Self-drilling screws with a piercing point and a thread suitable for drilling into timber with a hexagon head are designated as Type 17. A screw gun can provide sufficient torque for a type 17 screw to pierce metal roof cladding profiles and thread themselves into soft wood, but hardwoods may require a pilot hole.
|Threads Per Inch|
|Shank||Thread TPI||Thread Type|
|8||3||4.2|| ||18||32||1/4" HEX|
Coarser threads (less than 17 TPI) are used to fix roof and wall cladding. However, a finer thread, known as metal thread, is used when fixing to steel thicker than 2.4 mm. Different thread configurations are available for special applications and some shanks are provided with a gripping thread at the top of the shank.
All fasteners must be easily identified by a code stamped on the head to identify the manufacturer and the coating class. The size, type, length, head type and the standard to which the fastener is manufactured must be identified on the packaging and in the manufacturer's literature.
When counter battens are used, and the primary cladding fastener does not always penetrate the structure, the fastener used to secure the counter batten to the structure also becomes a primary fixing.
When solid sarking, insulation or decking is interposed between the cladding and the structure, the design uplift force on the decking should be resisted by the total number of fasteners per square metre, multiplied by their individual capacity. (See 3.12 Fastener Performance)
Screws that are to be used as a structural fastener, such as in a stressed skin roof design or structural purlin/rafter connection, should be made using a screw gun with an adjustable torque setting to ensure consistency of fastening.
Stainless screws should be driven with a new driving socket so that steel smear from a used or worn socket head does not contaminate the stainless steel screw head. Alternatively, a stainless socket should be used.
While the common roofing screw is a Class 4 coated steel screw, stainless steel and aluminium are alternatives. (Also see 8.7.3 Secret Fixing Clips)
The 14 gauge aluminium screws available are suitable for rib fastening of profile metal cladding into timber purlins and 12 gauge are suitable for pan fixing. They require pre-drilling into timber and are not suitable for fixing into steel. Care should be taken not to over-drive aluminium screws as their shear strength is less than steel.
Stainless screws are available in Grades 304 and 316, but only 304 should be used for fixing aluminium cladding. Grade 304 is suitable for very severe environments, provided the screw shank is isolated by an oversized hole and a load spreading washer made from 445M2 stainless steel is used with an EPDM sealing washer.