When selecting self-drilling screws for metal roofing, consider the drill point size, thread type, coating and corrosion resistance. #2 point drills up to 0.9mm steel; #3 for 1.2mm; #4 for 1.5-2mm; #5 for 2-4mm. Always match the coating to the environment: zinc for interior, CR-10 or bi-metal for coastal areas.