The National Design Specification for Wood Construction contains design provisions and design values for wood screws that conform to ANSI/ASME Standard B18.6.1. When wood screws are loaded in withdrawal, the design value for tension in the screws should not be exceeded. For single-shear wood-to-wood construction, the screws should be inserted in the side grain of the main member with axis perpendicular to the wood fibers.
Edge and end distances and spacing of wood screws should be sufficient to prevent splitting of the wood. If building-code requirements for such distances are not available, Table 10.38 may be used to establish wood-screw patterns. Spacing, or pitch, between fasteners in a row is affected by species, moisture content, and grain orientation.
Screws may be inserted in lumber or timber that has a specific gravity less than 0.50, without preboring a hole for them. In denser wood, lead holes should be drilled and screws inserted by turning, not by hammering. Holes for wood screws loaded in withdrawal should have a diameter of 90% of the screw root diameter in such wood as oak, with a specific gravity of 0.6 or more, and of 70% of the root diameter in such wood as Douglas Fir and Larch, with a specific gravity between 0.5 and 0.6. For wood screws subjected to lateral loads, holes receiving the shanks in wood with a specific gravity of 0.6 or more should have the same diameter as the shank. Holes for the threaded portion should have a diameter about equal to the root diameter of the screws. For screws in less dense wood, the part of the hole for the shank should be about seven-eighths the diameter of the shank. Holes for the threaded portion should have a diameter about seven-eighths the root diameter.
Soap or other lubricant may be used to facilitate insertion and to prevent damage to the screw.
Screws are designated by gage (diameter) of shank and overall length. For design purposes, it is adequate to assume two-thirds of the screw length is threaded.
