What Is a Lap Joint?
Jan 07, 2022
There are so many ways to join two pieces of wood together that the volume of options can be overwhelming. From mechanical fasteners and biscuit joints to dovetail joints and more, there can be a lot to know. The lap joint, or “overlap joint,” is one of the simplest of these joints, and it’s important to understand its strengths and weaknesses before using it.
In this article
What is a lap joint?
A lap joint is one of the simplest and easiest ways to attach two pieces of wood together. As the name suggests, this joint is when one piece of wood overlaps the other. Where those two pieces of wood meet is the “lap.” The carpenter, woodworker, or DIYer will fasten the boards to each other at the lap, using mechanical fasteners like nails and screws, glue, or wooden pegs and dowels.
The basic lap joint itself is not incredibly strong, and it’s not very impressive to look at. Though it does provide a bit of gluing area, just about any torsion or torque can cause the joint to fail. However, there are uses for this joint, and there are other versions that are much stronger.
Where are lap joints used?
The basic lap joint is not a joint you’ll often see in fine woodworking, cabinetry, or anywhere appearance matters. It’s rather clunky-looking, as they are simply two boards overlapping each other. However, these joints are extremely fast and easy to make, making them useful in wood framing roof construction.
For example, by marking and overlapping two 2×6 boards, a framing crew can assemble a set of roof rafters that they can lift into place. This joint itself is not strong enough, so framers will add another board across the span, which is known as a collar tie. Between the lap joint and the collar tie, these rafters are solid.
However, there are quite a few types of wood joints that use laps, many of which are much stronger and attractive to look at. Woodworkers, carpenters, and DIYers use these advanced lap joints for adjoining pieces of wood for furniture making, timber construction, cabinetry, and other joinery situations.
5 Types of lap joints
A lap joint describes any type of joint where two or more boards lay over each other. This is different than miters or butt joints where the boards simply mate up to each other at the joint. It’s also different from mortise and tenon joints, which require one board to slide into the other, receiving a pin for fastening.
Within the realm of joints where boards lay overtop of each other, there are several types of lap joints.
1. Basic lap joint
In a basic lap joint, the boards are simply cut to length and lapped over each other, with a few fasteners in the middle to hold them together. None of the wood is removed, and they’re extremely fast to make.
2. Half-lap joint
A half-lap joint is similar to a basic lap joint, but the carpenter removes half the material from each board before mating them together. For example, on a 2×4 (which is actually 1 ½-inch thick), the carpenter removes ¾-inch of wood from the thickness of both boards. When they overlap, they’re 1 ½-inches thick, meaning the joint is the same thickness as the rest of the board.
This technique takes longer but creates shoulders on both sides of the joint, increasing the surface area for glue and adding some lateral strength to the joint. Half-lap joints are popular in woodworking projects and basic cabinetry, but they’re also popular in construction where a plywood gusset nailed to the joint area can increase strength tremendously.
3. Mitered lap joint
As a more advanced lap joint technique, the mitered lap joint is very strong. In this joint, the carpenter or woodworker cuts a 45-degree angle on one board, and then removes half the thickness from the mitered section, creating a triangle-shaped relief in the back of the board. Then, on the opposing board, they cut a 45-degree angle halfway through the board before removing half the material between the 45-degree cut and the end of the board. This creates a corresponding space for the first board.
When the two mitered angles touch, the two boards overlap where the material was removed. This creates a shoulder, a mitered face, and lots of glue area.
Because this joint takes so long, it’s most popular in woodworking, cabinetry, and building picture frames.
4. Cross-lap joint
In a cross-lap joint, the woodworker removes half the material from both boards just like a half-lap joint. Essentially, it’s a half-lap joint that occurs in the middle of one or two of the boards, rather than at the ends. This creates 4 shoulders (two on each board) and plenty of surface area for wood glue or construction adhesive.
A cross-lap joint is an extremely strong joint compared to other lap joint types, and it’s popular in woodworking, furniture building, and even door construction.
5. Dovetail lap joint
The lapped dovetail joint is very similar to the cross lap joint, except it’s almost always between the end of a board and the middle section of another board. Also, the lapped section resembles a “V” or bird’s tail (hence the name). Because it has such a decorative look, it’s common in cabinetry and furniture making.
How to Cut a Lap Joint
Since the lap joint is most applicable in construction settings, the following steps will describe how to cut a lap joint for a rafter framing system.
- Lay the boards on a work surface. Lay both boards on a work surface (probably the plywood decking if you’re on a project site). Place the end of one board on top of the end of the other board. Apply a few dabs of construction adhesive and drive one screw through the lap to secure them.
- Adjust the boards to the appropriate angle. Adjust the boards so they form the appropriate angle. Most of the time, the lap joint is used for 90-degree angles, but if yours is different, you’ll have calculate the appropriate angle.
- Glue and screw through the lap. Drive at least 5 screws or nails through the lap joint to fasten them together.
- Trim the excess. If your boards form anything less than a 90-degree angle, there will be excess wood to trim past the joint. Use a power tool like a reciprocating saw or circular saw to trim the extra wood flush with the end of the joint.
MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like cabinetry basics. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.