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 (+).
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.
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.
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.
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.