Pullout strength are the axial force that is required to pull a screw straight out of wood. Pullout strength is an critical measurement because pullout strength determines if a screw will hold a joint together or if the screw will fail. Many peoples encounter issues when a screw fails to hold, and these issues often occur because the person didnt account for pullout strength.
The density of wood affects pullout strength because the density of wood determine how well the wood fibers grips the threads of a screw. Hardwood such as oak are dense, and dense hardwoods provide more pullout strength then softwoods such as cedar. A screw in oak will hold more weight than a screw in pine because oak is a denser wood and oak provide a tighter grip on the screw threads.
What Affects Screw Pullout Strength in Wood
Different species of wood provides different levels of pullout strength, and you must match the screw to the specific species of wood you are using. For example, Southern pine provide more pullout strength than Douglas fir, and spruce provides less pullout strength than Southern pine. There is a specific formula used to calculate the withdrawal load, which is another term for pullout strength.
The withdrawal load is equal to 1,800 multiplied by the specific gravity squared, multiplied by the shank diameter, multiplied by the penetration length. Specific gravity is a measurement of wood density, and a higher specific gravity will result in a much higher withdrawal load because specific gravity is squared in the formula. The shank diameter and the penetration length is linear multipliers, so increasing the shank diameter or increasing the penetration length will increase the withdrawal load in a predictable way.
Thread engagement is a factor that affects pullout strength. Coarse threads are useful in softwoods because coarse threads bite deeper into the wood fibers, and deeper thread engagement increase the pullout strength. Deeper penetration also increases pullout strength because deeper penetration provide more surface area for the threads to grip the wood.
If you increase the penetration of a screw from one inch to two and a half inches, the pullout strength will increase significant. The direction of the wood grain affects pullout strength. Face grain is the grain that is perpendicular to the axis of the screw, and face grain provides the maximum pullout strength because the threads cross the wood fibers direct.
End grain is the grain at the ends of the wood fibers, and end grain provides much less pullout strength because the screw slides along the fibers instead of gripping across the fibers. You should avoid using screws in end grain for load-bearing applications because end grain do not provide enough pullout strength. Pilot holes is necessary for maintaining pullout strength.
If you do not drill a pilot hole in hardwood, the screw will wedge the fibers apart and the wood will split, and a split board will have much less pullout strength. If you drill a pilot hole that is too large, the screw threads will not have enough wood to grip, and a large pilot hole will decrease the pullout strength. You should aim for a pilot hole that is approximately 75 percent of the shank diameter in softwoods, and you should use a slightly larger pilot hole in dense hardwoods.
Moisture content in wood affects pullout strength. Wood that is dry and has a moisture content below 19 percent provides the baseline pullout strength. Green lumber contain alot of moisture, and green lumber will swell around the screw initially, but the green lumber will shrink as it dries.
When green lumber shrinks, the grip on the screw loosens, and you must reduce your pullout strength calculations by one-third if you are using green lumber. Additionally, you can use coatings such as epoxy or ceramic to increase the friction between the screw and the wood, and increased friction can help maintain pullout strength. You can avoid common mistakes by following specific guidelines.
You should ensure that your screws have at least two inches of penetration, especially if you are performing structural work. You should use larger screw gauges, such as #10 or #12, for ledgers and hangers, but you may use #8 screws for sheathing. You should avoid relying on end grain for strength, and you should always account for knots in the wood because knots can reduce the real-world pullout strength by 15 to 25 percent.
Finally, you should always use the correct pilot hole size to ensure that the threads cut clean into the wood.
