Our black head galvnised screws are essential for screwing into heavy steel joists. Highly corrosion resistant. It is essential that you pre-drill a pilot hole (3mm) and countersink (the edging) for maximum strength. Predrilling also reduces the risk of splitting the substructure of the joists. Also ensure you have an adequate clutch / chuck setting to avoid over screwing and stripping the thread. Find out more on our Installation page and use our Deck Calculator Wizard to determine how many screws and other accessories you will need.