Half-lap joints: layout, depth and good uses

A half lap removes half the thickness of each of two boards where they cross, so they overlap with their faces flush. It is simple to lay out, can be cut with almost any tool, and gives a large face-to-face glue area. It shows up in face frames, workbench and base frames, grid dividers and quick shop furniture.

Four kinds of lap

• End lap (corner lap): two boards meet at a corner, each notched across its end.
• Tee lap: the end of one board laps into the middle of another.
• Cross lap: two boards cross in the middle of both.
• Edge lap: notches are cut into the edges of two boards so they slot together on edge, as in the dividers of a drawer organiser or an egg-crate grid.

Layout from the actual stock

Lay out a lap from the parts themselves, not from nominal sizes:

• Width of the notch equals the actual width of the mating board. Mark it by holding the mating board in place and knifing along its edges, rather than measuring.
• Depth is half the thickness when both boards are the same thickness. When they differ, the two depths must add up to the thickness you want the joint to finish at, usually the thicker board, so both faces are flush.
• Shoulders must be square to the edge. A shoulder that is off square twists the assembly.

Mark the waste on both boards, and check that you are removing it from opposite faces. Cutting both notches from the same face is the classic half-lap mistake.

Setting the depth

Getting both faces flush is the whole point of the joint, and it comes down to depth:

1. Set the blade, bit or marking gauge to a little under half the thickness.
2. Cut a notch in two offcuts of the actual stock, one from each face.
3. Fit them together. If the faces are proud, the notches are too shallow; if they are sunken, too deep.
4. Adjust by half the difference you see, since both notches change.

A marking gauge set once and used from the reference face of each board keeps the depth consistent by hand.

Cutting

• Table saw: a dado stack, or repeated passes with a standard blade, with the board against a mitre gauge or sled and a stop for the shoulder. Clean the bottom with a chisel or a shoulder plane.
• Router: a straight bit with a straightedge or a simple jig; good for wide or awkward parts.
• Hand tools: saw the shoulders to the depth line, make extra kerfs in the waste, chop out the waste with a chisel, and pare to the line.

Strength and wood movement

A half lap glues face grain to face grain, so a well-fitted lap is strong. In an end lap or tee lap with both boards running in the same plane, the glue surfaces often cross at right angles: one board’s grain runs along the joint and the other’s across it. On frame stock a few inches wide, that cross-grain glue joint holds well. On wide boards, the seasonal change in width works against it; see wood movement. Wide cross laps are better fastened with screws in slightly oversized holes, or designed with narrower members.

Laps can be reinforced with screws, dowels or pegs through the overlap, particularly on end laps in frames that carry load.

Good uses

• Face frames and simple doors where the joint is hidden or intended to show.
• Base frames and stretchers for benches and shop furniture.
• Grid dividers and organisers, using edge laps.
• Outdoor frames, fastened mechanically where glue will see weather.

In GrainDraft

Half lap is one of GrainDraft’s eight joint families. One relationship notches both boards, and the joint preview shows the overlap and the thickness left in each board, so you can check that the notches come from opposite faces before you cut.

Related: box joints, milling rough lumber.