Written by Dr. Jody Muelaner
Lag screws and metal screws are both types of self-tapping screws. Lag screws are primarily designed to be screwed into wood while metal screws are intended to be screwed into sheet metal.
A lag screw, known in the UK as a coach screw, is a sturdy screw often with an externally driven square or hex drive head. It features coarse threads and a tapered point. It’s typically much more heavy-duty than conventional wood screws that are fitted with slotted or Pozidriv heads.
The name lag screw derives from their original use in securing barrel staves, also known as lags. A lag screw requires a hole drilled at the same diameter as the shaft of the screw, a gimlet point helps pull the screw into the hole and tap its thread.
A lag screw can also be used to fix to masonry using a lag shield anchor or nylon anchor. Typical sizes available are from 1/4″ to 3/4″ diameter, and 1″ to 16″ in length.
Steel and stainless steel are the most common materials for lag screws. Steel screws are often finished with hot-dip galvanizing or zinc plating. A construction screw may be used instead of a lag screw, it has a thinner shank, an internally driven head, and a fluted tip which negates the need for pre-drilling. Construction screws have replaced lag screws in many applications.
A metal screw has a fully threaded shank, is self-tapping, and versatile in use. A metal screw has fine threads, which are sharp and hardened to allow thread forming. They feature a tapping or thread cutting point for use in a pre-drilled hole, or a self-drilling point. A metal screw also has a countersunk or non-countersunk head with any of a wide variety of internal and external drive types.
The size of a metal screw is usually shown with a series of three numbers representing the diameter number, thread count per inch, and length in inches.
One type of metal screw, commonly used in roofing, is a wood-to-metal self-drilling screw, designed for affixing soft materials, such as wood or cement board, to metal. Featuring a self-drilling point with wings further up the shaft, after the initial hole has been made, the wings follow cutting a clearance hole in the soft material but being destroyed when entering the harder metal. This screw provides a clearance drilling, tap drilling, thread tapping, and fixing solution all in one operation.