The 1.25" Self-Drilling screw for metal roofing and siding is designed for metal-to-metal applications. Use this screw if you are installing metal panels onto metal purlins.  Common uses would include installing metal panels over:
 
Hat Purlins
C-Purlins
Z-Purlins
Red Iron Framing
Galvanized Framing
Other metal framing
 
Flat-Top fasteners combine under-cut hex washer heads with EPDM cut-tubular sealing washers. They are a superior and cost-effective alternative to standard fasteners with metal back washers.
One-piece design eliminates damage to painted washer surface and eliminates dissimilar metal washer materials.
 
Flat on top stabilizes socket during installation
Higher hex provides positive socket engagement
Special undercut entraps sealing washer, even when driven at extreme angles
Assembled with an EPDM Cut Tubular Washer
 
Designed for use where self-tapping / self-drilling applications are needed.