How do I cut a half lap joint?

The easiest strong joint there is — half the thickness out of each piece so they interlock flush — and the one measurement that makes it work.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

Set a marking gauge to half the thickness and use that single setting on both pieces, working from the face side of each. Knife the shoulders, saw down to the gauge line, remove the waste and pare flat. Because both parts come from the same gauge setting, the assembly finishes flush even if your gauge is not exactly on half.

A half lap gives a large long-grain glue surface, resists twisting, and needs no more skill than sawing to a line. It is the joint for frames, workshop jigs, garden structures and anything where a mortise and tenon is more work than the job deserves. The trick that makes it reliable is refusing to measure half the thickness at all.

What you'll need

A marking gauge
A square and marking knife
A tenon saw
A chisel
Clamps

Step by step

Mark a face side and face edge on both pieces.Every gauge line is taken from those faces. This is what makes the joint self-correcting.
Set the gauge to roughly half the thickness — exactly is not required.Set it by eye from one face, check from the other, and split the difference until the two scribes coincide. Then leave the gauge alone.
Gauge the depth line on both pieces from their face sides.Because both cuts come from the same setting and the same reference face, the two halves add up to the full thickness whether or not the gauge is truly on half.
Mark the shoulder from the mating piece.Lay the other piece across, square, and knife along it. Do not measure a width — copy it, because the actual width is what has to fit.
Saw the shoulders down to the gauge line.Cut on the waste side. In a cross lap, both shoulders are internal; in a corner lap, one is at the end of the piece.
Remove the waste.For a corner lap, saw down the cheek from the end grain. For a cross lap, make several extra saw cuts across the waste and break them out with a chisel, then pare flat.
Flatten the bed to the gauge line.Router plane if you have one, chisel and straightedge if not. A hollow bed is fine; a high spot in the middle holds the joint open.
Fit dry and check flatness across the assembly.Lay a straightedge over the joint in both directions. If one piece stands proud, deepen the other side rather than planing the finished face.
Glue and cramp across the lap.Two clamps on a corner lap so it cannot rotate. Check the frame diagonals before the glue grabs.

Tips

Screw or peg a half lap in addition to gluing it if it will be loaded in racking, such as a gate or a workshop frame.
A dovetailed half lap — where the corner lap is cut with a single dovetail slope — resists a corner pulling apart and costs about five more minutes.
Cut all the shoulders on all the pieces before you cut any cheeks. Sawing the same cut repeatedly is what makes them match.

Common mistakes

Measuring half the thickness with a rule — Timber is rarely a round number and the two halves then do not add up. Gauging both from one setting removes the arithmetic entirely.
Gauging one piece from the face and the other from the back — Any variation in thickness lands in the joint. Always work from the marked face side.

Questions people ask

Is a half lap strong enough for a table frame?

For a light frame, yes, particularly if pegged or screwed. For a dining table or a chair, a mortise and tenon is significantly better because it puts far more long grain into the joint and resists racking in the plane that matters.

What is the difference between a half lap and a bridle joint?

A bridle takes the waste from the middle of one piece and the outside of the other, so the joint wraps round like an open mortise and tenon. It is stronger in racking than a half lap and shows end grain on two faces.