Wood joinery doesn't have to be complicated as long as it works. Let us teach you about lap joints, how it works, how its done and its different types.
Before the invention of useful tools, humans had to rely on simple physics to hold together structures. It turns out that when evenly distributing pressure throughout an entire material, a joint can withstand more pressure and weight than joints using external additions to hold them together.
Lap joints are a wood joinery method that has been perfected in Japanese and Chinese woodworking traditions, and these practices are still heavily in use today. In places that have such intense variations in temperature and humidity, wooden structures need to have the room to bend and flex. Since lap joints use only the pressure of the joining members to stay in place, this enables flexibility.
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Explaining the Lap Joint
The lap joint is the joining of two members using only overlapping materials. This joint can be used in wood plastic or metal, but most commonly with wood due to its give and flex.
Two long-grain members of wood are joined at either their ends or shins and sometimes glued together, depending on your design of the assembly. This method is spectacular for withstanding sheer force and weight, maybe even more so than the mortise and tenon joint.
The lap joint will be either full lap or half lap.
Full Lap Joint: no wood is removed from either of the wood members, and the entire thickness of the joint is that of the combined members.
Half Lap Joint: wood is removed from the joined members so that half of the thickness is removed from each. The resulting thickness of the joint is the thickness of the larger member.
The optional shapes for the lap joint can be done in either an L, T, or an X shape. For an L shape (the end lap), the members will be joined perpendicularly on the members’ ends. For a T shape (the half-lap), one member will be joined at the shin (center) of the other member. For an X shape (the cross lap), the members will be joined at an angle is the shin of each member.
The two joining members rely on only each other to stay erect, and oftentimes layered over and over again to create a nearly indestructible structure (much like how traditional log cabins are built).
How the Lap Joint is Made
This is probably the simplest wood joint to manufacture. Wood is chiseled away at any level of depth to receive another member with a corresponding notch. This is usually in the shape of a square or rectangle. This notch is divided into two components: the cheek, and the shoulder.
The Cheek is the floor of the notch. Anywhere from 1 centimeter to 5 inches (or truly any depth, depending on the width of your workpiece), the cheek is the bottom of this notch. It is parallel with the face of the wood member.
The Shoulder: this is the distance between the cheek and the edge of the workpiece. It is basically the wall leading from the notched floor to the top of the workpiece face.
The lap joint can be created using any simple woodworking tool. The need for accuracy is rather forgiving unless aesthetics is a big priority for your project. All that’s happening is the chiseling away of wood to make room for the width of another member.
Where To Find It
This is an extremely classic way of joining wood members together. The most recognizable place to find this joint is in a log cabin. Once you get good at recognizing wood joints, you’ll start finding the lap joint in coffee tables, antique chairs, vintage tables, and sometimes in cabinet making and frame assembly (this is the log cabin method I just mentioned).
Even if you’re building a campfire and using the kindling box method, this is a full lap joint!
Here we’re expanding upon the concept of the L, T, and X shaped joints a little further. As well as a couple of other methods of lap joining.
The End Lap (L): or referred to as the pull lap, this is the basic lap joint when members are joined end to end at a parallel or perpendicular angle. When the joint forms a corner it’s called a corner lap.
The Half Lap (T): when workpieces are joined at one of the member’s shins (center). The member does not continue to pass the joint.
The Cross Lap (X): the joint occurs in the center of one or both of the members, and joined at an angle. This requires more finesse and measuring when creating your notch. The crosses members continue past the joint (as opposed to the half-lap where the member ends at the joint).
The Dovetail Lap: this occurs when the notch is cut an angle on both sides in order to resist easy withdrawal. The members will have to be joined with one member being placed above the other and fitting together like a puzzle piece. This is used for framework purposes to prevent the joint from being pulled apart.
The Mitred Half Lap: the weakest of the lap joints, due to the angle at which mitered joints are cut and result in less surface area to be glued.
What is a lap joint flange?
This is used in metal welding lap joints. It’s an external ring added around your member for free rotation.
Why use a lap joint?
The lap joint is very resilient due to the fact that it distributes pressure and weight throughout the entire workpiece. When using wood, the lap joint is ideal if you exist in an area with high humidity and temperature fluctuations.
When is a lap joint used?
Lap joints are timeless and for good reason! They’ve been used in building log cabins, but today are used in table, chair, toy, and frame making.
Do members have to be the same size?
Not at all, as long as your lap joint notches are the same size. Just remember, if one member is significantly smaller than the other, chances are more strain is going to be put on the smaller one. The structure may not last as long if the workpieces aren’t the same size.