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.
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 (+).
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.
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.
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:
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.
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.
NZS 3604 divides New Zealand 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.
| Maximum Altitude as per NZS 3604 | ||||
|---|---|---|---|---|
| Zone | Up to: | 1.0 kPa | 1.5 kPa | 2.0kPa |
| 0 | Not Required | |||
| 1 | 400 | 600 | 850 | |
| 2 | 400 | 600 | 850 | |
| 3 | 400 | 600 | 850 | |
| 4 | 100 | 200 | 350 | |
| 5 | 200 | 300 | 400 | |
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.
