Extremely easy to form, lap joints also have the benefit of being extremely strong. By notching one board and placing it another board within the space, the walls of the notch prevents the other board from twisting free. Also, the gluing surface is entirely long grain to long grain, producing a very strong bonding surface to complemetn the mechanical interlocking of the joint.
By its nature, lap joints result in joints that form an L, X, or T, however there are many variations on how to accomplish this.
The depth of the joint is the depth of the notch measured from the face of the board to the bottom of the notch. When a lap involves the end of a board, the width is the width of the board. The length is the distance measured from the end of the board (the end grain), along the cheek, to the shoulder. For the portion of a lap that occurs in the middle of a board, the width is measured along the direction of the grain, the cheek is the bottom portion of the notch, and the shoulders are the two sides of the notch.
Half-lap joints are formed by both mating pieces receiving identical-depth notches, usually half the depth of the boards being joined.
Full-lap joints are formed by having only one thicker board receiving a notch the entire depth of the second board. The second board sits completely within the notch.
These joints are formed by notching the end of two boards, then overlapping the ends so the cheeks meet and the shoulders are square to the cheeks. It is essential that the cheeks be flat or the joint will not mate properly, which will weaken the joint.
These joints involve one board receiving a rabbet at one end, and another board receiving a notch somewhere mid-section.
These joints involve both boards receiving a notch somewhere along its length into which one board is placed into the other, notch-to-notch. This is an extremely strong joint due to its mechanical interlocking.
A pocket lap is a joint in which one board receives a rabbet at its end, but its length does not reach entirely across its mate. Its primary use is when you do not want to expose the end grain of the joint.
A mitered half-lap joint is one in which one board receives a normal rabbet at its end, but is then mitered to remove half the cheek. The second board would need to be measured from the first, then notched appropriately. This joint will give the appearance of a mitered joint, without any of the failings of a normal flat miter joint.
Though the standard half-lap joint provides a very strong mechanical joint, the dovetailed half-lap improves upon the design by preventing the lap from being pulled out due to the dovetail-shaped lap. To create the joint, first create the end lap one one board and trim the cheeks to a dovetail shape. Once done, simply transfer the shape to its mate and notch it accordingly.
In addition to its added strength, this joint benefits from its dovetail-like appearance.
This variation turns the end lap into a dovetail pin instead of the tail. The joint not only resists racking, but it also prevents one board from being twisted free from the other. The only direction the board can possibly be forced is out the direction it was inserted.
To produce this joint, a normal lap is created at the end of a board, then the sides of the lap are slanted to be wider at the cheek, and narrower at the face of the board. Again, keeping the standard dovetail angle of 8° to 12° is optimal for strength. Once the end lap has been created, the notch can be formed with matching angles. The least error-prone method to create this joint is to use a router to create both the end lap sides and the notch, then trimming the shoulder with a chisel.