Roofing Screws. Steel to Steel (For Light Section Purlins). Washered Self-Driller. Steel Sheet to Steel Purlin. Head: Hexagon Self-Drilling. Finish: 'Elementech' coating for superior anti-corrosion - salt spray tested to 1000 hours. The 29mm rubber washer beneath the aluminium flange expands under pressure and forms a permanent weather-seal. Designed for fixing cladding and sheet metal to light and heavy structural purlins as used when constructing industrial units.