How do I make a mitre joint stronger?

A mitre at 45 degrees exposes a surface that is half end grain, and end grain drinks glue and holds badly. That is why mitred boxes and frames come apart at the corners years later even though nothing obvious went wrong. Every good fix does the same thing: it introduces a piece of wood whose grain runs across the joint, so the corner is held by long-grain glue surfaces instead of by end grain.

What you'll need

• A saw or router for the slot
• Offcuts of contrasting or matching timber for keys
• A cradle or jig to hold the corner at 45 degrees
• Clamps and wood glue

Step by step

1. Glue and assemble the mitre first if you are using keys.Corner keys are cut into the finished corner, so the box or frame must already be assembled and square. Use a band clamp or corner blocks; a mitre slides under clamping pressure.
2. Size the glue before assembly if the mitre is bare.Brush a thin coat of glue diluted with a little water onto both mitred faces, let it soak in and go tacky, then glue and assemble normally. It stops the end grain drinking the glue out of the joint.
3. For corner keys, make a cradle that holds the corner at 45 degrees.A V-shaped carrier running against a saw fence or a router table fence. The corner points down into the blade and every slot lands at the same depth.
4. Cut two or three slots across each corner.Space them so the outer ones are well in from the edges. Depth should reach comfortably past the joint line but not through the inside face.
5. Cut keys with the grain running across the joint.This is the whole point. A key with grain running along the slot is end grain again and adds almost nothing. Cut them from a board so the grain runs the short way.
6. Glue the keys in proud and trim flush when set.Flush cut saw, then a plane or a sanding block. In a contrasting timber they become a feature rather than a repair.
7. For a frame, use a spline slot cut before assembly.Run each mitred face over a saw or router to cut a groove, and glue a loose tongue into both. Hidden inside the joint, with the tongue grain running across the joint line.
8. For a picture frame that will never carry load, consider pinning.Fine pins or a corner clamp with a V-nail add mechanical hold and no strength worth speaking of, but on a light frame with glass they are usually enough.

Tips

• A mitre cut a fraction under 45 degrees — so the corners touch first at the outside — closes visibly tighter than one cut a fraction over.
• Contrasting keys in a light box, or matching keys in a dark one, are the difference between a repair that shows and one that looks intentional.
• Painter's tape stretched across the outside of a set of mitres, then folded up into a box, is the cheapest and one of the best mitre clamps there is.

Common mistakes

• Cutting keys with the grain running along the slot — The key is then end grain to end grain again, exactly the weakness you were trying to fix, and it snaps off at the first knock.
• Relying on glue alone on a mitred box that will be handled — Half of the mating surface is end grain and it will fail with time and humidity cycling, even with a perfect fit.

Questions people ask