Fasteners

Fasteners are mechanical devices used to secure cladding components to structures or other components. The result of the proper use of a fastener is known as a fixing. There are two classifications of fixing: primary and secondary. An array of fasteners can be used for each, of which four or five may be used within any individual structure:

• Primary fixings secure the profiled sheeting or lining to the supporting structure, for example sheeting to purlin.
• Secondary fixings secure the laps of profile sheets to each other, but not to the supportingstructure, and are also used to attach lightweight flashings.

Fasteners used with pre-finished steel should be made from either carbon steel or austenitic stainless steel. All fasteners should have integral coloured heads and in more demanding environments, or where longevity is important, austenitic stainless steel fasteners should be used. There are many styles of fasteners used with prefinished steel roof and wall cladding systems. Some of the more specialised are:

• Stand-off or sheet support screw. This is used when afastener has to pass from the weather side of a construction through foam insulation, or if a fixing is made on the crown of a profile. The lower, smaller diameter thread acts as the primary fastener, whilst the larger diameter acts as a secondary fastener by providing spacing and supporting the external sheet and maintaining the seal beneath the washer head.
• Non self-tapping fasteners such as expanding rivets. The expansion of the rivet within the pre-drilled hole clinches the surfaces together to form the joint. Only the ‘closed’ type of this fastener should be used, since it has a sealed end to prevent the ingress of moisture through the central core. In exposed situations, rivets should incorporate a sealing washer under the head.
• Soft-bodied expanding rivets use an EPDM or neoprene body to clamp the fixing when the central screw is tightened. This type of fastener cushions the clamping force and thereby reduces the stress around holes in translucent sheets.