100, TERACOTTA Brown Corrugated Roofing Plastic Screw Cover CAPS - ONDULINE, COROLUX

These screw caps snap together firmly and do more or less what you'd expect.The plastic is quite stiff, and the surface of the bottom ring is essentially flat. As such these screw caps will work very well on box-section steel which has flat surfaces and is very rigid. They work a little less well on corrugated felt or other similarly soft and curved materials because as you drive the screw into the supporting timbers the roof material will tend to deform before the plastic cap does, and so the cap will not easily mould itself around the curves of the corrugation. Nonetheless, I've used them with corrugated felt sheets and the result is acceptably good even if not perfect.If like me you prefer to use hexagonal head screws for outdoor work (so much easier to extract after time than slot head or pozi-drive head screws) then you might like to know that the screw hole diameter is large enough for 5mm type coach screws such as these:  Bolt Base 5mm A2 Stainless Steel Coach Screws Hex Head Lag Bolts Wood Screw Bolt DIN 571 M5 X 60 - 20 . You can just about force a 6mm screw through the hole, but then the cap will not shut properly.

My husband has used these before so he knew that they would do the job ok.