Coach screw a large diameter screw with a coarse thread, providing a strong hold in wood. For fixing metal to timber or timber to timber where a nut fixing is not possible. e.g. Pergolas, carports, fencing, etc. Zinc plated finish. Various sizes available to sort your needs. when choosing the right screw length take into consideration the type of timber being used (softwood or hardwood) and what you are wanting to hold. Ideally the screw should be at least twice as long when fixing into a hardwood base than the top timber being held, and then from 2 to 3 times longer when fixing into softwood base, although this is not always possible.