Saflok 700 Featured
Today's trend for sustainable designs that reduce environmental impact is helping drive the metal roofing market to new heights. Understanding the engineering concerns fundamental to the proper design of metal roofing will enable specifiers to serve the long-term interests of their clients, and reduce liability for all concerned.
In large-span metal roofing applications, concealed fix roof systems are the logical choice – they are fixed to the purlin with a clip system, and are not pierced with any fasteners. The clips serve two purposes – they circumvent the need for holes in the sheet (which prevents the possibility of leakage through the perforation) and they allow the roof to expand and contract in a sliding movement over the clips.
Because the sheets can slide over the clips, they can be rolled in long lengths which can thermally expand without restriction. If the sheets were positively fastened through the sheet into the purlin, the fasteners would take the brunt of the force of expansion, and would tear larger holes in the sheet, or shear off. Consider that coated steel expands at a nominal 1mm per 1m, and it is apparent that a sheet of a mere 15 m will move by 15mm in heat. It is common best practice to not install a pierced fix sheet in lengths of more than 15m for this reason. Over 15m, a pierced fix sheet should be end lapped to allow for thermal cycling.
A concealed fix system allows sheets to be rolled in continuous lengths up to 120m or more. The lack of end laps further helps reduce the risk of leaks.
In these applications, with the size of investment concerned, engineering performance of the roof “system” is critical.
The first distinguishing characteristic of a concealed fix roof is the profile, which is both aesthetic and functional. For low sloped roofs, one is looking for a defined rib, with a wide deep pan to deliver excellent water run off at slopes as low as 2 or 3 degrees. The height of the rib is important as it also provides structural strength, acting rather like a mini I-beam.
Of equal concern is the design of the clip, as this is the “anchor” securing the sheet to the sub structure and holding it down in high winds. The material and thickness of the clip helps determine its inherent strength, which in turn helps to determine the minimum wind-uplift resistance requirements for a particular roof.
The overall “system” design defines the manner in which the clip locks into the sheet profile. The clip should engage every rib of the profile to prevent wind cushioning under the unsecured rib, and distorting the sheet off the clip. Generally, the more ribs that are secured with a positive clasp holding it to the purlin, the stronger the wind uplift resistance. If not positively secured, the ribs should be shaped to provide a strong and stable interlock with the clip at every point.
The system must be able to withstand the foot traffic and loads imposed by the installation of HVAC units, rooftop photovoltaic (PV) systems, or other utilities.
As a profession, you should demand full technical details of the system you intend to specify – and request technical support throughout the installation itself.