Wood screws and lag bolts have a lot in common. In fact, a lag bolt is often, and more accurately, called a lag screw, and it behaves much more like a wood screw than any type of standard bolt.
Both wood screws and lag bolts are specifically designed to fasten together two pieces of wood. They have pointed tips and steeply angled threads made to penetrate and grip wood fibres. They also have a smooth section of shaft near the head so that the threads grip only the rear piece of wood, creating a tighter connection.
Wood screws come in a variety of types, each suited to particular applications or types of wood. In general, screws with finer threads are used for hardwoods (such as oak or cherry), while those with coarser threads are for softwoods (such as pine or redwood). Lag bolts are commonly available with coarse threads only and are used most often with softwoods in the form of framing lumber and timbers.
The biggest difference between wood screws and lag bolts are the heads. Wood screws come in a range of head types, including flat, oval and round. Lag bolts typically have a hexagonal bolt head, or "hex head," designed to be driven with a socket wrench.
Pilot Holes and Washers
Wood screws and lag bolts almost always require pilot holes to facilitate driving the screws and to prevent splitting the wood. Wood screws can be countersunk into the wood and can be used with or without a washer. Lag bolts should almost always be used with washers; without them, the bolt heads dig into the wood, marring the appearance and weakening the connection.
Materials and Sizes
Wood screws are commonly manufactured in brass, stainless steel, zinc-plated steel, and silicon bronze, in sizes from 1/4 inch to 6 inches. Lag bolts come in hot-dipped galvanised steel, zinc-plated steel, and stainless steel, in sizes from 1 inch to 18 inches. Stainless steel and silicon bronze offer the best corrosion resistance.