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Lap joints can be used in wood, plastic, or metal. A half lap joint or a halving joint is a technique of joining two pieces of material together by overlapping them. A lap may be a full lap or half lap. In a full lap, no material is removed from either of the members to be joined, resulting in a joint which is the combined thickness of the two members. In a half lap joint, material is removed from each of the members so that the resulting joint is the thickness of the thickest member. Most commonly in half lap joints, the members are of the same thickness and half the thickness of each is removed.
Half lap joints are used extensively in traditional timber framing, construction and cabinetry for framing. They are quick and easy to make and provide reasonable strength through good long grain to long grain gluing surface. The shoulders provide some resistance to racking (diagonal distortion). They may be reinforced with dowels or mechanical fasteners to resist twisting.
Also known simply as a 'pull lap', it is the basic form of the lap joint and is used when joining members end to end either parallel or at right angles. When the joint forms a corner, as in a rectangular frame, the joint is often called a corner lap. This is the most common form of end lap and is used most in framing.
The main difference between this and the basic half lap is that the joint occurs in the middle of one or both members, rather than at the end. The two members are at right angles to each other and one member may terminate at the joint, or it may carry on beyond it. When one of the members terminates at the shin , it is often referred to as a Tee lap or middle lap. In a cross lap where both members continue beyond the joint, each member has two shoulders and one cheek.
This is a lap in which the housing has been cut at an angle which resists withdrawal of the stem from the cross-piece.
The mitred half lap is the weakest version of the joint because of the reduced gluing surface.