Screws

Wood screws are usually used for millwork and finishing rather than for structural framing. They are used in fastening millwork where resistance to withdrawl is a requirement.

Screws find some applications in structural framing as in the case of floor sheathing which is glued and screwed to the joists or the positive attachment of gypsum wallboard to support members. They are higher in cost than nails because of the machining required to make the thread and the head.

Screws are designed to be much better at resisting withdrawl than nails. However, when used for structural purposes, it is better that screws not be loaded in withdrawl as shown in Figure 5.1 but rather use the withdrawl resistance properly to produce and maintain close contact between the elements being joined.

The types of wood screws commonly used are shown in Figure 5.4 below.

Figure 5.4: Types of Screws

screwsFigure5.4

 

Download Figure 5.4 as a PDF

Figure 5.5: Wood screw Lead Hole Diameters and Depths

Diameter Depth
Countersink Same diameter as head Same depth as head
Shank Slightly smaller than shank diameter; 7/8 shank diameter for withdrawl loading. For softwoods, about 1/2 of screw length for shank and thread lead holes combined (may be same diameter); for hardwoods or soft screws, lead holes nearly as deep as screw.
Thread About 70% if loaded in withdrawl; about 90% of diameter for hardwoods. Overall depth to match screw length.

screwsFigure5.5