S.Drilling Screw - Nut-Head (1.5-inch)

Cavalry self-drilling screws, an inch and a half long, with a hex nut head. It fills the gap between the one-inch, which fixes sheet straight to purlin, and the two-inch, which is for sheet over insulation: an inch and a half suits sheet with a thin spacer or underlay behind it, deeper-profile roofing where the fixing lands part-way up a rib, thicker timber purlins, and cladding with a shallow cavity. Driven with a nut setter so the driver cannot cam out under torque, and the point drills its own hole and cuts its own thread with nothing pre-drilled. Stop the moment the head seats - running on crushes the washer and dimples the sheet around the fixing, which is exactly where a roof starts to leak.