Cutting Lap Joints in Wood

Lap joints are joined pieces of wood that overlap each other at the point of union. In doing so, the overlapping pieces of wood do not alter the thickness of either piece. They are cut in such a way so that the pieces are joined without adding height. There are half lap joints and mitered lap joints. The first is found at 90-degree intersections while mitered laps are cut and joined at angles. This process will walk you through the basics of making a simple yet effective 90-degree lap joint. When it is finished, you will be left with surfaces that are flush and uniform. 

Tools You Will Need

All you need to cut this type of lap joint in wood are clamps, a carpenter’s square, a circular saw, a well-honed chisel, hammer, vise and a sanding block. Some sort of measuring device such as a tape or a ruler is necessary as well. 

The Process

The first thing to do is take measurements and note them. Measure the thickness of the wood you are joining. Next, measure the width of one piece of wood. On either a work bench or somewhere stable, clamp the two pieces of wood to be joined side by side with their ends completely flush. Once secure, measure in from the ends the exact distance of the width of piece of wood. For instance, if the wood is 3 inches wide, measure lengthwise in 3 inches from the end and make a mark. Take your square and draw a line across both boards at that mark. If it helps, make a series of lines from that mark back to the end of the boards every ¼ inch. Use the square to make straight lines. If the first measurement was 3 inches in, you should be left with 12 lines ¼ inch apart from the end extending 3 inches in. 


The next step is to make the necessary cuts. Take the measurement of the wood’s thickness, halve it and set the circular saw blade to that depth. With the wood still securely clamped, run the circular saw over the 3-inch mark first. The blade will only go halfway through the wood. Repeat this cut for each of the other 11 lines. These cuts are called kerfs. Both boards will have identical kerfs, so when properly cleared out they will form a perfect lap joint. 

Chisel and Join

Unclamp the wood pieces and place them upright in a vise with the kerf edges to the top. You should be able to look down onto them. Set the chisel at the halfway point on the short edge, at the point the saw blade reached. Lightly tap on it with a hammer until the kerfs break away. If done correctly, half of the thickness of each board will be left to the 3-inch mark, leaving no splinters or splits. Sand the surfaces thoroughly, continually testing the lap joint. Once finished, the two pieces of wood should overlap each other perfectly, creating a flush joint. 

If the wood is not that thick, the pieces may be joined by the right length screws. For industrial or commercial applications, metal brackets are typically used.