The correct depth setting on a screw gun is provided either by the depth gauge or by a clutch torque adjustment, and an adjustment should be made every time a different screw or material thickness is to be drilled. Resilient washers under fastener heads will only seal properly with the right adjustment.
Experienced operators can, in most instances, drive screws correctly by using a variable speed screw gun; however, a depth set gun will give more consistent results.
Type 17 screws driven into timber will 'part' the fibres rather than cutting them which provides a self-locking action against withdrawal. Screws driven completely through timber will, therefore, not have the same pull-out resistance as screws with embedded tips.
Impact drivers and poorly fitting nut drivers can both damage the protective coating on the screw head which will affect durability. It is the roofer’s responsibility to ensure the method of installing screws does not cause damage.
Screw points, method of driving, and thread design all have an impact on pull-out capacity; so in critical situations, the specific screw and method of installation must be specified.
Fasteners should be of grade and type suitable for the application, installed at spacings required by design loads and manufacturer’s recommendations.
On buildings constructed to NZS 3604, a consistent fixing pattern should be used on all fastener rows; for other buildings, higher fastener density may be required around the periphery. All purlins must be fastened to so that they each contribute to resisting uplift loads. See 3.9 Fastener Requirements and Overhangs.
Rivets on flashings should be placed at 50 mm centres.
Roof fasteners should be placed at the crest of the profile. Wall cladding fasteners can be placed at the crest or the pan. Pan fixing of wall cladding is more popular as the screw lines are less visible.
N.B. The pullover design values established by testing for pan fixing are more than twice those for crest fixing.
Fasteners should be seated snugly to give a good seal, without distorting the roofing profile. Any “pigtails” created by the drilling process must be removed before seating the screw.
All roofing and cladding are subject to expansion and contraction due to temperature extremes. This is particularly evident with darker colours and long spans where the expansion may be as much as 8 mm for a 10 m sheet. Screws fitted with profiled washers to allow for thermal expansion must be installed centrally through a 9 mm diameter pre-drilled hole in the roof sheeting.
