Canadian Woodworking

Bird’s mouth lap joint: the simple approach isn’t always the best

Author: Rob Brown
Photos: Rob Brown
Published: October November 2022
Birds mouth
Birds mouth

There are times when a simple, strong joint is the right choice. There are also times when a bit of flair can go a long way. And, heck, a bird’s mouth joint is also a lot of fun to make.

Working in a small hobby shop allows a lot of freedom. With no deadlines you can be free to take extra time to produce a more time-consuming, unique joint just for the fun of it. Although I build professionally, I still have the opportunity to incorporate a joint that takes a bit of extra skill and precision to build from time to time.


Birds mouth joint

Simple Layout
Brown first marked the width of the stile onto the rails, then added a bolder, longer centreline.

Birds mouth joint

The Correct Notch Width
After cutting a V-notch into a piece of 1/2" thick scrap, Brown made test cuts on scrap to ensure that when the front edge of the V-notched template was flush with the workpiece, the routed notch was the same width as the mating rails.

Birds mouth joint

Get Into the Corner
A sharp chisel or plane iron is great for getting into the corner.

Birds mouth joint

A Precise Mitre
A mitre can now be cut on the ends of the rails. The tip of the joint must be located dead centre on the rails.

Birds mouth joint

Mark for Depth
With the parts on the same smooth surface, butt them together and mark the depth needed on the rails.

Birds mouth joint

Cut the Half Lap
Crosscut the waste from the underside of the mitreed rail. Be careful not to remove too much material or the gap between the rail’s end grain and the side grain of the stile will be visible. It’s best to leave a bit of material on at first and fine tune the joint later.

Birds mouth joint

Minor Adjustments
A block plane or other hand tools can have the joint fitting nice and tight.

Birds mouth joint

Additional Strength
Every joint is different, but Brown thought this joint needed a bit of additional strength, so he routed a groove in the rear face of the joint and glued in a piece of solid wood.

Birds mouth joint

Invisible
Once dry, the additional strip is planed flush and will never be noticed.

Birds mouth joint

While building a sideboard for a client I had the chance to take the easy way out and just join a pair of centre dividers to the upper and lower rails with a Domino-reinforced butt joint. It would have been strong and sim­ple, an approach I often appreciate. But I thought this sideboard needed a bit of flair, as it was otherwise fairly straightforward, and the drawings didn’t have specifications for joints.

The pine sideboard is distressed and is meant to look like an antique when it’s complete. The carcass features mitreed frame assemblies, so I opted for what I’m calling a bird’s mouth half lap joint, though there might be another term for this joint I’m not aware of. I could have opted to include a mortise and tenon that was integral to this joint, but I didn’t think this specific situation needed that elaborate of a joint.

Look for options for fun joinery

I’m not the type of woodworker who needs to have loads of showy joinery in order to impress clients, friends or family. I do, however, feel that there are times when going that extra mile on a joint will pay off in spades down the road and bring a bit of spice to an otherwise simple project.
When you’re planning your next project, think about what joint (if any) needs a bit of extra flair to bring the whole piece to life. It may mean replacing a standard rabbet joint with a keyed bevel joint, making variable-spaced hand-cut dovetails instead of typical machine-made dovetails or, as in my case, replacing a basic rein­forced butt joint with something unique.

When deciding on what to include you might want to do a bit of internet research or check out books or magazines. There are so many ways to join wood that it often pays to look to others for inspiration. Social media is also a great option for joinery inspira­tion. Craftspeople in Japan are great at making intriguing joints, though many other locations around the globe also bring a unique perspective to the art of joining wood.

Layout and a template

This bird’s mouth lap joint was simple to lay out. I marked the width of the dividers in the right locations on the rails, then extended a centre line to give me the centre point of the bird’s mouth. Next, I tilted my table saw blade to 45°, and with two passes, cut a notch out of a finishing sample scrap. A guide bush­ing in my router’s base plate would trace the two 45° edges and guide a 1/4″ diameter bit.

If I aligned the edge of my template with the edge of the rail the bird’s mouth notch would be too large, so I made a number of rips on my table saw, sneaking up to the exact dimension so that when the edge of the template and workpiece were flush, the width of the routed lap joint would be exactly the same width as the dividers.

I was fairly confident I could make this joint without any prob­lems so I decided to forego a practice joint to dial things in. If worse came to worst, I’d break out another divider or fill the negative space I routed in the rails and start over. If you’re at all unsure of the details regarding the joint you’re making, it’s always a good idea to make a test joint. It’s amazing how many things you learn when making the first joint, and how much better the second joint looks and fits. After all, the joint you make has to be strong enough to last the test of time.

Ready for action

I aligned and clamped the template, then adjusted the bit to cut about 1/3 of the rail’s thickness. The centre point of the template needed to be aligned with the extended centre line I marked.

I routed all four of the bird’s mouth notches, then turned my attention to the ends of the dividers. First, the length of the divid­ers had to be determined and cut. In theory, the length of the dividers is equal to the distance between the rails, plus the width of the divider, but it’s always a good idea to check. With this joint a divider that’s even 1/16″ too short is going to show a gap of that same dimension, and your fancy joint will be nothing more than an eyesore.

Mitre time

I cut the divider to exactly the finished length needed. This meant that I had to dial in the next four mitre cuts perfectly or the divider would be too short. I then marked a line on the end grain of the dividers at both ends dead centre on the width of the divid­ers. In theory, this line was to be split in half by each of the mitre cuts. If I was going to be cutting many of these joints, I would have set up a stop on my mitre sled to give me a consistent length, but with only four joints to cut I just marked and cut each of the joints.

I snuck up on all four lines to leave a sliver of the pencil line on the workpiece, yet create a point on both ends of the dividers. Removing 1/32″ too much material would have caused a gap in the final joint, so accuracy and patience are critical here.

Lap it

To mark the lap portion of the joint I butted the two parts up against each other directly at the routed notch and marked where the divider should be trimmed to obtain a flush lap joint. Back at the table saw, this time with my mitre gauge set to 90°, I snuck up to the proper height and width of cuts.

At this point the divider should fit in place between the two notches in the rails, though some fitting may be necessary. I used a sharp block plane to fine tune the mitreed edges of the divider so they fit in place nicely. One of my dividers was now fitting per­fectly, though there was a gap between the rear face of the parts with the other divider. To fix this I glued a piece of veneer to the end grain of one of the dividers. Once dry, I trimmed it flush and tested it for size.

Glue it up

It was now time for glue. An even layer on both faces of the lap joint, as well as the end grain on the divider, and it was brought into position and clamped. I made sure one clamp was used to bring force parallel to the divider (in order to minimize any gaps) while another clamp was used to bring the half lap faces together.

Add some extra strength

If very little stress was going to be applied to this joint, as would be the case in a small decorative box, I could have left it as-is. But this divider will be asked to provide some strength and rigidity to a full-sized piece of furniture so I wanted to be on the safe side. When the lap joint was dry, I flipped the assembly upside down so I could machine a groove across the joint and install a strip of wood. The piece of wood would act as a mechanical strengthener, as well as offer some glue surfaces to keep the joint together. As it was added at the back face of the assembly, it will never be seen.

I clamped a straightedge to the assembly and used a 5/8″ straight router bit to plough a groove about 7″ long and 1/4″ deep. I then fit and glued a piece of solid wood into the groove. When it was dry, I hand planed it flush. The result is a joint with a fair amount of glue surface area, as well as a decent amount of mechanical protection to prevent twisting, bending or racking. It also looks different than your typical 90° butt joint which was the main reason for taking the extra time to make it.

What joint will you choose?

The next time you’re designing a piece of furniture, see if it makes sense to add a unique joint to your project. It can certainly be showy, but even if it’s not you’ll appreciate the work that went into it. You could even keep the sample joint you machined first on the mantle or displayed on the piece you’re making to act as a con­versation starter. That way, friends and family can appreciate how much work goes into making solid, lasting joints that look great.

Please send me photos of the joints you add to your projects. I’d love to share them with readers of the magazine or online. A few photos of the joint being made, the final joint coming together, along with a shot of the fin­ished joint, would be great for myself and our readers to see and learn from.


Rob Brown - [email protected]

Rob is a studio furniture maker and the editor at Canadian Woodworking & Home Improvement.

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