Written by Jody Muelaner
A typical square headed lag screw (left) and a countersunk head metal screw (right).
Lag screws and metal screws are two types of self-tapping fasteners. A lag, known in the UK as a coach screw, is primarily designed to be installed into wood. And, as the name suggests, metal screws are intended to be screwed into sheet metal.
Lag screws are sturdy and often have an externally driven square or hex drive head, and features coarse threads and a tapered point. Lag screws are much more heavy-duty than other wood screws that are fitted with slotted or Pozidriv heads. Its name derives from its original use in securing barrel staves, also known as lags.
This type of fastener requires a hole drilled at the same diameter as the shaft of the screw. A gimlet point helps pull the screw into the hole to tap its own thread. It can also be used to fix masonry using either a lag shield or nylon anchor.
The typical sizes available are between 1/4 to 3/4 inches in diameter, and between 1 to 16 inches in length. The most common materials used are steel and stainless steel. Steel screws are often finished with hot-dip galvanizing or zinc plating.
In several cases, a construction screw can 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. In fact, construction screws have replaced lag screws in many applications.
Metal screws are versatile and have a fully threaded shank. These fasteners typically have fine threads, which are sharp and hardened to allow thread-forming. They also feature a tapping or thread cutting point, which is used in a pre-drilled hole or a self-drilling point.
A metal screw has a countersunk or non-countersunk head with the option of a wide variety of internal and external drive types, depending on the application. Its size is usually marked with a series of three numbers representing the diameter number, thread count per inch, and length in inches.
One type of metal screw that’s commonly used in roofing is a wood-to-metal self-drilling screw. It’s designed to affix soft materials, such as wood or cement board, to metal.
This type of fastener features a self-drilling point with wings further up the shaft. These wings follow the cutting clearance of a hole in the soft material but are destroyed when installed in harder metal. This screw provides a clearance drilling, tap drilling, thread tapping, and fixing solution in one operation.
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