Plywood is used to construct homes and a host of other buildings. Most of the plywood is installed on the roof, flooring and exterior sheathing over the home's frame. Contractors have found that nails have a tendency to pull out over time. The use of screws has largely eliminated this problem. Determining the pullout strength of screws takes several things into consideration.
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One of the major factors considered when determining the pullout strength of screws is the thickness of the plywood. Thinner plywood has less pullout strength for screws than thicker plywood. Screw Products Inc. and Fabral have done a battery of tests on this factor and have reported their findings. A half-inch piece of plywood has a screw pullout strength of just over 113 pounds per square foot (psf) if the screws are placed 9 inches apart. However, a three-quarter-inch piece of plywood provides the screw with a pullout strength of about 197 psf.
Distance Between Screws
Another factor that must be considered when determining the pullout strength of screws in plywood is the distance between screws. As the distance increases, the pullout strength of the same screws decreases. For example, screws set 18 inches apart in a half-inch piece of plywood have only 63 percent of the pullout strength (71 psf) of screws set 9 inches apart (113 psf). Those screws will have even less strength when placed 24 inches apart.
The depth the screw goes into the plywood must also be considered when determining pullout strength. Most pullout strength tests provide a result for screws that have penetrated through the plywood. If you use a shorter screw and do not penetrate all the way through, the pullout strength of the screw will decrease. A screw that has penetrated only halfway through the plywood will have only half the pullout strength of a screw driven all the way through.
Type of Screw
Screw Products Inc. has taken pullout testing to another level by providing information about the pullout strength of different types of screws the company manufactures. The different types include galvanized, self-taping, steel or aluminum. Their material composition helps determine the pullout strength. Fabral and Screw Products Inc. also consider the type of wood being penetrated. Plywood is made with different types of hardwoods, softwoods or a combination of both. The wood can be Douglas fir, pine, oak, cedar, spruce or redwood, which help determine their pullout strength in plywood.