What is a half-lap splice?
A half-lap splice is a wood joint used to connect two beams or planks end-to-end (lengthwise). In a half-lap splice, half the thickness is removed from each workpiece, so the two pieces overlap and together form the original thickness again.
How does it work?
Imagine you need a 6-metre beam, but you have two beams of 3.5 metres. With a half-lap splice, you can extend them:
- At the end of beam 1, cut away the top half over a length of approximately 20-30 cm
- At the end of beam 2, cut away the bottom half over the same length
- 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 splices
| Type | Description | Strength |
|---|---|---|
| Straight half-lap | Both cut surfaces are straight | Basic — good for compression, moderate for tension |
| Scarf joint (angled half-lap) | The cut surfaces run at an angle (diagonal) | Better — more glue surface area |
| Hooked splice (toothed half-lap) | With a hook or tooth in the joint | Strong — cannot slide apart |
When do you use a half-lap splice?
- 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 needs to be extended at corners or on long walls
- Extending battens — In timber frame construction or batten work
> Important: A half-lap splice should always sit on a support point (on a wall, truss, or column). Never make a half-lap splice in the middle of a span — the joint is too weak there.
Making a half-lap splice
- Determine the overlap length — The overlap is at least 2x the beam width (for a 75 mm wide beam: at least 150 mm overlap)
- Mark out — Mark the cut lines on both beams
- Cut — Cut to half the beam height, then horizontally to the end
- Test fit — Check that the two pieces slide tightly over each other
- Join — Glue + bolts or screws. For structural timber: at least 2 bolts through the overlap.
Related terms
- Half-lap joint
- Mortise and tenon joint
- Purlins
- Wall plate