Used mainly for fastening timber to timber and is extensively used for timber joist construction in house building and numerous decking applications.
A Carriage Bolt, also known as a Coach Bolt or as a Fully Threaded Cup Square Hex Bolt, has a domed or countersunk head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck Carriage Bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
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 substrate 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 aluminium and stainless steels by their oxide layers.