RESIDENTIAL // COMMERCIAL // INDUSTRIAL
REGULARS : QUALITY MATTERS
Steel : one size doesn ’ t fit all
Choosing the right roof sheeting profile for the right location and performance is more than choosing a colour , material or copying and pasting from a previous specification .
By Richard Polling , group director , MRC Group | Photos by Richard Polling
One of the major aspects that often is overlooked is wind loadings and how this affects the roof design . There are many different roof profiles available in the market place from many manufacturers , all promoting the strengths of their individual profiles and yet one profile cannot be ‘ one size fits all ’.
In terms of what to ask about wind loadings and how this affects the design ? ‘ Acts of God ’ are very often the point of reference when a roof system detaches itself from a structure and yet how often is this really the case ? Could it be that the roof system was not correctly specified , installed or maintained resulting in detachment from the structure ?
On most projects the design is done by the engineer on the structural steel and the roofing specification is left to the architect to decide . In essence the engineer should select the roofing profile as he is the professional who calculates the terrain category and the wind pressures on the structure .
Choosing material
So , what should designers look at when designing the building to ensure the roof will stay on the building ?
1 . The high-pressure zones on buildings in the built environment are generally the eaves , gables and ridge of the building as a rule of thumb . A designer will close up the purlin spacings in these areas to ensure the roof sheets can withstand the wind pressures . In the mid span areas , the pressures could only be one third as opposed to the ridge and eaves all depending on the shape of the roof .
2 . Careful consideration should be taken on selecting roof sheeting , such as total coated thickness , hot dipped coatings , base metal thickness , mechanical properties and overall performance of
There is a process to selecting a roof sheet profile .
the system in terms of ultimate and 5 . If a building has exposed eaves serviceability limits of the profile . overhangs again the engineer should
3 . It is because of this information that a determine the superimposed wind roof sheet should be selected on the loads that the eave is exposed to and performance criteria and not because select the correct roofing profile . of a brand name . The engineer will take 6 . An important point for the professional the serviceability and ultimate loads of team to consider is the suppliers the roof sheet into consideration to willingness to issue performance select the profile to be used . guarantees on the roofing system . In
4 . In retail design as a rule of thumb the requesting this information , the point loading on the roofing profile designers can be sure that the roofing becomes more critical as the roof profile that is selected is the correct sheeting is usually protected by parapet profile for the project . walls and it is expected that following 7 . Last but not least the applications of trades use the roof platform as a the solar installations on buildings play trafficable area when , actually , most of a major role on wind effects on the these structures are designed as nontrafficable areas . Again , the engineer is whole building design will change in
structure as well as point loading . The key for deciding if the areas require terms of loadings and must be walkways for maintenance teams to considered at the design stage even access areas . Point loading limits are though this application may be done at tested and published by the roofing a later stage . manufacturer for the engineer to decide on the serviceability .
See more on page 28
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JULY 2018