Quick Answer

A lap joint joins two pieces by overlapping them, with material removed from one or both pieces so the surfaces sit flush. The overlap creates face-to-face contact and a much larger glued area than a simple butt joint. The most common form is the half-lap, where each piece is notched to half its thickness so the joint lies flat.

How a Lap Joint Works

In a half-lap, each board is cut away by half its thickness for a short distance from the end. The two notched ends then nest together, overlapping and leaving the top and bottom faces flush.

The key improvement over a butt joint is surface area. Where a butt joint glues end grain to face grain, a lap joint glues the face of one notch to the face of the other — a long-grain to long-grain bond that is far stronger. The shoulders of the notches also register the pieces and resist sliding.

Where It Is Used

  • Frames and rails where pieces cross or meet at corners
  • Furniture frames and bracing
  • Door and window construction
  • Laminated or reinforced assemblies
  • Joining pieces of similar thickness

Why the Geometry Works

The joint works by replacing a weak end-grain contact with an overlapping face-to-face contact. The load is spread over the whole overlap rather than concentrated on a seam. The shoulders add registration, so the joint helps the assembly stay square during glue-up.

Performance still depends on fit, species, adhesive, and loading direction. A gap-filled half-lap loses much of its strength.

Main Variations

  • Half-lap, the basic notch-in-notch overlap
  • Cross lap, where two pieces cross in the middle
  • End lap and T-lap, depending on where the pieces meet
  • Mitered half-lap, used at frame corners

Advantages

  • Stronger than a butt joint thanks to face-to-face gluing
  • Registers the parts and helps keep them aligned
  • Relatively simple to cut
  • Flush surfaces when notched accurately
  • Good glue area for its simplicity

Limitations

  • Removes material, slightly weakening each piece
  • Not as strong as a mortise and tenon
  • Fit and flatness of the notches matter
  • Visible seam on one face
  • Less suited to thin stock

A butt joint has no overlap at all. A rabbet or dado joint gives one piece a recess to sit in rather than overlapping both. A finger joint is a more elaborate interlocking version of the same idea.

Related pages:

Common Mistakes

  • Confusing a lap joint with a simple butt joint
  • Assuming any overlap guarantees strength without good fit
  • Notching too deeply and weakening the stock
  • Ignoring grain direction in the overlap

References

See the References section for the sources used to write this page.