Self-Tapping Sheet Metal Screw, Wood Screw or Lag Screw? : Nuts and Bolts of Fasteners

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Self-Tapping Sheet Metal Screw, Wood Screw or Lag Screw?

by MaryLouise on 01/30/12

Self-tapping Sheet Metal ScrewSelf-Tapping Sheet Metal Screw, Wood Screw or Lag Screw?Even pros are confused at times on which fastener they are using on a job. When you are putting screws in to wood, then they have to be wood screws right? Well, possibly.......One good clue is to check underneath the head of the screws

If the screw is fully threaded with a sharp point, you most likely have a sheet metal screw. 

Wood ScrewIf the screw only is about 2/3 threaded with a smooth shank underneath the head, you are looking at a wood screw. Typically in the past, wood screws were not case-hardened like sheet metal screws and, unlike sheet metal screws, were plain steel and not plated.

But these days, there are many more options available.  Some companies carry case-hardened wood screws and many are now zinc plated which gives them more protection from corrosion.

The purpose of the smooth shank underneath the head was so that the two pieces of wood being fastened would draw together once the threads past through the first piece of wood. 

Lag ScrweThen there is the lag screw which is also used in wood.  Frequently a customer will ask for a lag screw when, in fact, he is wanting a #14x3 self-tapping  Hex Washer Head Sheet Metal Type A  screw.  A quick way to figure out which one you want is to ask: 1) does it have a smooth shank underneath the head and 2) does it have an attached washer on the hex head.  As you can tell from the pictures above, lag screws and sheet metal screws look quite different.

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