When building with any framing lumber, half-lap joints are often used on long runs and for 90-degree intersections. They keep the mating surfaces flush and the wood thickness uniform.
Although there are several ways to cut a half-lap, here's how to quickly mill one using nothing more than a portable circular saw and a chisel:
Clamp two boards side by side with their ends even. Measure a distance in from the ends that's equal to the width of one board, and mark a square line across both boards. Set the depth of cut on the saw to half the thickness of the boards and make a cut along the line to form the shoulder of the joint.
Next, cut a series of closely spaced saw kerfs across the boards. Use a chisel to clear away the waste wood and smooth out the broad, flat surface of the joint. Unclamp the boards and put them together cheek to cheek to form a 90-degree angle.
Then assemble the joint. The method of assembly depends on the thickness of the boards and the size of the completed frame. Typically, frames built with 2-by lumber or thicker require mechanical fasteners.
Courtesy of American Tool
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