In wood-to-wood connections, nails and screws are some of the most common types of fasteners that hold a structure together. Such options work reliably in wood, as it’s a relatively soft material compared to concrete, steel, iron, and brick. Those heavier structural materials are more commonly associated with fasteners like bolts and anchors. There is, however, some overlap, which is shown in bolts that are designed to work with wood.
Fasteners like timber bolts, plough bolts, and carriage bolts have special features that make them uniquely suited to installation in wood. The following information will explain their uses and features and clarify what makes each bolt distinct for specific structural needs.
Carriage bolts, which are also called coach bolts, can be used in some metal-to-metal connections, but they are mainly used to fasten metal parts to wood. They derive their names from their use on the frames of carriages. Today, they are applied to many wood-based structures that have metal hardware.
Carriage bolts have a dome or mushroom-shaped head. In addition to creating a smooth, unobtrusive profile, the head shape helps to broaden the bearing surface and eliminates the need for a washer. It also prevents the bolt from being manipulated from the exterior.
One of the most notable features of the carriage bolt is its square neck, which is positioned directly below the head and before the unthreaded portion of the shaft—also called the shank. The square neck fits with square holes and notches that are cut into ironwork, which prevents the bolt from turning after it’s tightened in place. The end of the bolt is threaded to accommodate the nut, while the shank fits within the wood.
Since carriage bolts are paired with metal and used in exterior applications, some degree of corrosion resistance is recommended. Galvanized carriage bolts, or HDG carriage bolts, are effective for preventing rust and other issues resulting from oxidation. Zinc carriage bolts are also corrosion-resistant.
Timber bolts are sometimes confused with carriage bolts but they have different features and are used for wood-to-wood connections instead of metal-to-wood connections. They also have a broad, mushroom-like head shape, but instead of a square neck, the underside has four metal ridges or teeth. The ridges act as a type of cleat that bites into the surface of the wood, preventing the bolt from turning after it is tightened from the nut side.
Timber bolts will hold together larger planks and timber structures. Like a carriage bolt, the threaded end accommodates a nut and the smoother shank sits within the wood.
Timber bolts are used in the building of docks, larger decks, and boardwalks. They are also used in smaller timber-frame structures, like wooden benches and footbridges.
Plough bolts are essentially a variation of carriage bolts. They are used to attach wood to metal but they are designed for a countersunk installation. They are used mainly for tools and equipment, like shovels. They derive their name from their use on mouldboards used on iron ploughs. They are installed from the nut side and will sit within the surface of the wood.
Lag bolts, also called coach screws or lag screws, are a type of wood screw, although they are commonly characterized as bolts by many fastener distributors. They are used to attach wood to wood or metal to wood.
They are hex-headed and have a threaded end that comes to a sharp point. They are not used with nuts but are instead held in place through tension. The sharp end means they will cut into the material as they are tightened, but a pilot hole should always be drilled to ensure an efficient installation and to reduce the likelihood of the head breaking as the fastener is torqued.