Master craftspeople make screw selection seem like it requires zero thought. Experienced artisans can quickly pluck the right fastener out of their stash just by eyeballing the size. Even when you've been woodworking for years, some projects can leave you baffled: how do you select the correct screw length for your project?
What Happens When You Use the Wrong Size Screw?
Using a screw that is too short means your project might eventually fall apart, especially when under heavy use or excessive pressure and force. If you're building a project to last through the ages or one subjected to heavy wear and tear, using a screw that's too small spells disaster.
Conversely, a screw that is too long can split the receiving stock. Using a too-long screw for your woodworking project can drive the tip of the screw through the receiving board, exposing the sharp screw tip.
Choosing the Right Screw Length for Your Project
There’s no real rule for selecting a screw length for your project. You'll want a screw that can be driven completely through one board and at least 1 to 1.5 inches into the receiving board – provided the depth doesn’t cause the screw to exit the receiving board.
Expert Bob Vila provides further guidance, suggesting that a woodworking screw should have enough length to drive into at least half the thickness of the receiving material. For example, a screw driven into a 2-by-4 should enter at least 3/4-inch into the receiving board. If you were screwing two 2-by-4s together, you'd need a screw at least 2.25- 2.5 inches long.
Taking Grain into Account
The thickness of your wood isn't the only factor to consider. It’s also important to account for how the grain of the wood lays. If you're fastening two boards straight along the grain or across the grain, you won't need as much screw length as when you're adding screws into the end grain.
When working with wood and inserting screws across the grain, you're met with less resistance and have less of a reason to suspect deterioration around the screw threads over time. When screwing into the end grain, you're working against the material and basically forcing the wood to comply to your wishes. Over time, with movement and use, the anchoring of the screw is less likely to maintain its integrity, so it’s advisable to add an additional half-inch in screw length when anchoring screws into the end grain of wood.
Finding Help Selecting Screws
Use some of the scrap wood from your project to test out various screw lengths and thicknesses before starting the job or committing to a screw length. You can get a feel for how the screw length sits by using your scraps as a test swatch.
Check out the Connections Calculator
provided by the American Wood Council's website. The calculator allows you to input the design method, connection type, fastener type and loading scenario to get a good idea of what type and length of screw to use.
Woodworking experts can eyeball a project and guess which length of screw to use because of years of trial and error. Gain this knowledge yourself by trying different screw types on your scraps. While there are some basic guidelines to help you select screw length, there are no hard and fast rules.