Wood joinery doesn't have to be complicated as long as it works. Let us teach you about lap joints, how they work, how they're made, and the different types.
Before the invention of useful modern tools, humans had to rely on simple physics to hold together the structures they built. They found that when evenly distributing pressure throughout an entire material, a joint can withstand more pressure and weight than joints that use external additions (like an adhesive) to hold themselves together.
Lap joints are a wood joinery method that was perfected in Japanese and Chinese woodworking traditions, and these practices are still widely in use today. In places that experience intense variations in temperature and humidity, wooden structures need to have room to bend and flex with changes in weather and temperature. Since lap joints use only the pressure of the joining members to stay in place, this enables the flexibility necessary in wood structures in these locations.
Explaining the Lap Joint
The lap joint is the joining of two members using only overlapping materials. This joint can be used with wood, plastic, or metal. However, the lap joint is most commonly used with wood, due to the material’s ability to flex and give.
When creating a lap joint, two long-grain members of wood are joined at either their ends or shins. They are then sometimes glued together, depending on the design of assembly. The lap joint method is spectacular for withstanding sheer force and weight, perhaps even more so than the mortise and tenon joint.
The lap joint will be either full lap or half lap. Read on for a more detailed explanation of these types of lap joints.
Full Lap Joint: In a full lap joint, no wood is removed from either of the wood members. The total thickness of the joint is that of the combined wood members.
Half Lap Joint: In a half lap joint, wood is removed from the joined members so that half of the thickness is removed from each. The resulting total thickness of the joint is the thickness of the larger member.
The optional shapes for the lap joint are an L, T, or X shape. For an L shape lap joint (also known as the end lap joint), the members are joined perpendicularly at their ends. For a T shape lap joint (the half-lap joint), one member is joined at the shin (center) of the other wood member. For an X shape lap joint (the cross lap joint), the wood members are joined at an angle at the shin of each member.
In a lap joint, the two joining wood members rely on only each other to stay erect. Oftentimes this type of joint is layered over and over again to create a nearly indestructible structure. A perfect example of this is the way log cabins are built.
How the Lap Joint is Made
A lap joint is probably the simplest wood joint to create. In a lap joint, wood is chiseled away at the correct level of depth to receive another member that has a corresponding notch. This notch is usually found in the shape of a square or rectangle. The 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 the notch. The cheek is parallel to the face of the other wood member.
The Shoulder: the shoulder 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.
A lap joint can be created using any woodworking tool. The need for accuracy is rather forgiving, unless aesthetics are a priority for a particular project. In essence, the creation of a lap joint is the chiseling away of wood to make room for the width of another member.
Where To Find the Lap Joint
The lap joint is a quintessential way of joining wood members together. The most recognizable place to find this joint is in a log cabin. The lap joint can also be found used in coffee tables, antique chairs, vintage tables, and sometimes in cabinet making and frame assembly (as in the case of a log cabin).
Even using the kindling box method when building a campfire is using a full lap joint!
Here we expand upon the concept of the L, T, and X shaped joints a little further. We’ve also included a few other methods of lap joining.
The End Lap (L): also referred to as the pull lap, this is the basic lap joint where 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): this occurs when workpieces are joined at one of the member’s shins (center). The member does not continue past the joint.
The Cross Lap (X): this joint occurs in the center of one or both of the members, and they are joined at an angle. The cross lap requires more finesse and measuring when creating its notch. The crossed members continue past the joint, as opposed to the half-lap where the member ends at the joint, creating an X shape.
The Dovetail Lap: this occurs when the notch is cut an angle on both sides in order to resist easy withdrawal of the members from one another. In a dovetail lap joint, the members must be joined with one placed above the other so that they fit together like a puzzle piece. This type of lap joint is used for framework purposes, to prevent the joint from being pulled apart.
The Mitered Half Lap: this is the weakest of the lap joints, due to the angle at which mitered joints are cut. Mitered joints also result in the creation of less glueable surface area.
What is a lap joint flange?
This method is used when metal welding lap joints. It involves an external ring added around a member for free rotation of the member.
Why use a lap joint?
The lap joint is quite resilient, due to the fact that it allows for pressure and weight to be distributed throughout the entire workpiece. When using wood, the lap joint is ideal for use in an area with intense humidity and temperature fluctuations.
When is a lap joint used?
Lap joints are timeless, and for good reason! In the more recent past they have been used in building log cabins,. However, today lap joints are often used in table, chair, toy, and frame making.
Do members have to be the same size?
Not at all, so long as the lap joint notches are the same size as the member. If one member is significantly smaller than the other, chances are more strain is going to be put on the smaller of the two. The structure may not last as long if the workpieces aren’t the same size as one with workpieces that are the same size.