Lap Joint in Wood: Half Laps, Strength and When to Use

A lap joint overlaps two members instead of butting them. In a half lap, half the thickness is removed from each piece so the assembled joint finishes flush at the original thickness. That is all there is to it, and it works for the one reason that governs all wood joinery: the overlapping faces are long grain against long grain, which is the bond glue holds best.

The contrast is stark. The USDA Wood Handbook puts a plain butt joint at only about 25% of the wood's tensile strength parallel to the grain, and says a well-manufactured scarf, finger or lap joint in end grain can have up to 90% of the tensile strength of clear wood and behave much like clear wood. The lap is in the good half of that comparison without requiring any layout skill.

• Cross lap - two members cross in the middle of their length, each notched half depth. Used for table stretchers, shelf egg-crates and any grid.
• End lap - two ends overlap to make a flat corner, common in simple frames and workbench bases.
• Tee lap - the end of one member laps into the middle of another, the workhorse of frame and ladder construction.
• Mitred half lap - an end lap with a 45 degree line showing on the face, so the front reads as a mitre while the joint behind it is a lap.
• Dovetail lap - the end lap cut with an angled shoulder so it also resists being pulled out, borrowing the mechanical lock from the dovetail.
• Lapped splice - two members overlapped along their length to make one longer member, with the handbook's warning that the overlap must be long relative to the section.

The bridle joint is the close cousin: a published survey of ancient Egyptian woodwork describes it as similar to a mortise and tenon except that the mortise and tenon are cut to the full width of the tenon timber, and records examples including a Ptolemaic-period try square in the Petrie Museum.

Two dimensions do the work: how much material you remove, and how long the overlap is.

Depth is usually exactly half in a half lap, because the flush result is the point. Cut both halves with the same setup from the same reference face so any error in depth cancels rather than doubling. If the members are different thicknesses, remove the same amount from each rather than half from each, and accept a step.

Length is where the strength lives. The handbook's rule for end joints is that to approximate the tensile strength of clear solid wood, the bonded surface should be at least ten times the cross-sectional area of the piece, because wood is roughly ten times stronger in tension than in shear. A short lap is a shear failure waiting to happen. For a splice lap under real load, make the overlap long and, if the load is significant, add bolts or a splice plate rather than trusting glue alone.

A lap is one of the few joints where machine and hand methods converge on the same result.

On a table saw or router table, set the depth with a test cut in scrap and check it by assembling two test pieces - if they finish proud or dished, adjust once and cut every piece in the batch without touching the setting again. Define the shoulders first with a clean crosscut, then clear the waste with repeat passes. On a router table use a straight bit and a fence, taking the waste in two or three passes rather than one deep bite.

By hand, saw both shoulders to the gauge line, chop or saw out the waste and finish the cheek with a router plane, which references off the face and gives a parallel bottom no chisel technique matches.

Glue both mating faces and clamp across the joint, not along it. Because the faces are long grain to long grain, a well-clamped lap needs no fastener; screws or a peg are there for assembly convenience and for joints that may be disassembled.

Removing half the thickness removes half the section, and a member is weakest in bending exactly where the lap is. In a cross lap in the middle of a stretcher that is usually fine, because the load there is mostly tension and compression along the length. In a tee lap carrying a bending load right at the joint, it is not fine, and a mortise and tenon that keeps more of the rail intact is the better answer.

The second cost is appearance: a lap shows end grain on the outside face of most versions. The mitred half lap exists to solve exactly that, and a dovetail lap solves it while adding a mechanical lock.

The third is the one the handbook attaches to this whole family. Because wood swells much more across the grain than along it, moisture changes in end-to-edge-grain joints produce large internal stresses, and such joints should be protected from changes in moisture content in service.

Wikipedia gives "overlap joint" as the main alternative name, and the family has several more:

• Half lap or halving joint - material removed from both members so the joint is the thickness of the thicker piece.
• Full lap - nothing removed, so the joint is the combined thickness of both pieces.
• End lap, pull lap or corner lap - two ends joined, usually at a corner of a rectangular frame.
• Half lap splice - an end lap with the members in line, an alternative to a scarf joint for joining short pieces end to end.
• Tee lap or middle lap - a cross lap in which one member ends at the joint.
• Dovetail lap and mitred half lap - the angled-shoulder and mitred-face versions.

Shiplap, the overlapping edge used on cladding boards, is a related but different joint.

A 2x4 actually measures 1-1/2 by 3-1/2 inches dry (USDA Wood Handbook, Table 6-6), so lay out from the real boards, not the nominal size.

• Decide the orientation. Crossing flat, each board loses 3/4 inch of depth over a 3-1/2 inch wide notch. Crossing on edge, each loses 1-3/4 inch of depth over a 1-1/2 inch notch.
• Mark the shoulders by laying the mating board across and knifing or penciling both edges, then square the lines around.
• Set the depth at exactly half from the same reference face on both pieces. School of Woodwork's two rules: the shoulder depth must not exceed half the thickness, and the cheek length should not exceed the width of the wood.
• Cut the shoulders to depth, then remove the waste. A table saw, bandsaw or handsaw all work (School of Woodwork); with a circular saw or handsaw, make several kerfs between the shoulders and knock out the waste with a chisel.
• Pare the cheek flat with a chisel or router plane and test-fit before gluing.

For outdoor 2x4 work, see treated wood for decks for fasteners suited to treated lumber.

School of Woodwork calls the lap "not particularly strong" because it relies on the long-grain glue area of the cheek, while Wikipedia, citing a Wood Magazine test, says a glued long-grain lap is among the strongest joints at resisting shear, ahead of mortise and tenon. Both are right about different loads:

• Shear on the glue line - strong, because long grain is glued to long grain.
• Racking - only the shoulders resist it, so Wikipedia suggests dowels or mechanical fasteners to stop the members twisting.
• Bending at the joint - each member is cut to half its thickness there.
• Mitred half lap - Wikipedia calls it the weakest version because the mitre reduces the glue area.

That is why School of Woodwork lists light uses - latticework, Japanese kumiko, box dividers, light door frames and glazing mullions - and Wikipedia adds cabinet frames, temporary framing and some timber-frame work.