Lap joint
Also called: lap, end lap, cross lap, full lap
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A lap joint joins two pieces of wood by overlapping them and cutting a recess in one or both pieces so broad faces can meet for glue or fixing.
The overlap and the recess
The lap is the portion where the two members overlap after the joint is made. The recess, or notch, is the material removed to create that overlap; its edge forms a shoulder, the step that locates the meeting face. In a half-lap, each member is reduced by half its thickness so the joined surfaces finish level. In a full lap, one member is cut to receive the full thickness of the other.
Where each form fits
Lap joints are used in framing, including connections between rails or aprons and legs. An end lap is quick to make, but Noll (2002) warns that this version should not carry force acting sideways across the members. Seike (1970) describes the broader faces of a lap joint as suitable for heavy construction. These are different situations, so do not transfer the end-lap limitation to every form or assume a broad lap is suitable without checking its load.
Cut and fit it
Mark the shoulders on both pieces before removing the recess. Saw down to the marks, then clear the waste with a chisel or plane and test the fit dry. The overlapping faces should meet across their intended width without a gap. If the pieces have to be forced together, correct the cut before applying glue.
Glue and clamp
Long-grain faces, where the fibres run along the length of the mating surfaces, provide useful area for adhesive. During clamping, place a wood block beneath the clamp head so pressure spreads across the joint. Keep the pieces aligned while the glue sets, and check that the overlap has not shifted.