A half-lap joint is a timber connection used to join two beams or planks end-to-end (lengthwise). In a half-lap joint, half the thickness is removed from each piece so that the two pieces overlap and together form the original thickness.
How does it work?
Imagine you need a beam 6 metres long, but you have two beams of 3.5 metres. With a half-lap joint you can extend them:
1. At the end of beam 1, cut away the top half over a length of about 20-30 cm<
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2. At the end of beam 2, cut away the bottom half over the same length
3. The two beams slide over each other and together form the full thickness again
The joint is secured with:
- Wood glue + clamps
- Bolts or screws
- A combination of both
Types of half-lap joints
| Type | Description | Strength |
|---|---|---|
| Straight half-lap | Both cut faces are straight | Basic — good for compression, moderate for tension |
| Scarf joint (angled half-lap) | The cut faces run at an angle (diagonal) | Better — more glue surface area |
| Hooked half-lap | With a hook or tooth in the joint | Strong — cannot slide apart |
When do you use a half-lap joint?
- Extending purlins — A purlin that needs to be longer than the available timber
- Extending a wall plate — The wall plate runs around the building and must be extended at corners or along long walls
- Extending rails — In timber frame or battening systems
> Important: A half-lap joint should always sit on a support point (on a wall, truss, or column). Never make a half-lap joint midway along a span — the joint is too weak there.
Making a half-lap joint
1. Determine the length — The overlap should be at least 2x the beam width (for a 75 mm wide beam: at least 150 mm overlap)
2. Mark out — Mark the cut lines on both beams
3. Cut — Cut to half the beam height, then horizontally to the end
4. Fit — Check that the two pieces slide tightly over each other
5. Connect — Glue + bolts or screws. For structural timber work: at least 2 bolts through the o
verlap.
Related terms
- Half-lap (cross)
- Mortise and tenon joint
- Purlins
- Wall plate