Halving joint
Also called: halving, half lap, half-lap, cross halving, corner halving, tee halving
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A halving joint is formed when each of two timbers has half its own thickness notched away, so that the two pieces overlap and sit flush where they cross or meet. Among carpentry joints it is considered one of the simplest, and it is often taught as a beginner's exercise.
Where it is used
Despite its simplicity, a halving joint holds up well and suits cross-shaped intersections, where one timber crosses another, and it has real uses beyond training: it turns up in skeleton grounds, the light timber framework built to support panelling or plaster before the finished surface goes on, and in general frameworks. Halving cuts also form sash bar joints, the joints between the slender bars that divide a window into panes, and groundsill construction, the sill timber that sits at the base of a frame, still commonly relies on halving joints in Japanese timber framing. One Japanese variant, hakodai-mochi tsugi, is a half-lap that has additionally been pinned and halved together. Seike (1970) ranks halving third in simplicity among the Japanese joints he discusses, including koshikake-tsugi and ai-jakuri.
Marking and cutting
A marking gauge, a tool that scores a line at a fixed distance from an edge, is set to half the stock's thickness to lay out the halving lines. Framing timbers should carry face and edge marks, reference marks showing which face and edge were used as the true datum during preparation, before a halving joint is cut. Cutting by hand starts with two lines marked where the pieces meet; the waste is then sawn down to the gauge line and pared out to the diagonal with a chisel. On a table saw, a half-lap is cut by matching the sawblade's height to the width of the mating board. Once the halving cut is made, either the edge it leaves, or lines squared out from it onto the underside of the stock, can be used to position a mitre block, a guide block used to hold work square for a following cut.
Named variants
The plain form, made by notching both pieces to half their own thickness and fitting them together, is also called a half-lap. An end lap is a halving rabbet, a stepped recess, cut at a board's end; a centre lap is a halving dado, a rectangular groove cut across the grain, cut somewhere along a board's length; and notching a deep dado into a board's edge produces half of what becomes an edge lap joint. Cutting a halving notch into the face of a board instead creates a frame lap, used to support frameworks or hold panels in frames. In a bevelled halving joint, the surfaces of the checks, the flat-bottomed notches cut into each piece, are angled rather than flat; a sloped joint surface of this kind improves shear strength considerably over a plain half-lap. The shoulder cut into a half-lap, the step where the notch meets the uncut timber, resists twisting.
Dovetailed halving and its alternatives
A dovetailed halving joint is used to lengthen timber; it is cut by making the halving cut first and the dovetail's angled cut afterwards. Fairham (1920) judged it 'probably the most difficult of the halving joints to mark out and construct with the desired degree of accuracy.' On glazing rails, the bars that hold panes of glass in a frame, a stub tenon, a short tenon that does not run the full width of the joint, can replace a halving joint, though it is weaker where the rail section is small. A slip joint goes together easily but, like the half-lap, still needs heavy clamping to close the glue line.