Chair repair: How to fix a broken tenon
David Johnson repairs a broken tenon on a Mid-Century classic; a Hans Wegner CH25.
When I began repairing chairs as opposed to concentrating on making new pieces, I was often met with wrinkled noses from my woodworking peers. It’s understandable, since when you repair a piece you take on responsibility for the quality of someone else’s work. However, with many chairs coming and going through my shop, I get to study how different styles of chairs age and break. Two of the most common repairs I do on chair frames are broken tenons and dowels. Since chairs and repairs vary, use these methods as starting points and adapt them for your specific chair.
This post demonstrates my methods for fixing broken mortise-and-tenons. I’ll cover dowel joints in a follow-up post.
This Wegner chair failed where the tenon on the front stretcher meets the mortise in the front leg.
Start by removing the broken tenon from the front leg mortise by drilling out the waste with an undersize drill bit. If the break is jagged, roughly create a flat area so the bit won’t wander. Be careful how deep you drill; usually you’ll feel the drill bit finding the bottom of the mortise.
Continue to drill a series of holes and remove the bulk of the tenon by leveraging the bit back and forth in the mortise.
Next, chisel out the mortise. Carefully pare down the walls of the mortise to remove the glue and expose fresh wood. Only widen the mortise to fit the router bit you will use to cut the mortise in the rail.
Test the fit with thickness spacers. After being pared clean, this mortise in the leg was exactly 3/8 in. I stacked a 1/4-in. and a 1/8-in. spacer to test the mortise thickness.
Next cut a mortise in the rail exactly over the broken tenon. Typically, the break will be near the shoulder and I usually leave the jagged edge, especially if the tenon extends to the edges of the rail. This will help conceal the repair.
Make a spacer for the thickness of a shoulder. On this chair, the new tenon will be slightly larger than the original so I made the spacer a little thinner to compensate for half of the extra tenon thickness. I eyeball this but it’s important to get it right so the rail lines up exactly to its original location. Use this spacer to set the router bit off the reference surface for cutting the mortise in the rail.
Mark the rail and leg so you know which side you are registering against. As a rule, I usually reference off the insides. Set the depth of the mortise and the width to around 1/4 in. from the edges of the rail. Cut the mortise square to the shoulder.
Mill some tenon stock to thickness to fit the mortises but oversize in width. Mark the depth of the rail mortise on one end of the tenon stock and if the two mortises are of different widths, notch this end to fit over the broken tenon ends or to the shoulder, whichever is appropriate for your piece.
Round the end. I mark it with a shopmade template and trim it down with a chisel.
In this example, the side of the tenon that fits into the rail is a different width because the original tenon went all the way to the top and bottom of the rail. I notched the slip tenon to match the narrower mortise I made in the rail. Then I kept the slip tenon wide to match the original mortise in the leg.
With the tenon inserted into the rail, mark where it should be cut to match the leg mortise. For this chair the tenon extends to the top and bottom of the rail. After cutting it down, round the edges of the end that fits into the leg.
As always, dry-fit to check before gluing up.
-David Johnson specializes in the conservation of Danish Modern chairs with woven seats from his home shop in Los Angeles. He also produces original work and teaches weaving. You can find him on Instagram (@sidecar_furniture) and on his website, sidecarfurniture.com .