These Forgefix Pozidriv, round head Corrugated Roofing Screws are Zinc plated for increased corrosion resistance. They come complete with a specially designed plastic fold over cap to suit installation of corrugated roofing. The plastic cap has a large surface area to improve weather resistance. Screw size: 5.0 x 50mm (Pozi). APPLICATIONSuitable for fixing corrugated sheeting to timber sub frames. Zinc coatings prevent oxidation of the protected metal to create a longer lasting finish. It does this by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not trigger a breakdown of the substrates surface integrity as it is formed. The zinc oxide, if left intact, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers. The Pozidriv screw is similar in appearance to the classic Phillips cross-head, but in fact is very different. The Pozidriv has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. The biggest advantage it offers is that, when used with the correct tooling in good condition, it does not cam out, allowing great torque to be applied. The Pozidriv screw can easily be distinguished by a line embossed in the screw head at 45 to the slots for the driver. Round heads have a flat under head surface making the screw ideal for laying flat on sheet material of all types, reducing deformation.