Lap joints can be found in all types of wood construction from framing to furniture pieces. When cut so that the two boards interlock (a half lap joint), they are strong and durable and the joint has very little movement. Half lap joints can be made in a variety of ways but the best results are often achieved using a stacked dado cutter and a table saw.
Adjust the height of the stacked dado cutter to one-half the thickness of a board.
To make a lap joint on the ends of the two boards, adjust the table saw fence so the distance between the fence and the outside edge of the widest part of the stacked dado cutter is the same as the width of one board.
Hold one of the boards tightly against the miter gauge and make sure the end of the board meets the fence, then make a pass through the cutter. Make multiple passes through the dado cutter, moving the board to the left before each pass. Continue until reaching the end of the board. Follow the same procedure to cut the second board. Each board should now have a wide groove or lap cut on one end.
To make a lap joint away from the end of the boards, use a square to mark a line across the boards where they will meet. Make another line one board width away from the first line. Making multiple passes through the stacked dado cutter, remove the material between the two lines. Repeat this process for the second board to complete the lap cuts.
Stacked dado cutters typically produce very clean, square cuts but you can use a sharp chisel to remove any waste material that was left behind and to square up the cuts.
Slip the two boards together, joining them at the lap you just cut. The boards should fit together snugly and the wood should have a uniform thickness right through the joint.
To add strength to the joint, coat each lap with wood glue and join the two boards together again. Depending on the application, use clamps, screws or nails to hold the joint firmly in place.