof and wall cladding. Given the same cross section area, the deflection of the profile will vary with the square of the depth.

 

 

3.5.5 Yield Strength 

Yield strength is the point beyond which material will permanently deform, and Ultimate strength is the point beyond which the material ceases to resist load. Yield strength is affected by the type, grade or temper of the metal measured in Megapascals (MPa). The yield point is more important than the ultimate strength when designing and using metal roof and wall cladding.

The yield point of any metal can vary depending on the temper. If the yield point is higher, it is possible to reduce the thickness of the metal of the same profile and still obtain equal performance.

3.5.6 Section Properties 

Various attributes or effects can be identified by looking at individual shapes or profiles of metal cladding, but their performance cannot be accurately determined by their section properties as their profile shape, and therefore the sectional properties, change during deflection.

Corrugated iron is a common profile traditionally used in roof and wall cladding. It has a symmetrical sinusoidal profile and is equally strong under positive or negative loading. Its ‘neutral axis’’ runs through the centre, or halfway-depth, of the profile. Symmetrical trapezoidal profiles have similar attributes.

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 or along the purlin line, and they have a lower run-off capacity.

Most profiles fixed on non-residential roofs in New Zealand are asymmetrical trapezoidal or ribbed profiles, with ribs formed at various spacings and different heights. The angle and height of the trapezoid rib determines the profiles performance under compression. A steeper angle generally gives improved performance, but raises cost per square metre as it lowers sheet effective coverage.

The rib spacing determines the position of the neutral axis, which is the point of zero stress. Bigger spacing between the ribs lowers the neutral axis.

Roof cladding should be designed to withstand both positive and negative design loads. Profile designers use computer software to compare the "moment of inertia" (deflection) and "section modulus" (strength) to find a compromise in different profiles.

Strength and deflection are interrelated but not interdependent, and the design strength is determined by the stress (f) at which permanent deformation occurs. Stress is determined by the section property known as the Modulus of Section (Z). Although it is possible to calculate the sheeting performance from the section properties of the profile, only physical testing can prove the actual capabilities of the profile.

Deflection under load depends on the profile section dimensions or section properties, and it is determined by the Moment of Inertia (I) and Young's Modulus (E). Various metals have different E-values, which means the same cladding profile made in aluminium will deflect more than steel. See 17.3 General Methods Of Testing Sheet Roof And Wall Cladding.

The strength or grade of the metal does not affect the deflection. A high-strength steel profile will deflect to the same extent as low-strength steel profile, but using high-strength steel will increase the point at which yielding or permanent damage occurs. See 14.6 Walking On Roofs.

3.5.7 Fatigue 

The performance of metal roof and wall cladding is affected by stress as all metals may be subjected to fatigue under repeated heavy load conditions.

Metal cladding can fail at a much lower point than the yield stress when there is movement under continued fluctuating stress. A cyclic load test is used to determine the performance of cladding under load reversal.

All metal joints will suffer stress because of movement caused by expansion, vibration, traffic or wind. A sealed joint should have enough fasteners to mechanically resist this stress, because sealant or solder alone do not offer enough resistance. Load spreading washers are used in areas subjected to high wind design loads to give greater resistance to any stress-cracking at fixing holes.

High-strength steel is subject to fatigue, which seldom happens in practice. Other metals, such as lead and copper, are restricted in length or the overall panel size to avoid cracking by fatigue. Sharply folded corners should be avoided on these materials and the minimum radii requirements should be followed.

3.5.8 Continuity 

It is important to understand the relationship of strength and deflection when using multiple span profiled metal cladding, because the performance of roof cladding under load depends on the continuity over several spans.

An adjacent continuous span assists the performance of profiled metal cladding, as continuity can reduce deflection up to 50%. In single spans the cladding is free to rotate at each support, but with continuity at a support the cladding is held down by an adjacent span; reducing the rotation and midspan‑deflection.

Profiled metal sheeting deflects less over intermediate spans because it has continuity at two ends, unlike the end span, having continuity at only one end.

 

Tests for point load have established the ratio between the end span length and the intermediate span length, when both spans will fail at approximately the same load.

The end span of profiled metal cladding should be no more than two-thirds of the intermediate span for optimum performance under both point load and wind load. Neglecting to do this can lead to failure of the end spans when are they subjected to foot traffic.

The endspan condition occurs at the eaves, ridge, roof steps and on both ends of penetrations where a full width sheet is cut. Cladding around penetrations may need additional purlin support, spaced at an equal distance to the endspan spacing.

3.6 Types of Load 

Loads acting on roof cladding are generally classified into two types: point load and uniformly distributed Load (UDL)

Cladding reacts differently to a point load and a UDL. A point load is applied to a particular area, but a UDL impacts on the total area of the roof.

In many cases the point load will govern; it is often the most severe of the actions and will determine the purlin spacing of roof sheeting. Uniformly distributed loads vary over the roof area. They are greatest at the periphery and corners of a structure. Purlin spacing may have to be reduced, or the fastener frequency increased, to cope with local pressure factors.

Manufacturer's roof and wall cladding design load data should be published with both point and UDL performance values.

3.6.1 Point Load 

Most roofing profiles will resist far greater point loads when the load is applied to the pan of the profile rather than the rib. When the load is applied to the pan, the load is shared by the adjacent ribs and is applied to the flange under tension, rather than the flange under compression.

Testing loads may be applied to the pan or the rib depending of the profile shape and the design criteria. See 3.6.2 Roof Traffic.

Roofs that may be accessed by foot traffic must be designed to withstand a point load which is representative of a worker with a bag of tools. It is calculated at 112 kg, which equals 1.1 kN force.

In the case of a superimposed load, such as an air conditioning unit which is supported directly by the roof cladding, the unit weight per support and area of contact is calculated to arrive at point loads.

A point load on a roof is always positive or downward (+).

 

3.6.2 Roof Traffic 

The designer must consider the degree and type of foot traffic that may be expected on a roof. The following requirements are subjective standards and must be considered in line with customer expectations, and building use and type.

More robust design than specified below (reducing purlin spacing or adding protection from mechanical action), is required for:
  • roofs that are regularly accessed; and
  • roofs used as staging by subsequent trades; or areas that are adjacent to access points, particularly step down access.
Type A (Unrestricted Access) will apply for roofs:
  • that need to be regularly traversed by the roofer for access during installation;
  • that will be accessed regularly by sub-trades;
  • that butt on to walls or windows that may require maintenance;
  • that have plant, chimneys, or solar installations requiring regular maintenance; or
  • that require regular access for clearing gutters or spouting of debris.
For Type A roofs, the cladding must resist the load of 1.1 kN applied to the pan or a single rib.
Type B (Restricted Access) will apply for roofs:
  • that are simple in design and do not have to be regularly traversed by the installer;
  • which are infrequently accessed by qualified trades people for maintenance; or
  • with a pitch of more than 35°.
For Type B, roofs the cladding must resist the load of 1.1 kN applied to the pan or over two ribs.
Type C (Non-trafficable) will apply for roofs:
  • where supports are required to be laid to support roof traffic;
  • which have a pitch of 60° or greater; and
  • including non-trafficable translucent roof sheeting.
For Type C roofs, the cladding must resist the minimum load of 0.5 kN applied to the pan or over two ribs.

3.6.3 Uniformly Distributed Load (Wind or Induced Action) 

A Uniformly Distributed Load (UDL) is commonly either a wind load or a snow load. These loads are variable and depend on factors such as the location, topography, and position on the structure, but do not often exceed 6 kPa. The most severe wind load is usually an uplift load, or negative (-), and snow load is a downward load or positive (+).

3.7 Wind Load 

The wind load imposed on a roof structure is taken to apply perpendicularly to the roof cladding over a nominated area. The design wind load is affected by the pitch of the roof and is modified using factors called pressure coefficients. Wind design load is measured in kilopascal (kPa); 1 kPa equals 1 kN/m². 

AS/NZS 1170.2 and NZS 3604 contain the basic wind speed regions for New Zealand and the modifying factors that govern the design wind load. The predominant wind speed for New Zealand is 45 m/s. The exceptions are either side of Cook Strait and areas in the lee of mountainous areas.

3.7A Sheltered Building

Scattered obstructions of a similar height or lower within 500 m from a building will considerably lessen wind speeds and lower design wind pressures.

 

3.7B Unsheltered Building

Structures in open land such as flat pasture and playing fields, or by water, will be subjected to higher design wind pressures.

3.7C Topographical Influences

Terrain also has a big effect. Structures near the crest of a rise or on flat land near a steep face will have increased design wind pressure.

Wind Design Load is affected by building design factors such as building height, shape, proportions, orientation, and roof pitch. Permeability can also be a big factor. Buildings with large openings on one side but completely closed on the other three sides will suffer high internal wind pressures. These internal pressures must be added to the suction load on the outside of the roof when calculating wind design load.

Local territorial authorities are usually able to give wind speed figures for a specific address in their area. All other factors, including topographical influences, internal, and local pressure factors must be considered by a suitably qualified professional to calculate the design wind load on a structure.

3.7.1 Local Pressure Factor (Kl) 

The local pressure factor (Kl) is an important design consideration required by the Loadings Code. The peripheral areas of roof and wall surfaces are subjected to greater uplift loads than the main body of the roof. Designers need to include local pressure factors in the calculation of wind loads on the cladding.

When determining fixing requirements, the engineer must prepare a roof map showing purlin spans and local pressure factors for each section of the cladding. See A Pull-Over.

When designing to NZS 1170 the local pressure factors are:

  • 1.5—applied to the edges of all buildings at a dimension equal to 0.2 or 20% of the width or height of the building whichever is the least.
  • 2.0—applied to the edges of all buildings at a dimension equal to 0.1 or 10% of the width or height whichever is the least.
  • 3.0—applied to roof pitches less than 10°, at the corners where the dimensions in (a) intersect. It also applies to corners of walls where the building height is greater than the building width.

 

 

 

 

 

 

 

 

 

3.7.2 Conversion of Wind Speed To Pressure 

The basic formula for converting a wind speed to wind load is: 0.6 x velocity²= wind load.  However, to get a true design wind speed is a lot more complex; various factors have to be applied including roof self-weight, internal pressure and local pressure coefficients. 

The most influential of these factors is generally the local pressure factor, but internal pressure can also have a profound influence—particularly on unlined structures. Even houses built to NZS3604 have internal pressure factors incorporated into their design load calculations.

3.7.3 Roof Weight 

The self-weight of light-weight profiled sheet cladding should be included in the calculation of net wind load, but is a minor factor.

3.8 Snow Loads 

Roof cladding design does not usually have to be altered for snow load, but  it may be necessary to increase the strength of the structure to allow for induced snow loads.

The maximum snow load in New Zealand (under NZS 3604) is a UDL of 2 kPa. Collapse under snow load would be a strength failure, since 2 kPa is less than the upwards load in a Very High Wind Zone. However, as it is a downwards load, restraint is linear by the purlins, rather than point restraint by the fasteners, so greater capacity is achieved.

Any profile-gauge combination that will resist a wind load of Very High or Extra High Wind Zone with fasteners at each crest, will adequately resist a 2 kPa snow load.

New Zealand is divided (in NZS 3604) into six zones where the maximum snow load is 2 kPa. Any areas above specific altitudes in these areas require specific design. Buildings designed according to NZS 3604 are designed to withstand a 1.0 kPa snow load. Buildings above a given altitude in areas 1 – 5 (see 3.8A Snow Loadings Map) must be designed to withstand the appropriate snow load.
Projections such as gutters, flashings and chimneys need additional fixings and detailing to resist loads from sliding snow. Doorways must be easy to keep clear.

 

 

 

3.8B Altitudes for Specific Design

Snow Loading Allowance is not required for Zone 0
ZoneUp to:1.0 kPa1.5 kPa2.0kPa
0 Not Required
1 400600850
2 400600850
3 400600850
4 100200350
5 200300400

3.9 Dead Loads 

Any permanent load added to the roof cladding or structure is termed superimposed; that includes air conditioning equipment, solar installations, television aerials, anchor points and walkways.

All permanent loads must be fixed to or through the rib of the cladding profile or directly to the primary structure. The rib, even at the purlin line, has limited capacity for an additional load and the roof cladding must be free to expand.
Any attachment to the roof cladding must be compatible with the cladding.

An air conditioning unit correctly installed on a roof, using durable and compatible materials.

 
No additional equipment must be directly connected to the cladding without considering the effect of increased dead and live loads.
When designing installations for placement on the roof, the roof traffic implications of installing and servicing such must be considered when determining point load resistance requirements

3.10 Construction Loads 

Construction loads on a building include the wind load on a partially clad or braced roof or building. Depending on the method and sequence of construction, it can be greater than the load on a completed building.

Other forces contributing to construction load include:

  • The intensity of internal wind pressures due to a temporary absence of ceilings, walls and glazing.
  • Storage of roof cladding on the structure. Bundles of roof cladding should be placed so they do not cause overstress in purlins.
  • Any scaffolding above an existing roof must be designed to avoid damage to the roof structure or coatings.

3.11 Fastener Loads 

Fastener design aims to avoid strength failure of the screw before failure of the sheeting or the structure. Most fastener failures happen due to negative load (or uplift) and testing procedures are designed to closely simulate these conditions.

Fasteners used to fix metal cladding can fail by pulling out of the structure or by shearing. The cladding can fail by pull-over or profile collapse.

Fastener design should be sufficient to avoid pull-out and prevent deformation of the cladding around the fastener heads that can cause leaks.

 

 

 

3.11.1 Load-spreading Washers 

Profiled load-spreading washers spread high wind uplift-loads over a larger area around the fastener head. Using load spreading washers under the fastener can increase the load resistance of each fastener by up to 50%.

The type, size and stiffness of washers are critical for performance. Where performance data incorporating load-spreading washers is used, the specification of the washer must be quoted with the fastener.

In general, load-spreading washers should have a minimum thickness of 0.95 mm for steel and 1.2 mm for non-ferrous metal.

 

 

Where oversized holes are used to accommodate thermal movement of the sheeting, load-spreading washers should be used with sealing washers to ensure weather tightness.

3.12 Fastener Performance 

Most fastener failures happen due to negative load, or uplift conditions, and testing procedures for fasteners are designed to closely simulate these conditions.

If every rib has a fastener, the only way to improve performance is to use load-spreading washers under the fastener heads, or place purlins closer together.

3.12.1 Screw Fasteners 

A fastener penetration of three threads through the steel member is sufficient for the fastener to meet its full design capacity.  Pull out failure is not normally a risk with high tensile steel purlins over 1.1 mm in thickness.

3.12.1.1 Steel Purlins Thinner Than 1 mm 

The pull-out values of screws into light gauge steel battens or purlins varies greatly with thread design, pitch and drill point shape. Pull out can be the mode of failure of light gauge steel battens depending on the cladding profile and the fastener design.  When fastening into battens less than 1 mm in thickness, the pull-out values of the specified screw must be considered and the installation must be completed with that type, gauge and brand of screw.

In light gauge steel, it is important to avoid stripping out the formed screw thread, therefore a depth setting screw gun is recommended.

3.12.1.1 Steel Purlins Thinner Than 1 mm 

The pull-out values of screws into light gauge steel battens or purlins varies greatly with thread design, pitch and drill point shape. Pull out can be the mode of failure of light gauge steel battens depending on the cladding profile and the fastener design. When fastening into battens less than 1.1 mm in thickness, the pull-out values of the specified screw must be considered and the installation must be completed with that type, gauge and brand of screw. In light gauge steel, it is important to avoid stripping out the formed screw thread, therefore a depth setting screw gun is recommended.

3.12.1.2 Steel Purlins Thicker Than 1 mm 

A fastener penetration of three threads through the steel member is sufficient for the fastener to meet its full design capacity.  Pull out failure is not normally a risk with high tensile steel purlins over 1 mm in thickness.

3.12.1.3 Timber Purlins 

Timber purlins generally require a fastener penetration of 30 mm. With this level of embedment, a screw equipped with a profiled washer though 0.55 material will fail by pull-through of the cladding before it fails by fastener withdrawal from the timber. Greater thicknesses of cladding may require specific design. For fastening into sarking or rigid air barrier less than 30 mm thick, the pull-out values for the specific screw and sarking material should be obtained from the supplier and required fastener pattern calculated.

3.12.2 Pull-over Values 

When metal cladding is subjected to uplift wind loads within the withdrawal capacity of the fastener, the failure mode will be pull-over or pull-through. The pull-over value is determined by the thickness and strength of the metal and the area over which the load is spread. See 3.12.2A Typical Pull over values for crest fixing (serviceability failure).

If the pull-over load is likely to be exceeded, the options are to increase the metal thickness or use a load-spreading washer. The pull-over value when using 0.70 mm aluminium is approximately the same as 0.40 mm steel.

Pull-over load depends on the head or washer size. For example, as 12# and 14# screw heads have approximately the same diameter, these screw sizes have the same design load value for pull over.

The pull over values for pan fixing cladding are higher than those obtained by crest fixing by a factor greater than 2, depending on the screw’s position in the pan. See 14.5.2 Pan Fixing.

3.12.2A Typical Pull over values for crest fixing (serviceability failure)

Thickness (mm)Screw Only (kN)Load Spreading Washer (kN)
0.400.40.7
0.550.50.9

3.13 Material Grade 

The most significant strength characteristic of metals used for profiled metal roofing is the tensile strength. 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.  For G550 material, the minimum yield strength requirement is the same as the tensile strength, but for more ductile grades the yield strength requirement is lower.

Tensile 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[MS1]  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. When manufacturing aluminium roofs,  aluminium trapezoidal and corrugate profiles normally manufactured from G550 steel should be manufactured from H36 aluminium rather than H34.

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.

3.13A 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.

 

 

 

 

3.14 Material Thickness 

Material thickness has a great bearing on strength. For residential buildings, 0.40 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, fastener patterns may have to be increased or purlin spacings decreased to accommodate wind loads (see 3.7 Wind Load) or thicker materials can be used.

Material with a 0.40 thickness is, however, 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 or chimneys that need servicing, or prestige housing 0.55 material should be selected.

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

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

G300 at 0.55 is the most common specification for spouting, flashings and ridging.

3.15 Profile Shape 

The geometry of the profile shape determines the strength performance of the profile. Variation of profile shape from that tested will produce different results under load in pierced fastened profiles and may produce vastly different results in clip fixed profiles.

 

 

3.15.1 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 performance of high strength steel corrugated cladding under point and wind loads is much higher than the more ductile grade (G300) still used for machine curving. G300 material of 0.55 mm has the same strength as 0.40 mm high-strength (G550) material; designs using G300 should take the lesser strength into account. G300 material should not be used in lieu of G550 unless there is good reason to do it.

Mixing the two grades of corrugate profile should be avoided when possible. If they are used on the same job, particularly when they are overlapping, the manufacturer should adjust the profile shape to provide an acceptable fit.

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

3.15.2 Trapezoidal Profile 

The trapezoidal shape provides greater water carrying capacity and provides greater spanning capabilities than corrugate (sinusoidal) profile. For nomenclature or description of the parts of the sheeting used in this COP, see 2.4 Product Geometry

The maximum available fastener density (fastener per square metre) on deep trapezoidal cladding profiles is usually lower than on corrugate, because of the wider rib spacing and longer spanning capability of stronger profiles. 

Trapezoidal profiles are available with different ribs heights (see 3.15A Profile Types).

 

3.15.3 Trough Section 

Trough sections generally have 2 to 3 pans or trays of 180-250 mm width, interspersed with steep ribs.  Older profiles may have a single pan. They are secret fixed to minimise fastener penetrations and allow for thermal expansion.

The wide deep pans on trough sections allow for greater water carrying capacity. Traditionally they were used for pitches as low as 1° but due to durability issues (caused by deflection and ponding) the recommended minimum pitch is 3° (see 7.1.1 Minimum Roof Cladding Pitch). Recent innovations in product design have blurred the lines between Trough profile and Standing Seam.

3.15.4 Standing Seam 

Standing seam roofs are similar to trough sections in that they have a wide pan and a vertical rib, and they are secret-fixed. They are usually wider, having a single tray of 300 mm to 500 mm wide, which gives a unique appearance.

Standing seam roofs are based on traditional manufacturing methods using folding and hand tools, rather than roll forming (see 15.4), but now they are also available roll-formed in most iterations. They are traditionally installed on sarking, but high tensile versions (that do not require continuous support) are available.

3.15.5 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. The accuracy of the framing will determine the quality of finish obtainable.

Mini-corrugate has been produced in New Zealand for many years to the imperial measurement of 1" pitch and 1/4" height, which converts to 25.4 mm x 6.3 mm in metric measurement.

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

3.16 Maximum Span and Fastener Requirements 

3.16.1 Purlin-Rafter Connections 

For sprung curved roofs, the purlin/rafter connection must be increased at the eaves.

Long lengths of pierce fixed roofing will impose added loads to the purlin connection due to thermal movement of the roof.

3.16.1A Purlin to rafter fastener requirements for SG8 Radiata pine, complying with NZS 3604:2011

 Use 70 x 45 mm radiata pine on flat minimum
 Use 90 x 45 mm radiata pine on flat minimum
Rafter SpacingPurlin SpacingLow Wind ZoneMedium Wind ZoneHigh Wind ZoneVery High Wind ZoneExtra High Wind Zone
0.90.6ABCCC
0.90.9BCCCD
0.91.2CCCDD
1.20.6BCCCC
1.20.9CCCDD
1.21.2CCDN/AN/A

 

 

3.16.1B Purlin to Rafter Fastener Fixing Capacity

TypeFastenerFixing Capacity (kN)
A1/90 x 3.15 gun nails0.55
B2/90 x 3.15 gun nails0.80
C1/10g x 80 mm self-drilling screw2.40
D2.10g x 80 mm self-drlling screws3.45
E1/14 g X 100 mm self-driling screws5.50

3.16.2 Sheet Overhang 

The maximum overhang for all corrugate and low trapezoidal profiles is 150  mm, unless a product has been specifically tested to withstand point load and design wind loads at a greater overhang.

The allowable overhang distance of various cladding profiles will depend on their section properties.

When using trapezoidal profiles, greater overhangs can be achieved by stiffening the edge of the sheet in various ways; the most common being using a square gutter with a horizontal flange, but this should be fastened on every pan to achieve continuity.

The limit placed on low trapezoidal profiles with a stiffened overhang is 300 mm but it is not suitable for corrugate.

The overhang distance can be increased for some trapezoidal profiles with a rib height greater than 28 mm, but this distance must be proved by testing.

Where the cladding is fixed at a ridge or apron, the overhang distance can be increased to 250 mm from the end of the sheet, as the cladding is not subjected to the same point load or UDL and the load is shared with the flashing.

Point of access and expected roof traffic loads must also be considered.

3.16.3 Maximum Spans for Corrugate Wall Cladding 

The maximum span for pan fastened wall cladding is generally governed by temporary deflection under load, rather than permanent deflection around the fastener.

3.16.3A Maximum Span for Wall Cladding: 5 Fasteners per Sheet (every second trough)

*Serviceability load governs 
The deflection criteria used in this table is span/120 + P/20, where P = the space between fasteners. Higher deflection limits may be acceptable in certain circumstances.
Wind ZoneLoad (kPa)*Maximum Spans
0.40 mm0.55 mm
Medium0.931.802.10
High1.321.501.80
Very High1.721.401.60
Extra High2.091.201.50

3.16.4 Fastener Patterns 

Fastening patterns for the most commonly used profiles are designated in the following manner.

 

 

 

 

These fastening patterns should be used in conjunction with load span graphs.

The load on a purlin and a purlin/rafter connection is determined by the wind load and the area of roof the load is acting upon. Roof fasteners transfer wind uplift-loads to the purlins, which in turn transfer them to the primary structure.

Fastening to every second purlin may be within the roof's load/span range, but will double the load acting on the fastened purlins. All purlins must be fastened to unless alternate purlins are specifically designed to take the additional loads

 

3.17 Steel Cladding Wind Load Span Graphs 

Wind Load span graphs should be read in conjunction with the constraint of access and the span at which the point load will be the limiting factor.

The performance of profiled metal cladding depends on the profile shape, thickness of the metal, the span, and the fastening type and pattern. These values can be greatly enhanced by using load spreading washers or thicker material.

All the tests from which these graphs have been derived used the 2:3 ratio of end to intermediate span and the graphs shown are for intermediate spans only. End spans must be reduced by two-thirds for these values to be assumed.

3.17.1 Corrugated Steel – 0.40 

 

3.17.1B

Recommended Point Load LimitSpan
Type AUnrestricted AccessN/A
Type BRestricted Access0.9 m

 

 

3.17.2 Corrugated Steel – 0.55 

 

3.17.2B

Recommended Point Load LimitSpan
Type AUnrestricted Access1.2 m
Type BRestricted Access1.5 m

 

3.17.3  Trapezoidal Five Rib Steel – 0.40  

 

3.17.3B

Recommended Point Load LimitSpan
Type AUnrestricted AccessN/A
Type BRestricted Access1.4 m

 

3.17.4 Trapezoidal Five Rib Steel – 0.55 

 

3.17.4B

Recommended Point Load LimitSpan
Type AUnrestricted Access1.5 m
Type BRestricted Access2.1 m

 

3.17.5 Trapezoidal Six Rib Steel – 0.40  

 

3.17.5B

Recommended Point Load LimitSpan
Type AUnrestricted AccessN/A
Type BRestricted Access1.5 m

 

3.17.6 Trapezoidal Six Rib Steel – 0.55  

 

3.17.6B

Recommended Point Load LimitSpan
Type AUnrestricted Access1.5 m
Type BRestricted Access2.2 m

 

3.18 G550 Steel Cladding Fastening Pattern Tables 

 

3.18A 0.40 mm Corrugate G550 Steel (Min Depth 16.5 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Restricted 0.6C2C2C2C2C4
Restricted 0.9C2C2C3C4C4
 Non-Trafficcable1.2C3C3C4C5N/A

 

3.18C 0.55 mm Corrugated G550 Steel (Min Depth 16.5 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.6C2C2C2C2C2
Unrestricted0.9C2C2C2C2C3
Unrestricted1.2C2C2C3C3C4
Restricted1.5C2C2C3C4C4

 

3.18E 0.40 5 Rib Trapezoidal G550 Steel (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Restricted0.65T25T25T25T25T2
Restricted0.95T25T25T25T35T3
Restricted1.25T25T25T35T45T4

 

3.18G 0.55 5 Rib Trapezoidal G550 Steel (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.65T25T25T25T25T2
Unrestricted0.95T25T25T25T25T3
Unrestricted1.25T25T25T25T35T3
Unrestricted1.55T25T25T35T35T4

 

3.18I 0.40 mm 6 Rib Trapezoidal G550 Steel (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.66T36T36T36T36T3
Unrestricted0.96T36T36T36T56T5
Unrestricted1.26T36T36T56T56T5

 

3.18K 0.55 mm 6 Rib Trapezoidal G550 Steel (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.66T36T36T36T36T3
Unrestricted0.96T36T36T36T36T3
Unrestricted1.26T36T36T36T36T3
Unrestricted1.56T36T36T36T56T5

 

3.19 Aluminium Cladding Wind Load Span Graphs 

3.19.1 Corrugated Aluminium – 0.70  

3.19.1A

Please Note - Requires use of applicable Load Spreading Washers in all cases.

3.19.1B

Recommended Point Load LimitSpan
Type AUnrestricted AccessN/A
Type BRestricted Access0.8 m

 

3.19.2 Corrugated Aluminium – 0.90 

3.19.2A

Please Note - Requires use of applicable Load Spreading Washers in all cases.

3.19.2B

Recommended Point Load LimitSpan
Type AUnrestricted Access0.8 m
Type BRestricted Access1.2 m

 

3.19.3 Trapezoidal Five Rib Aluminium – 0.70 

3.19.3A

Please Note - Requires use of applicable Load Spreading Washers in all cases.

3.19.3B

Recommended Point Load LimitSpan
Type AUnrestricted AccessN/A
Type BRestricted Access1.2 m

 

3.19.4 Trapezoidal Five Rib Aluminium – 0.90 

3.19.4A

Please Note - Requires use of applicable Load Spreading Washers in all cases.

3.19.4B

Recommended Point Load LimitSpan
Type AUnrestricted Access1.2 m
Type BRestricted Access1.7 m

 

3.20 Aluminium Fastening Pattern Tables 

 

3.20A 0.70 mm Corrugated Aluminium (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Non-Trafficable0.9C3C3C3C3C3
Non-Trafficable1.2C3C3C3C4C4

 

3.20C 0.90 Corrugated Aluminium (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.6C3C3C3C3C3
Restricted0.9C3C3C3C3C3
Restricted1.2C3C3C3C3C3

 

3.20E 0.70 mm Trapezoidal 5 Rib Aluminium (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Restricted0.65T25T25T25T25T2
Restricted0.95T25T25T25T45T4

 

3.20G 0.90 mm Trapezoidal 5 Rib Aluminium (Min Depth 27 mm)

AccessSpan (m)LowMediumHighVery HighExtra High
Unrestricted0.65T25T25T25T25T2
Unrestricted0.95T25T25T25T25T2
Unrestricted1.25T25T25T25T25T2
Restricted1.55T25T25T25T25T4