A lap joint is a combination of characteristics using rabbets and dadoes to join two board surfaces together. The lap joint is an alternative to the use of a butt joint in frame construction. They make a stronger joint because there is more gluing area to interlock the board pieces. This is because the thickness of the wood in a lap joint is equal in dimensions. The two pieces of a lap joint are lined up so the joint faces of each board are aligned with one another. This is accomplished by creating openings with either overlapping dadoes or rabbet tongues of equal width.
Several styles of lap joints can be cut on a table saw. These styles include corner half-laps, cross half-laps, “T” half-laps, full laps, bridle joints and angled half-laps. The word “half” in each one of these joints refers to the fact that dadoes or rabbet cuts are completed in half the width of the wood piece. This means that the blade on the table saw is set to cut the required depth for the lap joint and is used on both pieces of wood without any adjustments.
Cutting a lap joint on the table saw is similar with all types of lap joints. The corner half-laps use two wide rabbet cuts. The cuts are made across the end faces of two boards. The rabbet cuts on both boards will match each other in width when looking at both cuts. The “T” half-lap refers to a rabbet cut end of the board fitting into the same width of a center cut dado opening on a second board. When the two pieces are fit together they look like a “T”. A full-lap joint is similar to a “T” half-lap. The cut is very different but the result ends with two boards that resemble a “T”. The full-lap joint requires a rabbet cut to be made down the center width of the end of a board the exact face dimension of the insertion board. Two dado cuts the face width of the boards are made on both sides of the insertion board so the remaining width in the center of the board is the same dimension as the dado cut made down the center end width on the holding board.
Cutting a large size rabbet or dado cut is just like cutting any other size cut. Multiple passes are required to achieve the right width for each cut. The dado makes a broader cut and is the better piece of equipment to use when making a lap joint. Use a tenoning jig when cutting rabbits into wood standing on edge.
Lay the boards face down on either the table saw or dado table to make the cuts. Only make cuts in boards that are manageable. Ask for help on those pieces that are longer. The lap joint is still possible to make but it is very important to keep the wood square when making the cuts.
Bob Villa: How to Cut a Lap Joint
This Old House: Creating Half-laps
Wood Magazine: Build a Jig for Lap Joints