In carpentry—like in most things in life—the details matter. In the quick tips series, I'm walking through some of the things I've learned in my decades of experience as a master craftsman so you can be more efficient on the job site—and produce better results. Stay tuned for more.
Use a Shim for a Back-Bevel Miter Cut
This quick working tip is a way of creating a slight back-bevel for miter cuts—or any cut for that matter—so your joint will fit tighter because the face of the material meets first (instead of the backside of the material touching first).
To accomplish this quickly, take a scrap of wood and place it on the miter saw to lift the stock slightly. Once the cut has been made, there will be a slight bevel—the thicker the shim used, the more back-bevel there will be. This can save a lot of time from slightly setting the bevel on your miter saw every time or forgetting it was slightly set, making a bunch of cuts for something else, and only then realizing they’re all off.
This is particularly helpful in an application where the extension jamb of a window stands proud of the drywall surface for some reason. Back-beveling allows the trim to be slightly folded back against the wall to help tighten any gap between the drywall and the back side of the trim. The thing to remember, though, is how important glue is for modern joinery—too much back-beveling may lessen the effective glue hold by creating an undesirable void behind. What you’re looking for is a tight wood-to-wood joint for maximum glue hold.
Fitting and securing a joint (like on window trim) becomes a critical process to get that wood-to-wood joint tight through the depth of the cut. That’s why using a clamp can work so well—the pressure from the clamp will have a natural tendency to pull the joint together. You can also see how the glue comes out of the joint as a visual measure of how tight the joint is fitting. If you do feel that there will be a slight void behind any joint, then use Gorilla Glue—it expands and fills any space. Either way, if the back bevel is just right, the trim will fold slightly back toward the wall, helping to lessen any gap between the backside of the trim and the drywall.
Use a Shim for Stronger Miter Joints
As part of this process, I like to use shims on the backside of the joint for another layer of holding power if needed. This is how it works: First, the trim is nailed into the window jamb or liner, then, after the glue dries, the trim is nailed into the wall. I always feel that nails can pull the trim into the wall and put pressure on the miter joint by pulling it back further, possibly causing it to open up. Sometimes, I like to place shims at the miter joint to keep it secure while the trim is being nailed into the wall. This way, there isn’t any undue pressure on the joint. In this case, a shim is simply installed with a little glue and cut off with a utility knife. With painted trim (like what we’re doing in this project), the drywall-to-shim always gets caulked so the shim and any crack totally disappear. Don't miss the video above for a quick visual demonstration of how to use shims behind a miter joint.
In any case, this quick working tip is really a throwback to the days when we didn’t have miter saws that beveled. However, I think understanding and using this technique is still relevant today. We’re talking about minutia here, but in carpentry, understanding and improving the small things adds up quickly over the life of your career—and any given project.