Newsletter Article Lap Joints
by Luther Shealy
The lap joint is a simple but useful joint. When learning joinery, a lap joint should be the first joint you master because it is the simplest option for joining two pieces of wood together.
A lap joint is simply a notch cut out of one board, and a similar notch cut out of another. Then the two pieces are overlapped (hence the name) and glued together. Because there is a big long grain surface area for the glue to grab, the joint is pretty strong, however; it almost always comes with dimensional conflict since the lap joint creates a cross-grained connection (except for a scarf joint).
Wood swells and shrinks in width and thickness (perpendicular to the grain) as it absorbs and loses water according to the relative humidity. But humidity hardly affects length, so most of the movement is in width. Therefore, any joint construction that restricts wood movement by fastening one piece of wood cross-grain to another can cause problems. Because of this potential problem, lap joints should be no greater than about 3 inches in width to minimize the wood movement and thus keep the joint from breaking apart.
- The Full Lap Joint, where the two boards are overlapped in an unaltered fashion. In other words, the boards of wood overlap and thus create a joint with a thickness equal to the combined thicknesses of the boards. If one board is a half inch thick and the other is a half inch thick, then a full lap joint will be one inch thick.
- The Half-Lap Joint, where the sections of wood that will overlap are reduced in thickness by 50% prior to overlapping one another.
For cabinetmakers the half-lap joint is by far the most used of the two types of lap joints and the only ones this article discusses. Half-laps are commonly used to create frameworks or frame panels, crossed stretchers, sash moldings, shoji, chair backs, and grid work.
Common Types of Lap Joints
Mitered End Lap
Dovetail Lap (Type of a center lap)
Scarf Joint (No Cross-grain construction issues)