When Using Glue-ups in Joinery is Best
For anyone starting out in woodworking, some skills take time to master. Others come more quickly. Joinery ranges from utilitarian to decorative, but it must always be dependable. Joint failure is annoying at best and dangerous at worst. A carpenter’s skill level, available tools, and application determine the joint type. Hundreds of possible configurations exist, however. Along with the joint type, a woodworker must also choose the fastener. That can be a screw, nail, biscuit, staple, or he can choose a glue adhesive. Here are some thoughts from the Pros about when using glue-ups in joinery is best.
All Aboard The Grain Train
Wood adhesives are stronger than wood itself and stronger than many fasteners when applied correctly. But that doesn’t mean that glue is the best choice for a joint. The wood grain orientation is an important component of joint strength. Wood cells are structured like a bundle of straws. Imagine these straws drawing water from the roots to the uppermost part of the tree. When a tree is hewn into lumber, the orientation of the straws creates three types of grain: end, edge, and face grain. The end grain is like the open ends of the straw bundle, the edge the thinner side, and the face the broader side. When an end grain is glued, the straws draw the glue away from the surface as they did with water in the wood’s tree days. This results in a weak adhesion. End grain glue-ups have very little lateral strength. A good example is the butt joint where and end grain is joined to face grain.
When Using Glue-ups in Joinery is Best Continued
Glue-ups are strongest with face to face, face to edge, and edge to edge joints. A lot of surface area is in contact in these orientations and the glue isn’t drawn away by the straw-like wood characteristics. This assumes that the contacting surfaces are flat and square, of course. Some argue that cupping should be in the same direction while others argue it should counteract. But it’s best to profile all boards so that they are as close to flat as possible and oriented from bottom to top as if they were still in tree form.
Don’t fasten flat or miter end grain with glue only. It results in a weak bond that is likely to fail under load. But using glue alone for the long grain wood surfaces is certainly acceptable. You’ll create a strong, lasting joint.
Since carpentry has a rich history that spans millennia, we have only scratched the surface. Perhaps we’ll go deeper in a future joinery article, but this is a good start. You’ll certainly learn through trial and error when using glue-ups in joinery is best. Our hope, however, is to save the new guys some pain—and money. If you’re a Pro and you have glue-up joinery tips, add them in the comments below—or contact us with your own Pro tips.