In an equal-thickness half-lap, each member loses half its thickness at the overlap so the two faces finish flush. The fit depends on actual stock thickness and square shoulders, not on how accurately you remembered the board's nominal size.
Mark from one reference face
Plane or select stock of consistent thickness. Mark the joint position using the mating piece directly, then mark the waste. Set a gauge to half the measured thickness and carry the depth line around the relevant edges from the same reference face.
For two 20 mm pieces, the nominal recess is 10 mm in each. If one piece is 19 mm and the other 21 mm, blindly cutting both to 10 mm does not solve the mismatch; decide which faces must align and lay out the joint accordingly.
Saw the boundaries, then remove waste
Clamp the work securely. Saw on the waste side of both shoulder lines, stopping at the depth line. Add several relief kerfs within the waste if they help you remove it in controlled pieces.
Use a sharp chisel to pare shallow layers. Work from both edges where necessary rather than levering a large chip through the far side. Keep hands behind the edge and support the piece so a released chip cannot send the tool into your body.
| Fit problem | Likely place to inspect |
|---|---|
| Joint rocks | High spot on a recess floor |
| Ends close but middle gaps | Uneven floor or bowed stock |
| Faces step | Depth layout or thickness mismatch |
| Visible shoulder gap | Shoulder cut beyond the line or out of square |
Fit the floors before widening the joint
Bring the pieces together square and identify contact. Remove a small high spot with controlled paring, then test again. Sanding indiscriminately can round the shoulders and hide where the joint was actually tight.
When the dry fit seats without force and the required faces align, prepare the glue-up. Apply an appropriate adhesive to the mating surfaces, clamp with protective blocks and verify the overall assembly remains square.
A half-lap is useful in many frames, but cutting away half a member also changes its section. Suitability for a bench, ladder or other loaded structure must come from the complete design, not from this practice joint alone.
Related workshop guides
- The ten-minute dry run that makes a glue-up calmer
- How to install threaded inserts square in hardwood
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