Coach screws are heavy duty screws designed for metal to timber connections, or to join heavy timbers. Also known as lag bolts or lag screws, they have a square or hexagonal head engineered to be used with a wrench, spanner, or pliers. These screws come in a wide range of sizes, from the very small ones packed with furniture designed to be assembled by the consumer to extremely big ones used in large scale construction projects.
Coach screws consist of an externally threaded cylindrical shaft that tapers to a point, with a head at the other end. When they are driven into timber, the threads bite into the wood, while the head stops the screw from going all the way through as it provides compression. The essential form of the screw has been in existence for over 2,000 years, although metal screws for construction and woodworking have only been in use since it became possible to mass-manufacture them in the late 19th century. It was not until the Second World War that screw sizes became standardized, so that they could be manufactured and shared between Allied forces with ease.
These types of screws are often used at critical joins in construction because of their sturdy manufacture. Screws are often superior to nails for construction projects in general, because they can be easily removed and reused. Coach screws are preferred for joints that see heavy use and other areas in which additional support through the use of metal flanges is needed.
When working with coach screws, it is important to make sure to use the correct wrench or spanner so that the head of the screw is not damaged during installation. When checking the size of a wrench or spanner, the user should measure along both flat ends, not from point to point. When using these screws on wood to wood connections, or in other applications where the screw will pass through wood first, it is advisable to use a washer with the screw so that the head won't crush the wood and reduce the load handing capacity.