COP v3.0:structure; types-load

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.

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. Alternatively, loads may be applied over two or more ribs.

Testing loads may be applied to the pan or the rib depending of the profile shape and the design criteria. See 3.7.4 Roof Traffic.

Trafficable roofs 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 an imposed 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 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 (+). Wind Load 

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



Under wind loads, inwards loads are resisted by the purlin or girts, whilst outward loads are only resisted by the fastener head or clip. Therefore, outwards load is the more critical figure.

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.

Load/span data for standard corrugate, and low rib 5 and 6 ribbed trapezoidals is at the end of this section.

Refer to manufacturer's load/span tables for all other profiles, which should give the maximum recommended load for continuous spans when tested as described in 17.1.3 Testing Procedure. Sheltered by Trees, "Urban Terrain"

Scattered obstructions of a similar height or lower within 500 m from a building will considerably lessen wind speeds and lower design wind pressures. Surrounded by flat ground, "Open Terrain"

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

The local pressure factor (Kl) is an important design consideration required by the Loadings Standard. 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 to NZS 1170.2, the engineer should prepare a roof map showing purlin spans and local pressure factors for each section of the cladding.

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.
 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. Snow Loads 

Roof cladding design does not usually have to be altered for snow load, maximum snow load in New Zealand (under NZS 3604) is a UDL of 2 kPa.This 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. It may, however, be neccessary to increase the strength of the structure to allow for induced snow loads.

New Zealand is divided (in NZS 3604) into six zones where the maximum snow load is 2 kPa; areas above specific altitudes in these areas require specific design.

Projections such as gutters, flashings and chimneys need additional fixings and detailing to resist loads from sliding snow. It is normal to fit snow straps to residential gutters in snow prone areas. Altitudes for Snow Loading Design

 Maximum Altitude as per NZS 3604
ZoneUp to:1.0 kPa1.5 kPa2.0kPa
0 Not Required
1 400600850
2 400600850
3 400600850
4 100200350
5 200300400 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 and, typically, works in the opposite direction to wind loads.